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The Updated Plant Pest and Pathogens Glossary

Growing high quality crops takes expertise. It’s not something that’s learned overnight, and it can be very difficult to achieve on the first grow. In order to have the right approach, you need to have the right knowledge.

To help with a lack of experience, or lack of knowledge for new growers, we’ve developed this glossary of pests and pathogens. These are common infestations or infections that could affect your plants either in a small garden or a large growing operation. This list is also a great resource if you’re a more seasoned grower. Whether you’re looking to generally improve your pest management and understand possible plant diseases, or you need to look up a specific new disease that has surfaced in your plants, this is the perfect place to start your research.

This glossary is broken up into two main sections:

  • Pests – the larger organisms like insects and mites (although some of these aren’t very big at all)
  • Pathogens – the disease causing microorganisms like fungi, bacteria, and viruses

Often the first visible signs of pest or pathogen damage to your plants may not help you distinguish the underlying cause. But to properly treat the issue, you need to find the root of the problem. Being able to recognize these common pests and pathogens in your garden will get you one step closer to eliminating them, and cultivating a healthy growing operation.

(Didn’t find what you were looking for in this glossary? You may want to consider that what’s actually limiting the proper growth of your plants is a nutrient deficiency or otherwise poor soil health.)

Pests

Aphids

These little insects rely on the sap and juices of plants as their main food source. They are found in groups on a range of different plants, and are often found on new growth because it is easier to pierce. Aphids will feed on plant leaves, stems, buds, and fruit.

Sometimes known as blackfly or greenfly, these pests can blend right into green plants, making them hard to detect. Typical signs of an aphid infestation are yellow or curled leaves, or a steady trail of ants to and from your plants.

Aphids secrete a sticky substance known as honeydew when they feed on plants, which can attract ants and other pests. It can also lead to a plant disease called sooty mold.

Aphids reproduce a-sexually, which makes them especially problematic to deal with. They can multiply quickly to produce large populations. Aphids must be removed completely and as soon as they are discovered to stop the damage to your plants. Even one left behind can reproduce to create a high number of pests, and continue the infestation.

  • Physical Appearance: Small, soft-bodied insects varying in color, but often green or black.
    • Adults are less than ¼ inch in size and like to live on the underside of plant leaves.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Look for yellowing leaves that are stunted in growth. They may be shriveled or misshapen.
  • Affected Plants: Various plants can be affected by the different types of aphids. Some attack multiple different plant species, and others only feed on one type of plant.
  • Treatment Options: Beneficial insects, neem oil, horticultural oil, insecticidal soap, and insecticides have proven effective along with physically removing the aphids by pruning or spraying with water.

Barnacle Scale (Soft Scale Insects)

Closely-related to lobsters, these tough insects hide and lock on to the leaves and stems of plants, taking energy from them gradually over time. Soft scale insects like barnacles suck sap out of plants.

Some are soft to the touch, while others are much harder. They rarely move and can be difficult to notice until a large infestation has formed.

They can significantly weaken a plant over an extended period and will result in a lower yield overall if left untreated. Barnacle scale often attacks ornamental plants like gardenias.

Like aphids and other plant-sucking insects, soft scale insects leave behind a sticky secretion on leaves called honeydew, which can attract other pests and lead to sooty mold, turning plants black in color.

  • Physical Appearance: These pests look like marine barnacles that you might find in the ocean.
    • Small, mounded insects with an irregular shape. They have a white shell, speckled with a few darker spots.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Sticky honeydew left behind on leaves, yellow leaves, and leaves that fall off prematurely are all common symptoms of an infestation.
  • Affected Plants: Ornamental plants like gardenias, holly, and juniper, as well as citrus plants, are often attacked by barnacle scale.
  • Treatment Options: Soft scale insects can be picked off by hand, or infested branches can be pruned and disposed of. Beneficial insects, neem oil, horticultural oil, insecticidal soap and some insecticides can also be used for treatment of scale.
    • Insecticides and oils are not effective on adult stages of scale, since they’re covered in a protective wax coating.

Broad Mites

Broad mites commonly gather on the lower side of new leaves and lay eggs that leave behind a toxic substance. This substance can stunt plant growth and really impact bud yields over time.

They are very common in tropical climates and greenhouse growing environments. In more temperate climates, broad mites are not able to survive the winter months, so they tend not be as big of a problem.

Broad mites are very small pests, so they can’t be seen with the unaided eye. This can make detecting an infestation difficult. They can be identified using magnification to determine the underlying cause of plant damage and help make the right treatment plan for your crops.

  • Physical Appearance: Tiny, oval shaped insects. Usually, pale yellow or green in color, and only visible under a magnifying glass.
    • Adult mites are only fractions of a millimeter in size.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth. Causes distorted leaves, especially with new growth.
    • Look for dark brown edges at the base of young leaves. Older, and physically lower, leaves of the plant may appear fine, while the newer leaves at the top are severely damaged.
    • Damage from broad mites can easily be mistaken for heat stress, overwatering, or a virus attacking your plants.
  • Affected Plants: Vegetable and ornamental crops including peppers, tomatoes, cucumbers, azalea, begonia, gerbera.
  • Treatment Options: Predatory mites, neem oil, horticultural oil, insecticidal soap, and insecticides can work to kill broad mites. The best thing to do is remove any infected foliage from the plant, to contain the spread of the infestation.
    • Many common miticides (insecticides targeting mites) won’t work on broad mites.

Caterpillars and Inchworms

Caterpillars and inchworms both cause similar problems to plants. These long, soft-bodied insects chew on plant tissue, leaving behind distinctive holes on the leaves of your plants.

Moths or butterflies lay eggs on the leaves and buds of plants. Then those eggs hatch, unleashing hungry inchworms and caterpillars to do their damage.

Greenhouse environments typically provide some level of protection to your plants from caterpillars and inchworms, but adult moths and butterflies can still enter through doors and other openings, especially during the summer months. Outdoor crops have the added benefit of natural predators, which help control caterpillar populations.

These pests eat through leaves at an alarming rate and will eat through stems as well. Common signs of a caterpillar or inchworm infestation are holes in leaves and buds with dark areas or black sections. Some plants can sustain significant leaf damage from chewing insects without substantially affecting the health of the plant.

  • Physical Appearance: Long insects with cylindrical bodies and many small legs. They vary in color based on the species, to either blend in with their surroundings, or scare off predators.
  • Type of Damage: Chewing on and eating various plant parts, including leafy foliage, flowers, and fruit.
    • Plants either have obvious holes in the leaves, or rolled or webbed leaves. You might also see signs of caterpillar or inchworm activity from their excrement left behind on leaves.
  • Affected Plants: The various species of caterpillars and inchworms feed on many different types of plants. Some of them eat only one specific type of host plant, while others can attack a broad range of crops.
  • Treatment Options: Caterpillars and inchworms can be hand picked and removed from plants. Natural predators can also be introduced and insecticides targeting the young caterpillar stage can be applied to all areas of plants.
    • Adult moth and butterfly stages should be watched as well. Bright lights should be avoided because they can attract moths to your greenhouse or garden, and weeds should be removed to prevent possible host plants for egg-laying.

Crickets

These omnivores live underground and will burrow up to munch on plants that are accessible. Crickets will eat plants rapidly and can do serious damage in large populations if no treatment action is taken.

Crickets lay their eggs in soil at the end of the growing season. The eggs survive the winter months, and hatch in the late spring. Aside from plants, crickets also eat other insects, and have been known to eat fabric or clothing when they make their way into the house.

They’re known for the chirping sound made by the males, as well as their leaping or jumping movement made possible by their long back legs. Male crickets chirp to attract females or repel other males.

Crickets are sometimes mistaken for grasshoppers. Although they look similar, crickets can be distinguished by their longer antennae. They tend to come out at dusk and will eat more than just plant matter.

  • Physical Appearance: They have a cylindrical body with a round head, topped with long, thin antennae. Long back legs protrude up above the body allowing crickets to leap across your garden, and wings on their back enable them to fly.
    • Crickets, depending on the species, can be less than 1 inch in length up about 2 inches.
    • Often black or brown in color.
  • Type of Damage: Chewing on and eating various plant parts, including leaves and flowers.
    • The distinctive chirping of male crickets is a clear indication that they’re in your garden.
  • Affected Plants: Crickets can eat just about any plants.
  • Treatment Options: Natural predators like birds and spiders do a good job of controlling cricket populations, and some insecticides can be used to kill them as well.
    • Crickets are attracted to light, so reducing light in your garden at night can also help prevent

Fungus Gnats

Gnats are small, fly-like insects. They reproduce at an incredible rate and can terrorize the root systems of healthy plants.

Fungus gnats are very common in indoor growing environments, including greenhouses. They thrive in higher humidity and moist climates with damp soil.

Even though the adult gnats are typically the first indication of an infestation, because they can be seen flying around your plants, they are harmless. They don’t feed on plants, or even bite humans.

The damage comes from the larval stage which feeds on plant root systems in the soil. Fungus gnat larvae eat beneficial fungus and organic matter in the soil, which can limit the soil nutrients that are available to your plants.

The lifecycle of a fungus gnat, from egg to adult, can take place in about 3-4 weeks. Because of their quick growth and the high number of eggs that each adult gnat can lay, populations can multiply very quickly. It’s important when treating host plants for fungus gnat infestations, that the treatment continue until there are no surviving eggs left.

  • Physical Appearance: Adults are small, dark flying insects, about the size of a mosquito. Larvae have a long transparent or white body with a shiny black head.
  • Type of Damage: Feeding on root systems and fungi in the soil which can cause wilting and yellowing of plants.
    • Look for adult fungus gnats flying up from potted plants, especially while watering, as a sign of an infestation below the surface.
  • Affected Plants: Indoor plants and plants grown in humid environments with damp soil conditions.
    • Poinsettias, carnations, and geraniums are all susceptible to damage from fungus gnats.
  • Treatment Options: Insecticides can be used to kill fungus gnat larvae, and making sure not to overwater your plants can prevent the moist soil conditions that are ideal for adults laying eggs.
    • Be careful when purchasing plants. Avoid any plants that are surrounded by flying gnats, and check the soil at the base of the plant for any signs of larvae.

Grasshoppers

These powerful pests lay eggs that can last all throughout summer creating a common problem all throughout the growing season. Grasshoppers will eat plants quickly, and can seriously hinder bud yields if left unchecked.

Grasshoppers differ from crickets in that they are active and visible in your garden throughout the day. They have shorter antennae than crickets, and rather than chirping by rubbing their wings together, grasshoppers create more of a buzzing sound by rubbing their long back legs against their wings.

Grasshoppers are common in grasslands and drier, semi-arid climates. They lay their eggs in the ground in late summer, so tilling the ground in fall or early spring before the growing season can help destroy the eggs before they hatch.

  • Physical Appearance: Similar to crickets, grasshoppers have a cylindrical body with a round head. But they have shorter antennae. Long back legs that appear too big for their bodies give them the power to jump long distances and wings allow them to fly.
    • Often green or brown in color.
  • Type of Damage: Chewing on and eating various plant parts, including leaves and flowers.
  • Affected Plants: Grasshoppers will eat many plants, but they often prefer corn, grasses, alfalfa, cotton, rice, soybeans, and some vegetables like lettuce and carrots.
  • Treatment Options: Predators like birds, frogs, and snakes help control grasshopper populations naturally, but insecticides can also be used if crop damage is severe.
    • Removing weeds from the growing area or nearby land will eliminate food sources and help mitigate the growth of large populations.
    • Tilling soil between growing seasons can destroy grasshopper eggs and limit the growth of large grasshopper populations.

Leafhoppers

Leafhoppers are tiny, multi-colored pests that feed on plant sap and excrete honeydew, causing serious damage to their hosts. There are thousands of different leafhopper species throughout the world, and they cover a wide range of different colors including green, yellow, blue, black and brown.

Hoppers are common in outdoor gardens and in greenhouses with leafy plant growth. They are often found in tropical or temperate climates.

These insects typically suck the fluids from trees, shrubs, and grasses. Their excreted honeydew causes mold growth and over time, plants may exhibit common symptoms of discoloration and sagging.

Leafhoppers can fly, but are also adept at jumping. Their full lifecycle is short, at only a few weeks, so a single growing season would see multiple overlapping generations. Adults can overwinter in remaining plant residue, so it is important to manage debris around the growing area during the off season.

  • Physical Appearance: Small, wedge-shaped insects with wings.
    • Adult hoppers are only about ¼ inch in length.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Look for distorted or curling leaves, yellowing, spotting, and stunted growth.
  • Affected Plants: Common host plants include beans, corn, lettuce, potatoes, and roses.
  • Treatment Options: Beneficial insects, insecticidal soap, and insecticides can be used for treatment of large infestations.
    • Removing plant debris after the growing season will remove potential areas for the adults to overwinter until the next season.

Leafminers

Leafminer infestations are easy to recognize because they leave behind white zig-zag patterns on the leaves they feast on. They are fly larvae that eat plant matter between the top and bottom surfaces of leaves. The result is a clear, winding tunnel across the plant leaf.

Although damage from leafminers is not always detrimental to the plant, it can be visibly unappealing, especially in crops or ornamental flowers that are sold commercially. Damage can limit plant growth and reduce yields over time if the infestation isn’t treated.

Leafminers are typically found in landscaped areas, gardens, and greenhouses. The small, dark-colored flies lay their eggs on the undersides of leaves, just under the surface, where they hatch into the damaging larvae.

  • Physical Appearance: Adult leafminer flies are only about 1/10th of an inch long. They are gray or black in color with a yellow stripe. The larvae are small worm-like insects, pale-green or yellow in color.
  • Type of Damage: Feed on leaf material, weakening plants and reducing their attractiveness and value.
    • The resulting tunnels are easy to detect on leaves as a white zig-zag or winding path.
  • Affected Plants: Peppers, blackberries, beans, lettuce, elm, aspen, citrus plants, and ornamental flowers are some of the common host plants of leafminers.
  • Treatment Options: Beneficial insects and insecticides can be used to treat leafminer outbreaks.
    • Removing heavily infested leaves with signs of tunneling can help prevent the spread of the infestation.
    • Squeezing leaves between your fingers at the tunnel locations can squish the larvae and kill them. This works best at the first signs of tunneling before the infestation is too widespread.

Mealybugs

These insects are small, but tough, and they can pose a serious risk to most types of plants. Mealybugs are soft-bodied insects that look like small pieces of cotton. They are fluffy white, wingless insects that are found on the stems, leaves, and fruit of various plants.

Mealybugs suck the sap from plant tissues, which can lead to leaf curling and yellowing over time. Like other sap-sucking insects, they often leave behind sticky honeydew which can lead to mold growth or other insect activity.

They are usually found in warmer areas, and greenhouses and indoor growing environments because mealybugs are not able to survive cold winter months.

Females lay eggs in an excreted mass on the underside of leaves, which can sometimes be mistaken for downy mildew. Once the young nymph stages find a suitable feeding location on the host plant, they tend not to move much beyond that point.

  • Physical Appearance: Small, oval insects, white in color. They are soft and have a waxy coating.
    • Adult mealybugs are usually no more than ¼ inch in length.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
  • Affected Plants: Avocados, fruit, ornamentals and houseplants are all often attacked by mealybug infestations.
  • Treatment Options: Common treatment options for mealybug infestations include beneficial insects, insecticidal soaps, neem oil, and insecticides.
    • Pruning infested areas of plants, and spraying them down with water to physically remove mealybugs can also help to limit their spread.

Nematodes

Nematodes are microscopic worms. Like earthworms, they have a long, slender, and soft body, but these round worms are not visible to the naked eye. And more importantly, plant parasitic nematodes are not beneficial to your garden like earthworms. They feed on plants and can be very detrimental to your growing operation.

They can attack plants at the roots, stem, leaves, or flowers. Nematodes pierce plant tissues and feed on their fluids, which can lead to poorly functioning root systems and distorted leaves and buds.

Typical signs of a nematode invasion include wilting or a slow weakening of the entire plant, yellowing of leaves, and swelling of stems. You may also notice a slow decline in yield with no other visible symptoms.

Nematodes can spread to other plants by contact with things like gardening tools or farming equipment that was used on infected soil, or naturally by wind, water, or other carrier insects. Common nematode species in the United States include root-knot, soybean cyst, and root lesion nematodes.

  • Physical Appearance: Tiny worm-like pests with soft, slender bodies.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
  • Affected Plants: Nematodes can infect many different host plants, but some, like peaches, nectarines, radishes, and sugar beets are especially prone.
    • Some species of nematodes are host specific, while others can attack many types of plants.
  • Treatment Options: Planting resistant cultivars, or nematode resistant species of plants is an effective control method in areas with heavily infested soil. Rotating crops with non-host plants is also common.
    • Nematicides are effective at killing nematodes, but can be poisonous and damage plants even more. Fumigating or steam-pasteurizing soil can be an option for some growing operations, but they tend to be expensive.

Russet Mites

Similar in size to broad mites, these compact pests start at the bottom of plants and work their way up, doing serious damage to the stem and leaves on their way up. They feed on sap and drain nutrients leading to stunted growth over time.

They are not visible to the naked eye, so they are very hard to detect. Russet mites should be identified using magnification for accurate treatment.

Early signs of a russet mite infestation, like yellowing leaves, are often mistaken for other problems like water or nutrient deficiencies. But control and treatment at the first signs of damage are important to avoid significant damage to all parts of the plant.

The russet mite life cycle is very short, taking only about a week or two to develop from hatched eggs to adult mites. So, populations can grow large very quickly, before the root cause of the damage is even recognized. They can be spread easily by the wind, or by bringing infested plants into your garden.

  • Physical Appearance: Miniscule, wedge-shaped insects that appear yellow in color when large populations are present.
    • Magnification is required to identify them, but russet mites can be distinguished from other mites because they only have two pairs of legs.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Look for yellowing and curling leaves at the bottom of the plant that slowly moves up.
  • Affected Plants: Tomatoes, hemp, and cannabis are often attacked by russet mites, but they can also feed on potatoes, peppers, eggplants, and many other plant species.
    • Many species of russet mites are host-specific.
  • Treatment Options: Predatory mites, neem oil, insecticidal soap, and insecticides can all be used to kill or deter russet mites. Many insecticides targeting spider mites will also work on russet mites.
    • Treating a russet mite infestation can be very difficult, so prevention is the best method of defense for your plants. Removing and disposing of infested parts of plants, or entire plants, can help slow their spread.

Slugs or Snails

These pests are common in every garden. Slugs and snails feed on a wide range of plants, especially new foliage. They chew through stems and leaves, causing the same sort of damage as caterpillars and inchworms.

Snails and slugs are both slimy, gray-colored creatures with soft bodies. They crawl through gardens and across vegetation very slowly, leaving behind a slime trail. Snails have hard protective shells that vary in color from pale white to brown or black, and have a spiral shape.

They are both active at night and prefer warm, moist environments. Their eggs take months or even up to a year to develop, but they can lay up to 500 eggs each year, leading to increasing populations if left untreated.

  • Physical Appearance: Small, gray mollusks with two long tentacles with eyes that stick up from their head. Snails also have an external shell.
  • Type of Damage: Chewing on and eating various plant parts, including leafy foliage, flowers, and fruit.
    • Clear signs of damage from slugs or snails are holes in the leaves of your plants, and trails of mucus.
  • Affected Plants: Young plant growth is especially susceptible to damage from snails and slugs, but they will feed on many different types of plants.
  • Treatment Options: Birds, frogs, and snakes are natural predators that help keep slug and snail numbers low. They can also be picked by hand or deterred using common household items like crushed egg shells, coffee grounds, and salt.
    • Slugs and snails need hiding places around your plants to protect them from drying out during sunny days. By eliminating these dark, damp locations, you can make the environment less favorable for them.

Spider Mites

These small pests feed on the inner cells of a plant using a piercing mouth. Spider mites are small, spider-like pests that live on the underside of leaves. Once they invade gardens and growing operations, they can multiply rapidly and do serious damage in high numbers.

Although spider mites are usually too small to see without some magnification, they can be easily identified by the characteristic webs that they spin. Other signs of spider mite damage include yellowing and loss of leaves.

Spider mites thrive in environments with hot, dry summer conditions. They can be quickly spread by wind, which carries the mites on their webs to new host plants.

Two-spotted spider mites, Pacific spider mites, and red spider mites are all common species in North America.

  • Physical Appearance: Small, wingless pests with eight legs. Color depends on the species, but usually reddish brown or yellow.
    • Adults are about 1/50th of an inch in size.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Spider mite invasions are usually easy to identify by the fine webbing on host plants.
  • Affected Plants: Spider mites attack many types of plants, including tomatoes, strawberries, melons, and ornamental flowers like roses.
    • Dusty conditions and water stress make plants highly susceptible to spider mite damage.
  • Treatment Options: Predatory mites, insecticides, insecticidal soap, and neem oil can all be used to control spider mites. Pruning and removing infested plants is also an effective method to prevent the spread of these pests.

Thrips

Thrips are very small, slender pests with wings in their adult stage. The wingless larval stages feed on plants, including garden vegetables and flowers, and can transmit viruses to your plants too

There are thousands of different species of thrips, and they can be found in many different colors. Translucent, tan, dark brown, and black thrips are all common.

The adults will fly around when disturbed, making them easy to spot. Leaves that are infested with thrips can become distorted or twisted from the larvae sucking sap from the plants.

Adult thrips lay their eggs in plant tissue, including stems, leaves and flowers. The eggs can hatch very quickly in warm weather, and the entire life cycle might only take about two weeks from egg to adult, depending on the climate.

  • Physical Appearance: Tiny, slender winged insects, most often yellow or black in color.
    • Adults are less than 1/25th of an inch in size.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Look for pale, splotchy leaves, and scarred or twisted plants.
    • Thrips can also spread viruses to their host plants.
  • Affected Plants: Vegetables like onions, carrots, and squash, as well as roses and other flowers are common host plants for thrips.
    • Thrips are attracted to light colored blossoms.
  • Treatment Options: Beneficial insects, insecticides, neem oil, and insecticidal soap can help treat thrip infestations.
    • Removing nearby weeds and grasses can eliminate other possible host plants in your garden, limiting the spread of these pests.

Whiteflies

Whiteflies are small, white insects with wings. They are related to aphids and have soft bodies with a triangular shape. They are typically found in large numbers on the backside of leaves, but can be easy to identify on your plants because they will scatter and fly away in swarms when disturbed.

They are not able to survive cold winter months outside, so in colder regions, whiteflies tend to only be a problem in greenhouses and other indoor growing environments. But in warmer areas where whitefly populations can overwinter, they will continue to reproduce and multiply in your garden year-round.

Common species of whitefly include the silverleaf whitefly, greenhouse whitefly, giant whitefly, and bandedwinged whitefly. Most whitefly species target a narrow range of host plants.

Like other sap-sucking insects, whiteflies pierce plant tissues and feed on their fluids. They leave behind honeydew, the sticky residue that can cause other problems for your plants, including the growth of mold and infestation by other pests.

Large whitefly populations can be very hard to treat, so it’s best to prevent infestations by inspecting new plants from nurseries, and controlling their spread early once identified.

  • Physical Appearance: White, soft-bodied insects with wings.
    • Adults are about 1/12th of an inch in size. They’re active during the day, and easy to see flying around when disturbed.
  • Type of Damage: Suck fluids from plants, weakening them and preventing growth.
    • Plants may wilt and show signs of yellowing or shriveled leaves.
  • Affected Plants: Houseplants, ornamental flowers, citrus, and garden vegetables like tomatoes, sweet potatoes, and peppers are common host plants for whiteflies.
    • They especially like new growth.
  • Treatment Options: Insecticidal soap, beneficial insects, and neem oil can help reduce whitefly infestations, and water spraying can help knock them down from your plants.
    • Using reflective mulch in your garden can also repel whiteflies, making it harder for them to find host plants.

Pathogens and Plant Diseases

Black Spot

Black spot is a common fungal disease found on roses. It causes the leaves of the plant to develop round black spots, limiting photosynthesis. If left untreated, the infected leaves will eventually drop off.

Cool, moist conditions aid in the growth and spread of the disease. Black spot fungal spores can overwinter on infected plant matter until they’re able to germinate in warmer weather.

There is no cure for plants infected with black spot, so preventing the disease is your best option, which can be done through growing practices and the use of commercial fungicides.

  • Type of Pathogen: Fungal
  • Disease Signs and Symptoms: Black spots on the top side of plant leaves, which grow as the disease progresses, and often become surrounded by a ring of yellow.
    • Spots usually start on the lower leaves of the plant and spread upward.
    • Eventually will lead to loss of leaves.
  • Affected Plants: Roses are the most common host plant, but black spot can affect many different types of plants if the conditions are right for fungal growth.
  • Treatment / Prevention Options: Choose sunny locations for your plants with good air flow and soil drainage to avoid the wet or damp conditions that black spot spores need to grow. Prune plants to avoid thick foliage, remove any infected leaves, and try to not get the leaves wet while watering.
    • Many plants, including roses, are available in resistant cultivars.
    • Some fungicides can be applied to plants to prevent black spot infections, and slow their spread.

Botrytis Blight (Bud Rot)

Also known as gray mold, botrytis blight, or bud rot is another disease caused by a fungus which thrives in humid environments. It attacks ornamental trees and shrubs, and many perennial garden plants. It’s also very common in cannabis.

The fungus infects tender parts of plants, especially flowers and buds. Flowers suffering from botrytis blight can often be mistaken for old flowers that are just naturally fading. This fungal disease causes discoloration, spotting, and wilting in your plants, and infected buds might not open at all. You may see a gray mold develop on your plants, and fruit can rot and fall off.

Botrytis blight is usually noticed on plants after a long period of wet, rainy weather in the spring or summer. The spores can spread easily with the wind, and once they’ve found a host plant, moist conditions allow them to grow and begin damaging your plants.

  • Type of Pathogen: Fungal
  • Disease Signs and Symptoms: Browning and wilting of flowers, with buds that don’t open. Botrytis blight will also produce gray spores or brown lesions on leaves and shoots of plants.
    • Flowers typically start to brown from the inner petals first, unlike old flowers which have discoloration starting at the outer petals.
  • Affected Plants: Garden perennials and ornamental trees and shrubs are often attacked by botrytis blight.
    • Cannabis buds are also often affected by bud rot.
    • Lilies, peonies, roses, tulips, and geraniums are common host plants as well.
  • Treatment / Prevention Options: Prevent disease by watering as close to the soil as possible, to keep plant leaves dry. Prune foliage frequently to maintain good air circulation.
    • Fungicides can be applied as preventive measures and for early treatment of infections.

Downy Mildew

Downy mildew typically exhibits as spots or lesions on the surface of the leaves of infected plants. It’s caused by various different fungi, which cause different symptoms in different host plants. This can make it hard to diagnose.

Some common host plants for downy mildew include grapes, vine vegetables, roses, violets, and mint. Although disease symptoms vary by host, the underside of leaves often develop a downy growth of fungus, white or gray in color.

Downy mildew pathogens thrive in cool, wet environments. They can overwinter in soil and plant residue, so keeping your garden free of diseased debris from the previous season is critical for control of the disease.

  • Type of Pathogen: Fungal
  • Disease Signs and Symptoms: White or gray fungal growth on the underside of leaves, and yellow or dark discoloration on the surface of leaves.
    • Downy mildew is sometimes confused with powdery mildew because of the white colored fungus that grows on leaves. But the key distinction is that the cotton-like downy mildew fungus will be found on the underside of leaves, not the top.
  • Affected Plants: Grapes and vegetables that grow on vines are common host plants for downy mildew, as well as roses, violets, snapdragons, and pansies.
  • Treatment / Prevention Options: Avoiding wet conditions by not watering leaves from overhead, and maintaining good air circulation through pruning is the best option for preventing the disease.
    • Destroy any plant matter that is infected with downy mildew, even at the end of the growing season, to keep from spreading the disease in the next season.
    • Fungicides targeting downy mildew pathogens can be used as preventive measures, or at the first signs of disease.

Fusarium Wilt

The fungus causing fusarium wilt damages plants by limiting water flow. Plant roots take up the pathogen, which interferes with water distribution.

As the disease progresses up into the stem and leaves, plants begin to show the distinct symptom of wilting. Foliage turns yellow, and in some cases will shrivel and dries up entirely, causing it to fall from the plant.

Fusarium wilt develops best in hot, dry weather, in soil that has a low moisture content. The fungus can live in soil for years, so it can be easily spread.

  • Type of Pathogen: Fungal, soil-borne
  • Disease Signs and Symptoms: Wilting and dry, shriveled leaves. If left untreated, fusarium wilt can kill the plant.
    • Lower, older leaves are usually affected first as the disease spreads up the plant from the roots.
  • Affected Plants: Fusarium wilt often attacks tomatoes, potatoes, peppers, and eggplants.
    • Many weeds can also act as host plants for fusarium wilt.
  • Treatment / Prevention Options: Fungicides and planting wilt resistant varieties of plants are the best preventive measures against fusarium wilt. Diseased plants should be removed and destroyed immediately, and all gardening tools should be sterilized to prevent spreading this soil-borne disease.
    • Infected soil can be heated to kill the fusarium wilt pathogen by covering the soil with a plastic tarp for several weeks during the hottest part of the season.

Leaf Septoria (Yellow Leaf Spot)

This damaging disease diminishes plant yields over time, affects foliage, and stunts plant growth. Leaf septoria, or yellow leaf spot as it’s commonly known, can be characterized by yellowing and spotty leaves.

Cannabis crops are very susceptible to yellow leaf spot, as are tomatoes, parsley, and potatoes. Hot, humid conditions encourage fungal growth that causes the disease, and it typically attacks plants in their flowering stage.

Leaf septoria can be prevented by avoiding the optimal climate that allows the fungus to thrive, especially in a greenhouse growing environment where temperature and humidity control are more readily available. Fungicides and neem oil can also be used to treat infected plants.

  • Type of Pathogen: Fungal
  • Disease Signs and Symptoms: Circular spots, yellow or brown in color, that form on both the top and bottom side of leaves.
    • Lower leaves are usually affected first, and the disease surfaces when plants are in their flowering stage.
  • Affected Plants: Cannabis plants are frequently attacked by yellow leaf spot. Tomatoes and parsley are also common host plants.
    • Some weeds can also act as host plants for leaf septoria.
  • Treatment / Prevention Options: Fungicides and neem oil are effective at controlling leaf septoria outbreaks.
    • Controlling growing environment temperature and humidity when possible is an effective method of preventing the disease. Pruning, keeping leaves dry, and removing weeds and infected plant foliage can also help to prevent infection.

Powdery Mildew

This fungal disease looks like a white powdery mold covering your plants. It is common throughout many different areas and growing environments, but really thrives in warm, dry daytime climates, with cool, moist nights.

Powdery mildew spores spread through the air, and can infect various host plants. Squash, cucumbers, and roses are all prone to infections, but many of the disease causing pathogens are host specific.

Powdery mildew infections weaken plants, but don’t typically kill them on their own. This will make your plants more susceptible to damage from other pests and pathogens. Diseased plants may also lose foliage and experience stunted growth and decreased productivity if left untreated.

Prevention of powdery mildew is much more effective than treatment because the damage that’s done to the plant can’t be reversed. The use of fungicides and other disease management techniques to avoid ideal fungal growth environments are effective at controlling powdery mildew infections.

  • Type of Pathogen: Fungal
  • Disease Signs and Symptoms: A white powdery growth on plant foliage, often starting on the underside of leaves and spreading to the top of leaves and even plant stems or buds.
  • Affected Plants: Most plants are susceptible to this disease.
    • Squash, pumpkins, cucumbers, and roses are often attacked by powdery mildew.
  • Treatment / Prevention Options: Fungicides can prevent disease emergence and stop the spread of an infection by killing the fungus.
    • Maintaining good air circulation, avoiding overhead watering, and removing any diseased plants from the growing area will also limit the chances of a powdery mildew infection from manifesting in your garden.

Root Rot

Caused by overwatering, root rot happens when plant roots begin to decay from a lack of proper aeration. This is especially common with indoor houseplants.

Root rot typically becomes evident when plant leaves start to turn yellow and dull after a period of overwatering. Even after the soil returns back to a normal moisture content, the plant does not recover and will eventually die.

To avoid this issue, only water plants as the surrounding soil becomes dry.

If you have a plant suffering from root rot, it can be treated by removing it from the soil, washing and trimming off all decaying roots, and replanting it in fresh soil with proper drainage. This will only work if the damage has not spread throughout the entire root system, and there are still some healthy roots left to regrow the system.

  • Cause of Damage: Overwatering, leading to a lack of oxygen for the root system.
    • Root rot is caused by a fungal pathogen in the soil that becomes active when the plant is overwatered.
  • Disease Signs and Symptoms: Yellowing and wilting foliage is often the first visible sign of root rot. Stunted growth and general signs of nutrient deficiency precede plant death.
    • The root system, if inspected, will be soft and mushy, rather than firm like it should be in a healthy plant.
  • Affected Plants: All plants are susceptible to root rot, but it is very common in indoor growing environments with potted plants and even outdoor gardens where there is not adequate drainage of soil.
  • Treatment / Prevention Options: To treat root rot, remove the plant and root system from the soil, remove decaying roots, and plant in fresh soil to allow the plant to regrow the root system.
    • Prevent root rot by avoiding overwatering.

Rust

Rust disease in plants often affects tomatoes, beans, roses, and snapdragons, among many other plants. It’s a fungal disease that’s spread by the wind or by splashing water.

Plants suffering from rust will have distinct red-orange spots on the surface of leaves. These fungal growths, or spore masses, begin as small white bumps on the bottom of leaves, and progress to the typical rust spots that give the disease its name.

If left untreated, rust can lead to yellowing of leaves, distortion, and even defoliation.

The fungal spores that cause rust disease in plants can overwinter inside infected plant matter. Ideal climate conditions for the disease to thrive are generally mild. The fungus grows well in warm, moist environments.

  • Type of Pathogen: Fungal
  • Disease Signs and Symptoms: Rust colored spots of fungal growth on leaf surfaces are characteristic of this disease.
    • It often attacks the lower leaves of mature plants.
  • Affected Plants: Tomatoes, beans, roses, and snapdragons are all common host plants for rust.
  • Treatment / Prevention Options: Removing infected plant tissue by pruning and raking around your plants will help prevent the spread of rust infections. Fungicides work well as a preventive measure, and can be applied at the first signs of disease to treat your plants.
    • Planting rust resistant plants is also an option to prevent disease.

Sooty Mold

This disease develops on plant surfaces that are covered in honeydew. The sticky secretion is left behind on plants by certain sap-sucking pests, like aphids and mealybugs, and creates an optimal environment for sooty mold fungi to grow.

Sooty mold is a dark fungal growth on plant surfaces, that resembles soot. But it doesn’t grow exclusively on plants because it doesn’t actually infect them. This mold can grow on any surface that might have honeydew left behind on it.

Damage to plants from sooty mold is primarily from covering leaf surfaces which inhibits photosynthesis. This can lead to stunted growth. The fungi feeds on the sweet honeydew, and doesn’t actually damage the plant itself.

The growth of sooty mold on your plants is a clear indication that there is a pest infestation as well. And these pests are likely causing their own damage.

The best way to prevent sooty mold in your garden is to have good pest control. If there are no insects secreting honeydew, there will be no sooty mold growth on your plants.

  • Type of Pathogen: Fungal, relying on honeydew from plant pests
  • Disease Signs and Symptoms: A dark, thread-like mold that grows on plant surfaces.
    • The presence of plant-sucking insects, honeydew, and ants are a precursor to a sooty mold outbreak.
  • Affected Plants: Any plant or surface with honeydew on it will sustain sooty mold growth.
  • Treatment / Prevention Options: Sooty mold should be controlled by managing the insects that create honeydew.
    • The sooty mold fungal growth can be washed off of plant surfaces.

Tobacco Mosaic Virus (TMV)

This rod-shaped virus affects plants at the cellular level. It causes a pattern of discoloration on leaves and will significantly reduce the growth rate of most plants. It is transmitted mechanically by contact with plants by growers, tools, or other plants.

The visible symptoms of a tobacco mosaic virus infection depend a lot on the host plant. This virus can cause leaf curling, stunted growth, mottling, and discoloration of plant leaves as well as fruit.

TMV often affects tobacco and tomato plants. There is no treatment for the disease, so prevention and control of spreading the virus are necessary to maintain healthy plants.

  • Type of Pathogen: Viral
  • Disease Signs and Symptoms: Discoloration of leaves and fruit, stunted growth, and mottling in a mosaic pattern are all common symptoms of a tobacco mosaic virus infection.
  • Affected Plants: Tobacco and tomato plants are very susceptible to tobacco mosaic virus, as are peppers and some ornamental flowers.
  • Treatment / Prevention Options: TMV resistant cultivars are available for both tobacco and tomato plants.
    • Good sanitation practices should be developed to prevent spreading the disease, like cleaning all gardening tools and washing your hands before having contact with plants. Any diseased plants should be removed and disposed of immediately.
    • Cross protection can also be an effective method of preventing an infection by a more damaging strain of the virus, by infecting plants with a weaker strain. Very much like a vaccine.

Verticillium Wilt

Verticillium wilt is a fungal disease that can be hard to identify because the symptoms are very similar to other plant diseases and nutrient deficiencies.

The soil-borne fungi that causes the disease enters plants through the root system. It travels through the plant, and causes leaves to wilt, yellow, and curl. If left untreated, leaves will brown and drop from the plant.

It can’t be cured, so removing diseased plants from your garden is essential to prevent spreading this fungal pathogen.

  • Type of Pathogen: Fungal, soil-borne
  • Disease Signs and Symptoms: Wilting, yellowing and curling of plant leaves.
    • Trees suffering from verticillium wilt can be diagnosed by observing dark discolored rings in the cross section of the wood.
  • Affected Plants: Verticillium wilt can affect many different types of plants including annuals, perennials, vegetables, trees, and shrubs.
  • Treatment / Prevention Options: Removing diseased plants from your garden and destroying them is the best method for controlling the spread of the disease. Verticillium wilt cannot be cured once plants are infected.
    • Heating infected soil through solarization can help kill the fungus.

By Alli Klein

Alli is a copywriter with a passion for sharing knowledge. She has her Bachelor’s degree in Chemical Engineering from UCLA and contributes articles as a hobby outside of her day job. She loves being outdoors, spending time with her dogs, and is working on cultivating her own garden in Idaho.

1 comment

  1. Disease caused by fungus on different field crops is serious problem in our region. This publication is very important to know and understand the behavior and epidemiology of disease to take appropriate control measures. Thanks for your contribution.

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