Garden pests and diseases


The good, the bad, and the ugly

Accurately diagnosing problems before considering any treatment will ensure you do not solve your problem by creating a bigger one. The following tips and resources will help you tackle your bug and disease problems using the least toxic and most beneficial approach.

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Diseases Common in Vegetable Gardens


Numerous 1/4 inch spots appear in the foliage. In bad attacks, the spots join up and the leaf is destroyed. Central areas of each spot is pale brown and may drop out. It first appears as irregular water soaked spots that eventually turn brown.  This disease prefers cool temperatures (50 to 75˚ F).

Bacterial Leaf Spot

Treatment: Pick off and burn diseased leaves.

Prevention: Practice crop rotation. Apply a balanced fertilizer before sowing seed or transplanting. This bacterial disease is frequently seed borne, so if it is a serious problem you should treat seed with hot water (122˚ F for 25 minutes) or bleach (be careful however or you can damage the seed). If you are buying transplants make sure they are disease free. Bacterial leaf spot can also be spread by water splashing from soil to plant, so use mulch and drip irrigation. You should also ensure that leaves don’t stay wet for long periods (water early enough in the day so plants can dry out quickly). It’s also good to rotate crops on a 3 year rotation. If you see infected plants remove them immediately to reduce the spread of infection.

Smart Gardener, Inc. (2009-2013). Bacterial Leaf Spot. Retrieved from

Bacterial blight affects many different types of vegetables and can be a devastating disease. The first symptoms to appear are small, angular, pale green, water-soaked spots on the leaves. These gradually enlarge and merge to form large brown blotches with dry centers (commonly surrounded by a narrow yellow zone). In extreme cases the leaves may become scorched and withered and drop off. The stems may also develop brown blotches, as well as cracks or water-soaked cankers that ooze a yellowish liquid (this is very infectious).  

Credit D. Malvick (

Credit D. Malvick (

Bacterial blight is usually caused by infected seed (a single infected seed can result in a widespread infection). Minimize the spread of this disease by removing infected plants as symptoms show themselves (and don’t touch other plants before washing your hands thoroughly). The bacteria can also survive in crop debris, so this should be removed, or incorporated into the soil, where it can break down quickly. This disease needs moisture for transmission and reproduction, so avoid getting the leaves wet (use drip irrigation) and avoid touching the wet leaves. It can also be spread via soil or sap on tools.

To minimize the effects of this disease, give the plants plenty of water and nutrients, good air circulation and well drained soil. Rotate bean crops every 4 years.


Smart Gardener, Inc. (2009-2013). Bacterial Blight. Retrieved from

This fungus disease can appear in many different ways. The best known type of Damping Off first manifests itself as a fuzzy whitish mold on the surface of the soil and then goes on to girdle the stems of newly germinated plants. These develop shrunken black stems and eventually fall over and die, though the stem may remain upright for a while afterward. This type of Damping Off mainly affects very small seedlings and becomes is less of a problem as they get older and their stems get tougher.

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Other kinds of Damping Off kill the seed before it germinates, or rot the roots, causing the tops to turn yellow and die.

Prevention: The best defense against Damping Off is to avoid giving it the growing conditions it needs. Damping Off fungi need high humidity, so thin your seedlings promptly to prevent overcrowding, avoid overwatering (especially on cool sunless days when water doesn’t evaporate quickly) and make sure there is good ventilation and air circulation.

Treatment: Sulfur powder, or a tea of Seaweed, Garlic or Chamomile has been used to treat small areas of infection and prevent it spreading. Covering the seeds with compost or Sphagnum Moss (instead of sowing mix) may also help.

Smart Gardener, Inc. (2009-2013). Damping Off. Retrieved from


Large yellowish patches appear between the veins of older leaves. Whitish moldy areas develop on the underside. Later, diseased patches turn brown and die. This serious disease is worst in cool, wet conditions.

It is fast growing and may appear at any time of the season. It enters the plant through wounds and natural openings. Because of the whitish patches of mildew it is sometimes confused with powdery mildew or gray mold. Remember that Downy Mildew appears down on the underside of the leaf, while powdery mildew is on top of the leaf too.



Control Downy Mildew by improving air circulation and keeping the leaves dry (if you must use overhead sprinklers then water early in the morning or evening, so plants don’t stay wet all night). Spores overwinter on crop debris, so clean up the beds in fall. Also rotate plants and remove any infected plants promptly. The spores can travel long distances on the wind, especially in moist air. Some varieties are resistant to downy mildew.

Smart Gardener, Inc. (2009-2013). Downy Mildew. Retrieved from



Wirestem is a common soil fungus that can causes several different problems, some of which have different names. The disease in seeds and seedlings is known as Damping Off. If the plant germinates successfully it may succumb to the phase known as Wirestem, which shows itself as an area of constricted, dark colored infection around the soil line. This damages the outer layer of the stem, leaving the woody inner core intact (hence the name Wirestem). Unlike damping off, the stem doesn’t collapse and it isn’t necessarily fatal, however the plant won’t be as productive as it could have been.



Wet soil favors the disease, so keep it well drained and have good air circulation.

Smart Gardener, Inc. (2009-2013). Wirestem.Retrieved from


This is a serious fungal disease that causes the roots to swell up (like clubs) and become unable to function properly. A serious infection will often kill a plant, while a milder one will reduce the harvest. The most susceptible crops are Cabbage, Chinese Cabbage and Brussels Sprouts, though all Brassicas are vulnerable to some degree. An infected plant won’t show symptoms above ground until the root damage becomes serious enough that it affects growth.



You will then see wilting and eventual yellowing of leaves as they can’t get enough water and nutrients (wilting may also be caused by the even more common Cabbage Root Maggot, which also damages roots). Inspect for Clubroot by uprooting a plant and inspecting the roots to see if they are swollen and misshapen. If you find seriously infected plants you should remove them.

Prevention: The best strategy against Clubroot is to avoid importing it into your garden, which means not importing soil or any Brassica seedlings (it’s easy enough to grow your own). If this disease gets into your soil it can stay there for 10 years or more, even without any Brassicas to infect.

Treatment: Fortunately there is a good strategy for dealing with this potentially serious pest. Clubroot can only thrive in acid soil, so if you raise the pH of your soil it won’t be a problem. You have to get it up to neutral or slightly alkaline (pH 7.2 is ideal).

Smart Gardener, Inc. (2009-2013). Clubroot. Retrieved from


This fungal disease most often affects tomatoes, but may also be found on eggplant, peppers, potatoes, peas and squash family crops. It enters a plant through natural openings and wounds in the roots and grow up into the stem, where it blocks the supply of nutrients and water to the leaves. The first indication of infection is when a part of the plant starts to wilt on sunny afternoons, though it usually recovers when the temperature drops (this often starts to happen when plants begin bearing fruit).



Eventually the infection spreads through the whole plant, lower leaves turn yellow (and may eventually die) and the stem becomes discolored. Plants don’t always die, but it slows growth and reduces yields. Fusarium is rarely a problem for commercial growers because most modern tomato varieties have been bred to be very resistant. If you stick with resistant varieties you don’t have to worry about it either. Many of the older heirlooms don’t have any resistance, so if you grow these then you should keep an eye out for it. Fusarium Wilt is most problematic in warm and wet conditions. To minimize its effects you should keep the plants well fed and watered. Mulch can also help by keeping the soil cool. If any plants start to show symptoms of partial wilting you should remove them immediately to reduce the spread of this disease. The spores can survive in the soil for up to 7 years.

Smart Gardener, Inc. (2009-2013). Fusarium Wilt. Retrieved from


"Anthracnose is a fungal disease of corn, cucumber, beans, peppers, squash and tomato. It can spread very quickly in warm (80 degrees F), wet weather, especially if air circulation is poor. Fortunately for California gardeners it doesn’t thrive in our hot dry summers.

This disease first appears as small, variously colored, circular spots (those on watermelon are angular) on the older leaves, though it eventually spreads to younger leaves, stems, pods and fruit. The spots enlarge and merge, getting darker until the leaves drop off and the plant is defoliated (or the stem is girdled) and dies. Sunken, round, water-soaked spots appear on fruit.



Anthracnose prevention is easier than cure. Remove diseased plants promptly to minimize its spread. Keep the plants off of the ground on stakes or cages to provide good air circulation. The spores overwinter on volunteers and crop debris, so clear up the beds in fall and rotate your crops. The spores are most often spread via water, when soil containing spores are splashed onto the plants by rain or irrigation. You can reduce this by mulching around the plants and by using drip irrigation. They may also be spread on the hands if the gardener, so don’t touch wet plants (especially not after removing infected plants). Some crop varieties are resistant to Anthracnose.

Anthracnose can also be carried on the surface of the seed, in which case treat them with hot water (127 degrees F for 25 minutes) or bleach solution (1 part bleach to 10 parts water for 30 minutes) to kill the spores."

Smart Gardener, Inc. (2009-2013). Anthracnose. Retrieved from

"This cute sounding little virus disease is most prevalent on beets and chard, but may also affect beans, tomatoes, peppers and other vegetables. Infected plants wilt during the day and their leaves are commonly curled, or thickened, cupped, yellowed or rolled inward. Sometimes the veins on the undersides are purplish in color. All plants are stunted and don’t grow well, or produce any more fruit.



Curly Top is commonly spread by leafhoppers, which makes it hard to control because these creatures are very mobile and travel long distances. Only the individual plants visited by leafhoppers will be affected, unlike bacterial and fungal infections which may spread to every plant you have. If curly top becomes a recurring problem you could use row covers to keep sucking insects away. It may also help to interplant another crop between your plants, as this makes them less visible and can provides some shade (leafhoppers tend to feed in the sun). The virus can remain in infected plant debris for years, so keep the soil clear. Also remove diseased plants to reduce sources of infection."

Smart Gardener, Inc. (2009-2013). Curly Top. Retrieved from

"Root Rot is caused by a variety of fungi and can attack many different plant species (including trees and shrubs). The first clue to a problem is that plants aren’t very vigorous. As this progresses plants show typical signs of root damage, leaves turn yellow and wilt easily and growth is slow. Small plants and seedlings succumb quickly, larger plants (even shrubs and trees) eventually die too.



Check for root rot by looking at the roots. If they are rotten it is probably Root Rot (no surprise there). The fungi that cause Root Rot are found in all soils. Make sure the soil is well drained and has warmed up enough for your crop."

Smart Gardener, Inc. (2009-2013). Root Rot. Retrieved from