What's Wrong with My Garden? Part 2: Demystifying Common Diseases
Garden diseases can be difficult to diagnose—many of them have similar symptoms. They may have different treatments that are appropriate at different times in the disease lifecycles. And of course, you need to know when to try to save your plants and when it’s best to pull them out and minimize risk to the rest of your garden. We’ll cover all that and more in this simple guide to preventing, identifying, and managing garden diseases.
There are basically three primary vectors for plant diseases: bugs, moisture, and poor rotation.
Pests: Infestations of garden pests easily and quickly distribute spores from fungal diseases from sick plants to healthy ones. So staying on top of pest populations in the garden will greatly improve the health of your garden over the long term.
Moisture: The second vector can be even trickier to control—if humidity is high, drainage is poor, airflow around plants is limited, or you’re dealing with monsoon conditions, there’s a pretty good chance you’ll end up with some diseased plants. However, even after planting there’s still plenty you can do to help keep your plants healthy, like pruning.
Rotation: If you plant members of the same plant family in the same spot over and over again, you’re asking for trouble by providing a constant food supply for diseases. One of the best methods for discouraging diseases from taking up residence is to rotate your crops on a 3-year cycle. If you have no choice but to plant things in the same place, then you may want to read up on companion planting, which makes it harder for disease to spread in the garden.
One more thing – I can’t say enough about sanitation. It’s hard to try to do and even harder to actually do, but it’s really important. All you need to do is follow some simple rules. 1) Never touch a sick plant and then a healthy plant without washing your hands in between. This goes for pruners too – 2) Never prune a sick plant and then a healthy plant without sterilizing your pruners with alcohol. Keep a small container with you that you can quickly dip them in between plants. 3) Never put diseased plant tissue into the compost. Unless your compost pile gets really, really hot, you’re just risking inoculating all your plants when you spread the compost. Bag diseased tissue and throw in the trash or burn it right away.
The Cucurbits: Winter and summer squashes, zucchinis, cucumbers, and melons.
Powdery Mildew (PM) is a very common disease for this family and is also commonly found on garden phlox, lilac, bee balm, and many other plants. PM looks just like it sounds – powdery, as though the leaves have been dusted with flour. It is a fungus and tends to occur in warm, humid, shaded conditions and where pests are present. PM rarely kills a plant, though it can definitely reduce yields and result in premature fruit ripening. To prevent PM in the first place, choose resistant varieties, stake, tie, or trellis plants to ensure good air circulation, and never touch plants when they’re wet. Once you spot the disease, you can try either of these two homemade solutions: Mix 1 teaspoon baking soda with 1 drop of dish soap and 1 quart of water and spray on plants to raise the pH of leaf surfaces, discouraging diseases. You can also mix 1 part cow’s milk with 9 parts water and use this as a foliar spray after each rain. Severely affected leaves should be pruned off, bagged, and thrown away. A counter-intuitive but effective solution is to use a strong jet of water to knock the spores off leaf surfaces—if it looks like you’re starting to lose the battle, try this method on a sunny day to slow its progress. Sulfur, neem oil, jojoba oil, and biological fungicides may also be used in severe cases.
Angular Leaf Spot is caused by the bacterium Pseudomonas and generally occurs in periods of wet weather. Again, it looks how it sounds – like spots that turn brown, then transparent, then disintegrate and fall out of the leaf, forming a rectangular hole. On squashes and watermelon it tends to look more variable in size and is surrounded by a yellow halo. Eventually it will infect fruit with small, circular lesions. To prevent the disease, avoid overhead watering and handling plants when wet, and rotate crops. Always clean up diseased crop residues by plowing under, bagging and throwing away, or burning immediately after harvest. Once the disease has been spotted, you can try the baking soda spray above. Like PM, Angular Leaf Spot does not usually kill plants, and they will generally outgrow it if conditions dry out.
Downy Mildew (DM) is like PM and most other diseases in that it prefers moist conditions. It has a strong preference for cucumber plants, especially late in the season. It starts as yellow spots on the leaves, which look gray or purple and fuzzy on the undersides, and progresses rapidly to form angular lesions followed by brown, dry, curled leaves. It needs living tissue to survive, but it can be blown into northern areas by summer storms. Prevent the disease by maintaining good air circulation and never handling wet plants. Resistant varieties are few since the disease adapted and overcame previous genetic resistance. When it appears, quickly dispose of affected plants to avoid transmitting the disease to healthy ones, or try the baking soda spray.
Bacterial Wilt is a common disease caused by the bacterium Erwinia tracheiphila transmitted by cucumber beetles. If you see them, keep an eye out for wilt—first one leaf or part of one will wilt suddenly, starting where there is significant cucumber beetle damage, then the disease will spread outward until the whole plant wilts (unlike the wilt caused by squash vine borer, which will only affect the chosen vine. Before you do anything else, check around the base of the plant for small holes in the vines and sawdustlike droppings caused by vine borers). Bacterial wilt mainly affects pumpkins and squash, and there’s a test you can do to find out if you have it: Cut a wilting stem. Push the two cut ends together, then slowly pull apart. If the two ends are connected by a “string” of ooze, you probably have it. The disease should be prevented by choosing resistant varieties and excluding the beetles—keeping plants covered with row cover until they flower and spraying with a kaolin clay product. You can also use Baby Blue Hubbard Squash as a trap crop. It is very attractive to the beetles, the seedlings are vigorous and it is tolerant of bacterial wilt. Once the beetles have fully colonized the trap crop, bag and destroy the plants in the early morning when the beetles are sluggish.
The Nightshades: tomatoes, potatoes, eggplants, peppers, tomatillos, and husk cherries
Early Blight, or Alternaria is a severe fungus that affects tomatoes and potatoes. It likes warm, moist conditions and appears as one or two brown 1/4"-½” spots per leaf on older (lower) leaves that enlarge, developing tan centers with fine concentric rings similar in appearance to tree growth rings, and a yellow halo around the spot. Eventually the spots will grow together, causing leaves to turn brown and dry. Dark, sunken spots will develop on tomato stems, fruits, and potato tubers. To prevent the disease, maintain good air circulation by keeping tomatoes pruned and begin spraying baking soda solution or fermented compost tea two weeks before early blight normally appears. Remove and dispose of leaves in the trash as soon as the spots appear and spray with compost tea or baking soda solution after each rain. In severe cases, copper- or sulfur-based fungicides can be used. Carefully dispose of all infected plant material at the end of the season.
Late Blight has in the last few years become one of the most common and serious diseases of tomato and potato in the northeast. It is caused by Phytophthora fungi, especially likes cool wet weather, and causes distinctive irregular “water soaked” gray to purple spots on the older (lower) leaves. The spots enlarge and have a white downy texture on the leaf underside, and cause dark blotches on potato tubers that cause them to rapidly liquefy in storage (this is the same disease that caused the Irish Potato Famine). One trick to avoid the disease altogether is to plant a very early crop and a very late crop of tomatoes in a hoophouse, as vigorous young plants can better stave off the disease than mature ones. Infection can be slowed or prevented altogether in dry weather by keeping plants pruned and spraying foliage with fermented compost tea, which creates a physical barrier of beneficial microorganisms that outcompete the fungi. In wet weather, however, the disease will spread and rapidly cause rotting of the fruit and foliage. In dry weather it may be possible to slow the disease using copper-based fungicide sprays, but in most cases you will want to immediately pull up and destroy plants infected with the disease to prevent it from spreading to other tomato or potato plants. Always dispose of infected plant material quickly and properly. Choose varieties with resistance such as Matt’s Wild Cherry, Wapsipinicon Peach, Juliet, and Iron Lady F1 tomatoes, and Yukon Gem potatoes.
Septoria Leaf Spot is a very common disease of tomato that is usually not deadly.
The symptoms are numerous tiny brown spots on the lower leaves. The spots have no yellow halo, and do not cause stem or fruit damage. Simply prune off the infected foliage, disinfecting pruners when moving from plant to plant as you should always do when pruning.
Blossom Drop occurs in tomatoes in cool rainy weather as well as very dry conditions. Mature flowers simply fall off the plant and fruit (especially large fruit) fails to develop. This is usually caused by a magnesium deficiency and occasionally by a bacterial or fungal infection. Foliar spray with Epsom salts. Fruit set can be encouraged by gently shaking or tapping the plant on a sunny day.
Blossom End Rot looks like a water-soaked spot close to the blossom end of tomato fruits when they are about 1/3 developed. The spot enlarges, turning brown and leathery. It is caused by calcium deficiency which can occur through uneven watering. Use mulch and water regularly to keep soil evenly moist.
Scab in potatoes results in dry, corky spots on tubers and generally occurs where soils are dry and/or alkaline. To avoid scab, keep soil pH low and mulch to maintain even moisture in the soil.
The Brassicas: broccoli, cabbage, cauliflower, kohlrabi, and kale, as well as more distant relatives like radishes and mustards. Brassicas tend to be more disease resistant than cucurbits or nightshades, since they have thick waxy leaves that provide good protection from foliar disease. There are a few serious diseases that can affect them, however.
Black Rot causes leaves to yellow from the bottom of the plant upward, and causes distinctive V-shaped yellow lesions on the leaf margins. The disease can be prevented through crop rotation and good airflow, or by spraying a copper-based, sulfur-based, or biological fungicide.
Fusarium Yellows causes lower, then upper leaves to yellow and wilt, and causes broccoli heads to be stunted and bitter. Pull up and destroy plants with fusarium to prevent spread of the disease.
Club Root is a common fungus affecting brassicas. The symptoms are weakened, yellowed plants, leaves wilting on hot days, and older leaves yellowing and drooping. The disease can survive for many years in the soil. Upon inspection roots will be swollen and distorted. Pull up and destroy plants to prevent spread of infection. Choose resistant varieties and maintain a soil pH around 6.8.
Damping Off is caused by a wide variety of soil fungi. Usually seedlings rot at the soil line and fall over. Be sure to wait until conditions are warm enough before planting, provide even watering and good airflow. Drenching soil with Trichoderma harzianumbefore planting may prevent the disease.
Stewart’s Wilt is transmitted by overwintering flea beetles and causes leaves of corn plants to develop streaking and wavy edges. Broken leaves will ooze bacterial slime, and plants will become stunted or die. Plant resistant varieties, combat flea beetles, and destroy infected plants.
A Note on Basil Downy Mildew
Downy Mildew in basil has become a very serious problem in recent years, particularly wet years. The spores can overwinter, especially in protected spaces like greenhouses. The disease starts as a lightening of the leaves before the leaves develop
transparent or brown patches. Stems will then turn brown around the soil line – once you see this the plant will usually be dead in a matter of days. Keep an eye out for light, wrinkled, or transparent-looking leaves. Biological fungicides such as oxidate and actinovate, if used early and often, can prevent the disease. Destroy plants that have woody or brown stems and those that have wilted. Provide full sun, good air circulation, even watering, and choose resistant varieties as they become available.
To learn more about garden diseases in your area, contact your local Cooperative Extension Service.
And check out our previous articles on managing diseases: