Estimated reading time: 10 minutes
Every gardener has experienced the disappointment that comes with discovering a disease in a once-healthy plant. Pests, fungi, bacteria, and viruses can play havoc with our flowers and vegetables. And, what’s worse is that some plant issues can infect the soil and spread to the rest of your garden.
This article explores some of the most common plant diseases, including how to identify them as well as preventive steps to take.
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1. Black Spot
Black spot is a fungal disease found on roses and other flowers and fruits. It first appears as small black spots on leaves, which then can turn yellow and drop off in severe cases. Bloom size is often affected. In addition to being unsightly, this disease weakens plans and makes them more vulnerable to other problems.
This black spot fungus usually overwinters in leaves and canes, so removing and disposing of diseased plant parts before winter is essential. During the growing season, water the roots, not the leaves. Use mulch to retain moisture and plant new plants where they will get full morning sun. Try dusting with sulfur to prevent and treat black spot.
2. Powdery Mildew
Powdery mildew shows up as a dusty coating on stems, leaves, and flowers. This fungus can affect lilacs, roses, phlox, daisies, apples, sage, grapes, squash, cucumbers, and peas, to name a few.
This fungus can thrive in high humidity and garden areas with poor air circulation, so pay attention to plant spacing. It can even stay active in dry conditions. Remove and destroy plant parts to prevent the spread of the disease. Water at plant roots during the cool morning hours.
3. Mosaic Virus
Mosaic viruses cause curled, yellowed leaves that curl. Some plants experience stunted growth and reduced yield. It can affect tomatoes, peppers, potatoes, pears, apples, and cherries.
The virus can survive in dry soil for several years, so removing and destroying infected plants and avoiding planting susceptible plants in the same area for a few seasons is important.
4. Damping-Off Disease
This condition, which often appears in greenhouse plants and new transplants, is caused by soil-borne fungi. It thrives in wet, humid conditions and causes young plants to decay.
You can prevent damping-off disease by using new pots, packs, or trays or disinfecting old ones with a 10 percent bleach solution. Also, use fresh potting mix and space seedlings so that they get adequate ventilation.
5. Rust
As its name indicates, rust is a fungus that forms reddish-orange spots on plant leaves and stems. As the disease progresses, the spots darken, turning brown and black. It typically affects roses, hollyhocks, rhododendrons, daylilies, and tomatoes. It also can show up on your lawn.
This fungus can overwinter, so removing affected materials is critical to getting rid of the disease. Many organic gardeners use a baking soda spray to help control rust.
6. Late Blight and Early Blight
These fungal diseases can affect tomatoes, potatoes, and other plants. Early blight first appears in the spring as dark leaf spots with concentric rings on stems and as large, dark, sunken spots on fruit.
Late blight shows up in late summer as irregular, dark blotches on older leaves or stems that can then spread to the fruit. The disease works quickly, literally destroying plants overnight sometimes.
Proper spacing and pruning for adequate air circulation is a preventative measure, as is watering at the roots rather than from above. Pick off and destroy leaves affected by early blight. Remove and destroy plants affected by late blight.
7. Grey Mold
Also called sooty mold, grey mold is a fungus that develops on dead or dying plants and can spread to healthy plants in wet conditions. Roses, peonies, geraniums, hostas, strawberries, and raspberries are susceptible to grey mold.
This fungus looks ugly, interferes with photosynthesis, and impedes plant growth. The best preventative step is to remove dead or dying leaves, flowers, and fruit and to mulch the soil so that rainfall does not spread the spores onto plants.
8. Snow Mold
As many areas of the country got large amounts of snowfall in 2023, gardeners should be aware of snow mold. This fungus can thrive in lawns in cold, wet conditions beneath the snow. When the snow melts, you may see threads of yellowy mold forming a barrier, preventing new grass from growing.
In the spring, lightly rake the grass lightly to break up the mold growth. In the fall, cut the grass short when you mow for the last time for the season.
9. Verticillium Wilt
Verticillium Wilt is a fungal disease that can infect berries, vegetables, deciduous trees, and herbaceous perennials. It is a common enemy of flowering cherry trees.
The fungus enters roots through the soil and then travels upwards. Symptoms include yellowing leaves and dying twigs and branches (often on one side). When you cut into a woody stem infected with verticillium wilt, you will notice black or brown streaks just under the bark.
To control this disease’s spread, removing infected plants, including their roots, is critical. Sterilizing cutting and pruning tools with a 10 percent bleach solution is also essential.
10. Canker
Canker can occur when fungus or bacteria enter a plant’s open wound. It is most common on woody plants with symptoms including cracked, sunken, swollen, or dead areas found on limbs, stems, or trunks. Situations that can lead to or spread canker include insects, rodents, extreme cold, drought conditions, root rot, and soil imbalances.
Treatment of canker usually includes removing infected branches with sterile pruning tools. Cauterizing the wound can help prevent a reinfection of bacterial canker. Disposing of any infected material immediately can help stop the spread of the disease.
11. Root rot
Root rot is a decaying disease that affects the roots of plants and trees. Symptoms include stunted growth, wilted leaves, branch dieback, and early leaf drop. Left untreated, the plant will die of this disease.
The main cause of root rot is poorly drained or overwatered soil that prevents roots from absorbing oxygen. Roots become thin and weak, causing the plant to decompose eventually.
To prevent root rot, make sure your garden has adequate drainage, avoid overwatering by checking moisture levels, and loosen the soil to allow aeration.,
12. Fusarium Wilt
Fusarium is a fungus that can lead to plant leaves wilting, yellowing, or getting dark spots. It thrives in hot weather and acidic soil. Susceptible garden plants include tomato, pepper, and eggplant. It also affects flowers like carnations and gladiolus.
This fungus works its way into a plant’s root system from the ground up. As it travels, it blocks the plant’s cells from receiving water and nutrients. Sometimes, one side of the plant will be affected, with the other side appearing normal. However, once the fungus is in your soil, it can keep attacking plants.
Discard infected plant debris, and do not put it in your compost pile. Disinfect all tools to help prevent disease spread.
General tips to prevent plant disease
Fungicides and insecticide sprays were once the go-to means to control plant disease. These days, however, many gardeners know the risks certain chemicals pose to their health and to the overall environment. For example, insecticides can kill pollinators and other helpful insects and can be harmful to our children and pets.
Here are some general tips to help prevent or cut down on the spread of disease in the organic garden.
- Make sure your garden soil can drain well. Use raised beds if needed.
- Water plants at their roots by hand or use a soaker hose to avoid getting leaves wet.
- Pay attention to the nutrients in your soil, adding appropriate fertilizers as necessary.
- Select disease-resistant plant varieties when possible.
- Avoid overcrowding your seedlings and young plants. Read seed packets for spacing guidelines.
- Prune and stake plants to improve air circulation.
- Remove and destroy infected plant parts to keep the disease from spreading. Place the debris in sealed bags and place them in the trash, not the compost pile.
- Water plants in the early morning hours when it is cool and use a mulch to help them retain moisture.
- Inspect all new plants for symptoms of disease or pests before introducing them to your garden.
- Disinfect all garden tools after cutting or pruning diseased plants.
Helpful resources on garden plant disease
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