It seems like our gardens are going to be under attack by one enemy or another no matter how hard we work to manage them. This year I faced new invaders: anthracnose on my tomatoes, white flies on my kale and collards, and powdery and downy mildew on my cucurbits (cucumbers, squash and pumpkins). I don’t know how or why these issues attacked my vegetable garden this year, or if others fought the same battles, but will offer some background and treatments for each problem in the hopes that it may help others deal with them in the future.
Anthracnose
My tomatoes were off to a great start until about mid-August when I started seeing rot spots (see photo) on every tomato, on every plant, as they started to turn color, worsening as they ripened further. While I had seen anthracnose invasions on tree leaves, our dogwood for example, it had never shown up on vegetables before. After some investigation I concluded that anthracnose was the culprit and ended up pulling and trashing all my tomato plants, losing my home grown tomatoes this year. Bummer.
Anthracnose is a family of fungal diseases that can impact shade trees and a variety of vegetables including tomatoes, beans, peppers, eggplant, cucurbits, spinach and peas. It spreads via spores in cool, moist, breezy weather and forms soft lesions with dark centers that penetrate fruits as they ripen. The fungus overwinters in seeds, soil and plant residue.
Best management advice includes:
- Mulch beds with shredded leaves or newspaper and cover with straw
- Pick fruits quickly and remove any showing infection
- Avoid soil-fruit contact
- Don’t overwater during humid/cloudy weather. Water at ground level, not on leaves.
- Remove plant residue at the end of growing season (or sooner, as in my case). Don’t save seeds of infected fruits.
- Bag and trash all infected vegetation; don’t compost it.
- Clean and sanitize tools that touch infected plants
- Rotate vegetables to avoid re-exposure year to year
- Try to plant resistant varieties
- There are fungicides that fight anthracnose, but thorough coverage is required to be effective, so it isn’t really recommended for home gardeners.
- Find additional information in the Cornell Extension article “Anthracnose on Tomatoes”.
White Flies
White flies are an insect related to aphids, mealy bugs, and scale. They feed by sucking sap from plant vegetation. The many different white fly species can damage vegetables, greenhouse plants, and ornamentals. They die in cold climates but can reproduce year-round in warmer environments, including greenhouses. As the photo shows, they are 1/10 to 1/16 inches long with a tiny moth-like appearance. Adult females can lay 2-400 eggs that hatch in about a week. They hatch as flathead nymphs that crawl around plants, inserting mouthparts to feed. After molting, they lose their legs and antennae and attach to leaf undersides for about 4 weeks, looking like oval shaped scale. The adult stage lasts about a month. All life stages can be present and generations often overlap.
White flies damage plants by sucking plant juices. Large populations of developing nymphs weaken plant leaves, often turning them yellow and causing premature drop. They tend to suck more plant liquid than they can digest. This leads them to deposit a honeydew on the leaf, providing a growth medium for sooty mold that can harm plant health. Some species can transmit plant viruses.
Control actions include:
- Regularly inspect plants and remove heavily-infected leaves
- Encourage bio-control by supporting spiders, lady beetles, and lacewings.
- Note that pesticides will also kill predatory insects that help control whitefly infestations.
- Whitefly traps are available, but the traps can catch predators as well as the flies.
- Flies can be vacuumed off plants using a small hand vac. Put the vacuum bag or container into the freezer (not the entire vacuum) for 24 hours to kill the flies.
- Chemical controls are available and include neem oil, insecticidal soap, horticultural oil, pyrethrins, and permethrin. Be sure to target only the infected areas on target plants and follow label directions carefully. If chemicals are used, rotating them is advised to counter resistance that develops.
- Remove all infested plants and vegetation and keep soil clean to minimize year to year carryover.
Powdery and Downy Mildew
Powdery mildew on pumpkin. Photo: R Morini
Powdery mildew is a fungal disease that releases microscopic spores that spread across plants and extract nutrients. It looks like flour dusted on the leaf surfaces. Young leaves are the most susceptible, but it spreads from the crowns to lower leaves, including leaf undersides. In the autumn round black spots form that become fruiting bodies that release spores that spread the mildew. The effect is to slow plant growth, cause leaves to yellow and whither, distorting and stunting plants, and affecting fruit quality.
Powdery mildew tends to form during warm, dry weather but high humidity encourages spore formation and spreads the infection. It slows at temps below 60 and above 90 degrees. It can infect hundreds of plants from trees to edibles to grasses, with different species impacting different plants.
Downy mildew is a water mold disease, rather than a fungal disease. It thrives in cool, humid conditions and starts as a fluffy growth on the undersides of leaves. It grows into yellow dots on the upper leaf surface, causing leaves and ultimately full plants to wilt and whither. It moves fast, sometimes killing plants in a week. Also, some spores are released and can infect other plants. Spores can move into soil from rotting plants and survive for up to 5 years, making good bed maintenance a must. It can affect many and various plants and plant families including basil, cucurbits, brassica, roses, berries, impatiens and others.
Management practices for both powdery and downy mildew include:
- Look for and select resistant cultivars
- Regularly inspect at-risk plants
- Remove plant suckers and other new growth that is most susceptible
- Promote good air circulation by staking, pruning etc.
- Avoid plant crowding, carefully water at ground level, avoid overwatering, maintain good soil drainage and remove weeds.
- Dispose of any and all infected parts of plants up to and including entire plants and plant debris in a closed garbage bag or container.
- Sterilize garden tools that touch infected vegetation
- Certain fungicides may help, including neem oil and copper or phosphorous based fungicides. There is no guarantee, however, and fungicides can harm plants and pollinators. In all cases, follow label directions if fungicides are used.
Conclusion
These issues were my main garden battles in 2024. Regrettably, the anthracnose ruined my tomato crop. While the mildews prematurely killed my pumpkin plants, there were several fruits that were far enough along that, while a little smaller than we would have liked, gained color and were in full display on Halloween evening. Meanwhile a second planting of greens in a different area kept us supplied with kale and collards. Fingers crossed that the management practices above will help me do better next year. Gardening is truly a never-ending learning experience.
I hope readers find the material in the article and sources below helpful and wish you all luck in fighting these nemeses in the future.
Sources:
Anthracnose of shade trees | UMN Extension
Anthracnose diseases of trees | NC State Extension Publications (ncsu.edu)
Anthracnose Disease of Vegetables | University of Maryland Extension (umd.edu)
Anthracnose of cucurbits | UMN Extension
Anthracnose of Pepper | NC State Extension Publications (ncsu.edu)
Managing Whiteflies on Indoor and Outdoor Plants | MU Extension (missouri.edu)
Whiteflies Found on Flowers and Foliage | NC State Extension Publications (ncsu.edu)
Powdery mildew on trees and shrubs | UMN Extension
Addressing Downy Mildew and Powdery Mildew in the Home Garden (psu.edu)
Powdery mildew in flowers and vining vegetables | UMN Extension
https://www.extension.purdue.edu/extmedia/BP/BP-5-W.pdf
Most Troublesome Pumpkin Diseases in Virginia | VCE Publications | Virginia Tech (vt.edu)
Featured Photo: Early downy mildew on pumpkin leaf. Photo: R Morini