In last month’s web article, I talked about the myriad of fruit diseases that I’ve been seeing at the clinic. This month, I thought I’d provide a summary of the vegetable diseases that I’ve been seeing. Many of the vegetable samples that I’ve received have been from commercial growers, but diseases don’t discriminate and can affect both commercial- and home-grown vegetables alike.
Root/Crown rots
Given our regular (and often plentiful) rains this year (it’s pouring as I write this article), it’s not surprising that vegetable root and crown rots have been an issue. I have seen these diseases on peas, snap beans, and vine crops thus far this season. Typical symptoms of root/crown rots can include stunted growth, leaf yellowing and browning (caused by inhibited water and nutrient uptake), and discolored and mushy roots. The menagerie of organisms that I’ve recovered from root/crown rotted plants has been quite impressive. I’ve seen pathogens such as Phytophthora, Aphanomyces, and Pythium (all fungi-like water molds), as well as Rhizoctonia and Fusarium (two true fungi). Management of root rots in home gardens relies heavily on proper crop rotation, [i.e., making sure not to grow the same vegetable (or even vegetables in the same plant family) in the same area of your garden year after year after year]. This technique helps prevent the buildup of root/crown rot pathogens in the soil (where they tend to survive quite happily). In particular, water molds produce thick-walled, long-lived resting spores (called oospores) that can survive for years in the soil. For details on how to use crop rotation effectively in your home vegetable garden, check out the fact sheet here.
Black rot
This disease of cruciferous crops (e.g., cabbage, broccoli, cauliflower, kale) seems to have exploded. I have had four cabbage samples with this disease show up at the clinic in the past week. Symptoms of black root often start at the edges of leaves with yellowing that progresses into wedge-shaped necrotic (i.e., dead) areas with yellow borders. The points of the wedge-shaped diseased areas usually follow veins. This disease can be very destructive and lead to substantial losses in commercial vegetable production. The bacterium that causes this disease (Xanthomonas campestris pv. campestris) is typically introduced into a garden on contaminated seed or transplants. Once the disease has occurred in a garden, the bacterium can survive in small bits of debris from infected plants that filter into the soil. Crop rotation is a useful technique for managing this disease. In addition, make sure you plant high-quality seeds that are less likely to harbor the bacterium. If you have seed that you suspect may carry this pathogen, there are hot-water treatments that you can use to decontaminate the seed.
Interestingly, in 2024, I have seen a large number of Xanthomonas diseases (aside from black rot) on a wide range of plants including lettuce, tomato, pepper, begonia, zinnia, hydrangea, amur honeysuckle, peach, and winter wheat. This bacterial genus contains many common and destructive plant pathogens.
Verticillium wilt
I have only seen a single vegetable example of this disease so far this season (on melon), but Verticillium wilt affects a wide range of vegetable crops including tomato, potato, pepper, eggplant, and vine crops such as cucumber, melon, squash, and pumpkin. The fungus involved (typically Verticillium dahliae) survives readily in soil or associated with other plants (e.g., broad-leafed weeds) and infects susceptible vegetable plants through roots. It then colonizes the xylem (i.e., the water-conducting tissue) in the plants and blocks it, preventing water flow from the roots to the aboveground plant parts. This results in wilting and eventual plant death. Use of nonsusceptible vegetables (e.g., corn, peas, snap beans) and use of Verticillium wilt-resistant varieties of susceptible vegetables (e.g., tomatoes) are common means of managing this disease.
Veriticillium wilt is not only a vegetable disease, but it can also be a serious problem on woody and herbaceous ornamentals. So far in 2024, I have diagnosed this disease on Japanese maple (a very common host), catalpa (another common host), gas plant, and buttonbush. My buttonbush detection this year is only the second time in my 26+ year career that I have seen Verticillium wilt on this host. And, I have never seen the disease on gas plant before.
Powdery Mildews
As with Verticillium wilt, I have only seen a single case of vegetable powdery mildew this year, and that was on cucumber. I have also seen limited powdery mildew on other non-vegetable plants this year, with detections only on pear, serviceberry, and delphinium. Part of the reason I’ve likely not seen a lot of powdery mildew samples in the clinic is that many people are familiar with these diseases. They readily recognize the powdery white growth associated with powdery mildews and don’t need my help with an ID. In addition however, we’ve had a very wet growing season in 2024. Powdery mildew fungi don’t perform well when there is a lot of rain and leaves are wet. These fungi prefer dry leaves and high humidity. Limited rain and muggy conditions (which favor powdery mildews) are typical in Wisconsin in July and August, but not so much in 2024. We’ve had regular rains this year, so powdery mildew infections have been suppressed.
Interestingly, in one of the powdery mildew samples that I received, I found Ampelomyces, a fungus that parasitizes powdery mildew fungi. Ampelomyces provides natural biological control of these diseases. Interestingly, this fungus has been commercialized and is the active ingredient in certain fungicides that are used for powdery mildew control.
What vegetable diseases are you seeing in your garden?
If you find something interesting and need help in identifying the problem, please contact me at (608) 262-2863 or pddc@wisc.edu for advice. As always, for more information on plant diseases and their management, be sure to check out the UW-Madison PDDC website. To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe).