PRAIRIE DU SAC — Although Michelle Marks’ doctoral research focused on Wisconsin’s hop industry and its top disease, hop downy mildew, she also had advice for tomato growers in the crowd at the Wisconsin Fresh Market Vegetable Growers Association Fall School on Nov. 28, where she gave a disease outlook for 2019 for those growing them.
Fungal pathogens will remain the cause for the most common tomato diseases, whether they are grown inside a high tunnel system or outside in an open field.
Tomato leaf mold and tomato gray mold both thrive in high humidity environments, specifically high tunnel systems. And while tomato leaf mold only affects tomatoes, gray mold can affect a variety of vegetables, fruit and ornamental crops as well.
There are some varieties of tomato with leaf mold resistance, Marks said, adding that if a farmer has experienced problems before with leaf mold, selecting one of these varieties to grow instead may be valuable. Other management tips she offered included increasing air movement, reducing leaf wetness, removing the lower leaves of the plant to prevent spreading and practicing good sanitation, such as removing debris from the plants.
She had similar recommendations for gray mold; however, there are no resistant varieties of tomato currently on the market. She encouraged farmers to limit the damage done by gray mold by removing debris from the growing area as both leaf and gray mold are residue diseases, living on decaying plant material.
In the open field, where fungal spores can travel up to 40 miles in a day, diseases such as late blight, septoria leaf spot and early blight often plague tomato growers. Late blight in particular has “a high potential to do a lot of damage,” Marks said, “defoliating and melting plants in a matter of days.”
Late blight spores are produced in the spring and carried by wind or water to the plant. The fungus lives throughout the summer and into the fall, and when leaves are culled, the pathogen can live on into another year. Unlike the common high tunnel pathogens, late blight favors cool, moist weather and can affect tomatoes and potatoes.
Most finds of late blight in 2018 were in New York and Pennsylvania, although there were some reports in central Wisconsin. There are several lineages of late blight, with the most common one reported in Wisconsin being the US-23 lineage; last year, however, a new lineage of late blight was discovered in New York.
Marks provided the following management recommendations for late blight: grow resistant varieties and use preventive fungicides when the disease is present in your region. USAblight.org is a website that tracks late blight finds throughout the growing season and can be a good resource to find out if the disease has been recorded in your region, she said. Farmers can even set up an account and receive an alert when the disease has been detected in their area.
She also mentioned that the Plant Disease Diagnostic Clinic at UW will process suspected late blight samples for free as there is lots of interest at the UW in the area of late blight research. This push for a better understanding of late blight is coming from the state’s potato industry, Marks added.
Septoria leaf spot is a fungus that can survive over winter and is also a residue disease. It is dispersed through splashing water and favors moist and warm weather. Its appearance can mimic early blight, which has a similar disease cycle to late blight — sticking to debris and growing in the spring.
“Rotate, sanitize correctly and keep your plants dry — those are my three key points today,” Marks said.
There are also two bacterial pathogens that affect tomatoes in Wisconsin, although they are less common to find: bacterial spot and speck. These diseases favor high humidity and wetness, but are seed borne pathogens, so having clean transplants and clean seed is important to ward off these diseases.
Resources are available to help tomato growers identify diseases, including the Compendium of Tomato Diseases and Pests, a book that Marks said every professor that is in the field at UW has in their office; and the Commercial Vegetable Production in Wisconsin guide, released annually by UW-Extension and available online for free. She also suggested using the new UW Vegetable Pathology website, accessed at https://plantpath.wisc.edu/wivegdis.
Farmers can also use the Vegetable Disease and Insect Forecasting Network, an online resource that serves as a tool to find pests and diseases in your region. It, too, is a relatively new resource from UW-Extension, and can be found at https://agweather.cals.wisc.edu/vdifn/maps.