Killing white mold on seedlings soil1/7/2024 ![]() Dead infected plant tissue may take on a dry, bleached papery look (called “timber rot” in solanaceous crops like tomatoes, fig. Initial infection may show up as a small, circular, water-soaked lesion, but it typically grows in size rapidly. Because of these requirements, white mold infections often start on crop debris, such as fallen flower petals, that have landed in spots that also tend to collect moisture, such as leaf axils (where leaf meets stem) or stem joints (fig. These conditions produce the most rapid development of disease, and white mold infection severity is greatly reduced in very hot weather. Successful germination of airborne spores is favored by relatively long periods of wetness (16 to 72 hours) and moderate temperatures (68 to 77 F). This can be as minimal as a fallen flower petal, or a leaf that has dead leaf margins. Modified from image by Anthony Rittmeyer, courtesy of Wikimedia CommonsĪirborne spores require injured or dead tissue as a food source to initiate infection of healthy plant tissue. ![]() Apothecia produced by white mold (Sclertotinia sclerotium), with a dime for reference. If these spores land on a plant under appropriate environmental conditions, they can infect the plant. The majority of apothecia grow from sclerotia in the top 1 inch of soil. Apothecia are most likely to be formed after the plant canopy closes, creating humid conditions that favor spore survival. 5), which releases thousands of airborne spores, called ascospores. ![]() When soil remains cool and very wet, sclerotia also produce a mushroom-like fruiting body at the soil surface, called an apothecia (fig. This is the primary pathway of initial infections in the earlier part of the season. Germinated sclerotia in the top 4 inches of soil produce mycelium, which may infect nearby plants directly. Sclerotia of white mold (Sclerotinia sclerotium) formed inside an infected tomato stem. Sclerotia will not germinate, however, unless they have been stratified (undergone a prolonged moist cold period in temperatures below 40 F). In the spring and early summer, overwintered sclerotia in the top 2-4 inches of soil are triggered to germinate in moist soil as temperatures reach the 50s (F). 4), which are very tough and can survive in the soil for five to eight years. White mold overwinters as sclerotia - dark, hardened masses of mycelium, said to resemble rat droppings (fig. White mold (Sclerotinia sclerotium) infection of green beans. “Soft cottony rot” of carrot, caused by white mold (Sclerotinia sclerotium) infection. “Timber rot” symptoms of white mold infecting tomato. A related species, Sclerotinia minor, causes “lettuce drop” in lettuce, and organic management considerations are similar. White mold infections can occur in most, if not all, vegetable crops, but damage is most commonly noted on lettuces, green beans, cucurbits, brassicas, carrots, potatoes and tomatoes, especially when grown in higher-humidity environments like high tunnels and greenhouses (fig. Understanding the disease organism’s lifecycle is critical to successfully managing it. It is only a sporadic problem in “normal” and dry growing seasons however, white mold can be very damaging in wet years and, if it goes unrecognized or is not well-managed, it can continue to be problematic in following seasons, regardless of weather. White mold (Sclerotinia sclerotium) is a widespread disease of over 300 species of plants. Pest/disease lifecycle, most common damage symptoms and crops affected: Pest: White mold (Sclerotinia sclerotium) This means you can put them into fresh compost and space them out, which will help improve airflow and reduce humidity.This resource is offered as part of MOFGA’s Pest Reports Fact Sheet Series ![]()
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