![]() ![]() In 2009 Cornell University started collecting information on basil downy mildew occurrences, to try to determine whether the pathogen could move northward through the eastern U.S., now that it is considered established in Florida. Since basil downy mildew affects many ornamental types of basil as well as those grown as culinary herbs, it can spread even faster once gardens get going. every summer since 2008 by wind-dispersed spores moving north following the warm weather. Most likely it has spread throughout the eastern U.S. It may also spread by marketing infected basil leaves or plants from a warm region or from another country. The pathogen can be seedborne and so be unknowingly distributed by contaminated seed. Since it does not seem to produce resting spores in Maine, and all basil dies during our winters, how does it spread? During the growing season it is dispersed via airborne spores, but how did it get here to begin with, and how does it get here each year? It is always present in warm climates such as Florida or California. The basil downy mildew pathogen is an obligate parasite, which means it can live only in living tissue (similar to the pathogen that causes late blight). Only one mating type of the basil downy mildew pathogen seems to exist in Maine now, so asexual reproduction via sporangia protruding from leaves seems to account for the spread of the disease here. Sexual reproduction requires the presence of two mating types. Sexual reproduction results in a resting spore (oospore), which is very tolerant of cold, hot, dry and wet conditions and can persist in soil for years. They reproduce asexually by sending out sporangiophores from stomata. ![]() A different but related genus causes downy mildew of lettuce. Other species of Peronospora cause downy mildew in many other plants, including spinach, strawberries, crucifers and onions. The pathogen responsible is Peronospora belbahrii. Vast numbers of sporangia are produced under cool, moist conditions, and the disease spreads very quickly. (See photos.) If your basil begins to look sick, flip over a leaf and look for downy patches. Downy mildews send out the spore-producing branches of mycelia through leaf stomata (pores in the epidermis), and patches of violet-gray sporangia (spore-producing structures) form. The symptom that shows that basil downy mildew is a disease caused by a pathogen is sporulation on the lower leaf surface. Eventually the yellowing spreads over the leaf, more leaves become infected, and the plant looks like it has been stressed by heat or a nutrient deficiency. The bands are there because at first the major veins in the leaves block the spread of the pathogen. The first symptom that catches your eye appears as bands of burned yellow on some leaves. Last year was the first year I saw it on most farms I visited in Maine. By 2008 it was in the Northeast, in Pennsylvania, New Jersey, New York and Massachusetts. In 2007 it was first reported in the United States in south Florida. It seems to have been spreading faster in the last decade. In the 20th century it was generally confined to African countries. Still, I want to highlight it, because last summer I received many calls about the mystery of basil dying midseason.īasil downy mildew is a new disease to our region, and it seems to be here to stay. ![]() One of these new problems is basil downy mildew – a difficult disease to discuss because I have no good solution for it. I don’t know if the weather, the climate or some other factor is to blame, but for the first time I am hearing about enough previously unheard-of problems to fill an entire hour of discussion. This winter I have been lecturing at organic growers’ meetings about new and emerging insects and diseases. Photo by Eric Sideman.Īnd on the lower leaf surface. Basil downy mildew on the upper surface of a leaf. ![]()
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