![]() Note the yellow or chlorotic splotches on the upper side of the leaves. Note the chlorotic areas on the upper side of the leaf (left), which relate to mycelial growth on the lower side of the leaf (right). The upper and lower sides of sweet basil ‘Nufar’ infected with downy mildew. ![]() Note the yellow (chlorotic) blotches on the leaves and the black mycelia on the underside of the leaf seen on the opposite side of the plant. Young basil plant with a downy mildew infection. These dark, “fuzzy” threads produce spores. In very conducive conditions, the dark hyphal-like threads may emerge on the upper side of the leaf. They may take on a “fuzzy” appearance (Figures 2 and 3). Further disease development produces dark-colored (gray) “thread-like” structures similar to fungal hyphae, on the lower side of the leaf (Figure 1). The first symptom is a slight chlorosis (yellowing), usually in the central area of the leaf around the mid-rib. Recently, seed companies have started to decontaminate their seeds by steaming. Basil plants grown from known infected seeds were systemically and latently infected (plants did not express symptoms), so contaminated seeds were harvested from these plants. Movement of the pathogen between continents is probably a result of the unintentional distribution of contaminated seeds. Its spores can move many miles on winds.Īnother key point that is crucial to the spread of basil downy mildew is it is very difficult to manage and control.The rapid spread of basil downy mildew is due to two key characteristics: By 2008, it was found in at least nine U.S. The disease was in several African countries in 2005. By 2004, it was found in most Italian growing regions and in France. Pathogens in this group thrive in wet or moist environments, and the plant-pathogenic members develop resistance to chemicals used for their control.īasil downy mildew was first noticed on a significant commercial basis in Switzerland in 2001. Other well-known water mold plant pathogens are Pythium and Phytophthora. A Very Short History of the Pathogen (Peronospora belbahrii)ĭowny mildews are members of the water mold group. In very conducive situations, fuzzy growth may be on upper leaf surface, as well. ![]() May develop fuzzy gray growth on underside of leaves. Quick symptomsĬhlorotic (yellow) area in center (mid-rib) of leaf. Colored types and other species are not so susceptible. Basil types affectedĮspecially bad on sweet basil. WeatherĪ number of days with rains and high humidity, especially originating from the Gulf of Mexico. Greenhouse Basil Downy Mildew Most common seasons She suggests making pesto on the spot with the healthy leaves when you see the first signs of blight.Greenhouse grown basil plants. McGrath says that if you see the blight, you could get rid of the leaves, but if you remove one, you'll probably knock around the spores and they could get on other parts of the plant. "The wind will pick those up and blow them off and the disease just keeps multiplying like crazy." And there are incredible production of them on the underside of the leaves," she says. "You'll see a grayish, almost purplish dusty growth on the underside, sometimes turning to almost black - and that is all of the pathogen spores. ![]() But to really know the state of the plant, you have to flip the leaves over and look on the undersides. The sign of the blight, McGrath says, is the top of the leaf will start yellowing. McGrath says basil blight was reported in Uganda in 1933 and wasn't reported again until 2001 in Switzerland. "If you have it in your garden, you can affect other gardeners and farmers because the pathogen can jump onto their crops and plants," she says. ![]() Margaret McGrath, associate professor of plant pathology at Cornell University And it's new so people don't know about it.ĭr. It wipes the leaves right out - and that's what you want to use. ![]()
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