Basil downy mildew9/22/2023 ![]() ![]() Downy mildew developing on single plants like this one documents the abundance of spores of the pathogen in the air. But the basil plant in the landscape planting in the picture below still experienced enough high humidity during rain and overnight for downy mildew to develop. Only these two leaves were found with symptoms while pathogen sporulation was observed on many leaves on plants that stayed outside at the same time.Ī sunny, open location seems like a good spot for growing basil to minimize humid conditions favorable for downy mildew. Infection was able to occur because these plants were left outdoors sometimes when humidity was high. Under high humidity in the bag, the pathogen produced spores confirming the symptoms were those of downy mildew. To determine if it was this disease, the leaves were put upside down on wet paper towel in a sealed plastic bag over night. Yellowing on these leaves, especially where in bands delineated by veins, looks like symptoms of downy mildew. Leaves below were on a gardener’s potted plants that were being brought inside at night to minimize time the plants were exposed to conditions favorable for disease development. When humidity is low the pathogen does not produce spores. Under humid conditions the pathogen produces an abundance of dark spores as shown in photographs below from Fran Smith in Pennsylvania who submitted them for confirmation of her report to the monitoring program. More information.īelow: Sporulation on both surfaces of small leaf.īelow: Sporulation on upper surface of large leaf.īelow: Sporulation on wet leaf with dew drop. Next two images: Sporulation of the basil downy mildew pathogen abundant on the lower leaf surface and also present on the upper surface of leaves of infected commercial basil plants kept in high humidity (inside a plastic bag) over night.īelow: Sporulation on container grown plant kept outside (left) but not on plant brought inside at night (right). (Photo courtesy Maire Ullrich, Cornell Cooperative Extension-Orange Co., N.Y.) ![]() Pathogen growth and spores on lower leaf surfaces.īasil downy mildew in a commercial crop. ![]() First three photographs taken on 9 August 2009, next five taken on 12 August 2009. Scientists suspect that this will also happen with basil downy mildew pathogen.Īdditional and more cutting-edge control methods are also gaining traction, such as keeping leaves dry to prevent pathogen spores from germinating, turning on red or white lights at night to prevent germination, and dehumidifying the air.Find more information about basil downy mildew and its management on the Basil Downy Mildew Extension page.įollowing eight images show first symptoms of downy mildew found on untreated plants in a home garden. Other downy mildew pathogens have shown they can evolve quickly to develop resistance to fungicides. Scientists recommend that growers alternate fungicides and begin applying weekly before symptoms appear to minimize opportunity for the pathogen to develop resistance. Research trials have shown that there are some fungicides that are suitable for downy mildew management. There are now several resistant basil varieties that are commercially available, but recent observations are finding that downy mildew is more severe than expected.įungicides are also an important part of downy mildew management. "Currently an integrated management approach with multiple practices is recommended to achieve the very high level of downy mildew control," explain Patel et al, authors of the recently published article on basil downy mildew management in Plant Health Progress Focus Issue.įirst, after determining that spice types of basil were less susceptible to downy mildew than the more commonly grown sweet basil, breeders were able to start finding resistance genes to breed into basil, a process that can take many years.
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