Basil - Downy mildew
Tuesday, 17 August 2010 10:16
This information was summarized from a recent article by Rob Wick and M. Bess Dicklow, Plant, Soil and Insect Sciences, UMass Extension published in Floral Notes, 23(1), July-August 2010
During 2008 and 2009, Downy mildew (Peronospora species) of basil occurred throughout the east coast in epidemic proportions both in the field and in greenhouses. Although the downy mildew pathogen has been detected in basil seed; seed transmission is probably a rare event. Air-borne dissemination from infected plants is more likely.
Symptoms on the top of the foliage occur as yellowing as if the plants are nitrogen or nutrient deficient. When spores are produced, a characteristic gray, fuzzy growth on the underside of the leaves is evident. Symptoms of downy mildew on basil can easily be mistaken for nutritional deficiency.
The most important environmental factors favoring disease development are high humidity and extended leaf wetness. Cultural controls are directed at reducing the amount of time leaves remain wet by reducing plant density and watering early in the day. In the greenhouse, horizontal air flow can be increased and condensation reduced by fans. Removing and destroying severely infected plants can slow disease development.
Field trials conducted in southern New Jersey in 2009 determined that commonly-grown sweet basil (Ocimum basilicum) cultivars such as ‘Poppy Joe’ and ‘Nufar’ were the most susceptible to downy mildew. The least susceptible basils include the lemon and spice types such as O.x citriodorum and O. americanum cultivars, “Lemon Std’, ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’.
Few fungicides are labeled for herb plants. Copper products and phosphites are labeled for use on basil. It is the grower’s responsibility to read and follow label directions. The label is the law and any recommendations made here are superseded by the label.
At the University of Massachusetts, we are investigating methods to control this disease with biological control agents. We are interested in collecting live, infected plants.
If you think your basil plants are infected, please call or email Rob Wick, Dept. of Plant Soil and Insect Sciences; 413-545-1045, This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Photos: DM symptoms on top of Basil foliage, DM symptoms on underside of leaf
Robert Wick and Bess Dicklow, UMass Extension
Note: Downy mildew has also been observed recently on ornamentals in the Lamiaceae family, which includes basils (Ocimum spp), mints (Menta spp.), sages (Salvia spp) and other aromatics. According to Margaret McGrath, Cornell University, the coleus and basil downy mildew pathogens have now been demonstrated to be genetically different; therefore, these ornamental plants are not considered potential alternative host. However, there are many ornamental basils that are also hosts to pathogen affecting basil grown for use as an herb.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension
Fact Sheet with photos and more details: Basil Downy Mildew (Cornell University)
