Downy mildew caused by Peronospora behlrabi was first reported in Uganda in 1930. The disease did not attract international attention until it recently appeared in several new locations; Italy (2004), France (2005), South Africa (2006), Iran (2007), United States, in Florida (2007) and Argentina (2008). During 2008 and 2009, the disease occurred throughout the east coast in epidemic proportions both in the field and in greenhouses. Many states reported 100% infection of greenhouse and field basil, often with 100% yield loss. Considerable economic losses occurred in Massachusetts during that time and basil downy mildew will be a major disease of basil in the US in the foreseeable future. The pathogen is active in southern regions year round and long distance transport from FL to MA occurs by aerial dispersal of spores. Rapid transcontinental transport probably occurred via infested seed sold internationally. Although the downy mildew pathogen has been detected in basil seed; seed transmission is probably a rare event. In July 2011, infected transplants were observed in some large retail stores selling plants to homeowners in Massachusetts, which increases the likelihood of widespread infection. This means of dispersal will continue to threaten commercial production of basil.
Infected leaves develop diffuse yellowing on the top of the leaf but distinctly vein-bounded patches on the bottom. 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 a nutritional deficiency. The fuzzy growth of spores on the underside of the leaf looks as if soil had been splashed onto the leaf under-surface.
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 included 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'.
Management: The most important environmental factors favoring disease development are high humidity and extended leaf wetness.
In the greenhouse, these factors can be reduced by:
• The most important management tactic is to heat and vent the greenhouse, especially when warm days are followed by cool nights. This practice allows water saturated air to escape and draws in less humid air that is further dried by heating.
• Improve horizontal air flow by the use of fans.
• Reduce plant canopy density by spacing to speed leaf drying.
• Water in the morning, if practical, or sub-irrigate rather than overhead.
In the field, these factors can be reduced by:
• Plant in sites with good soil and air drainage and orient rows with the prevailing winds.
• Control weeds and space plants to enhance leaf drying.
Few fungicides are labeled for herb plants and there are differences in registrations for field grown plants versus greenhouse plants. In the field, Quadris (azoxystrobin), Ranman (cyazofamid), Fosphite (phosphorous acid), and Armicarb (potassium bicarbonate) are registered for Downy mildew on basil. Greenhouse registered fungicides include Ranman, Fosphite, and Armicarb. Alternative control products include MilStop (potassium bicarbonate), OxiDate (hydrogen peroxide), Actinovate (Streptomyces lydicus), and Sonata (Bacillus subtilis). Control obtained with these materials may be commercially unacceptable. Research trials have demonstrated that a mixture of azoxystrobin plus phosphorous acid fungicides gives the best control of downy mildew. The addition of phosphorous acid fungicides to any other fungicide enhances their efficacy. It is recommended that fungicides be tested on a small portion of the crop for adverse effects before applying to the entire crop. It is the grower's responsibility to read and follow label instructions. 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 from residential gardens, greenhouses and field grown basil. If you think your basil plants are infected, please call or email Rob Wick, Department of Plant Soil and Insect Sciences; tel. 413.545.1045, email@example.com
Photos: Dr. Robert L. Wick, University of Massachusetts
MB Dicklow 2013