Downy mildew was first reported on basil 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. Considerable economic losses occurred in Massachusetts during that time and we anticipate basil downy mildew will be a major disease of basil in the US in the foreseeable future. Long distance transport from FL to MA might be explained by aerial dispersal of spores but 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. Air-borne dissemination from infected plants is more likely.
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.
Cultural Controls & Prevention:
The most important environmental factors favoring disease development are high humidity and extended leaf wetness. These factors can be reduced by:
- Toward evening, heat and vent the greenhouse, especially when warm days are followed by cool nights.
- 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 water subirrigation rather than overhead.
- In the field, plant in well drained sites with good air drainage and orient rows with the prevailing winds.
- Control weeds and space plants to enhance leaf drying.
Relative susceptibility of basil types:
- 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’.
Chemical Controls & Pesticides:
Few fungicides are labeled for herb plants and there are differences in registrations for field grown plants versus greenhouse plants. Copper products, phosphites, and azoxystrobin are labeled for use on basil. 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.
Crops that are affected by this disease:
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.
--Robert L. Wick and M. Bess Dicklow, University of Massachusetts, Amherst