September 2015 Plant Disease Advisory

basil downy mildew closeup
Yellowing and browning of basil leaves with downy mildew.

Is your basil turning brown long before it should be shutting down for the season? If so, your plants may have downy mildew, a disease whose symptoms can fool you. Leaves initially show patches of yellow that later turn brown, leading growers to believe that plants are either suffering from a nutrient deficiency or showing early, end-of-season leaf senescence. Take time to turn a leaf or two over to look at the lower leaf surface, where downy mildew may be visible as a grayish, fluffy mold. The gray growth may resemble soil that splashed onto the leaves, but it is actually composed of sporangia (reproductive structures) of the pathogen, Peronospora belbahrii. Under conditions of high humidity and leaf wetness, sporangia of the pathogen can spread quickly, causing new infections and rapid plant decline. 

The best way to prevent the disease is to reduce humidity and leaf wetness in the canopy, but this can be hard to accomplish in home gardens. One option is to plant basil in pots that can be moved indoors on nights with high humidity and heavy dewfall. This practice has been reported to delay the onset of downy mildew and prolong the life of basil plants compared to basil planted outdoors. If you do plant basil outdoors, be sure to space plants adequately to promote good air circulation around the plants. 

basil downy mildew
Gray sporulation of downy mildew on the underside of an infected basil leaf.

Basil downy mildew can also be transmitted on seed. Purchasing steam-treated seed can help to avoid introducing the disease on seed. Although some fungicides are registered for control of downy mildew on commercially produced basil, most fungicides registered for home use have not proven effective against the disease. Some specialty basils, such as Thai basil, lemon basil, and the red leaf types, are less susceptible to downy mildew, but most of the currently available varieties of sweet basil, the most popular type of basil, are very susceptible to the disease. An exception is the hybrid ‘Eleonora’, which has shown less severe symptoms from downy mildew than other varieties in field trials. Some newly developed hybrids also show promise. To provide the best insurance of harvesting a healthy basil crop throughout the summer, however, we advise using a combination of the disease management practices described above.