Basil Downy Mildew

Sporulation of Peronospora behlbarii, causal agent of basil downy mildew.

Basil downy mildew is a new disease that has been causing headaches for growers in the eastern US for the past two seasons. Basil downy mildew was reported in South Florida in October 2007. This disease has the potential to be very destructive to basil producers. European producers have been dealing with the disease since 2001. The disease is thought to have been introduced in the US from infected seed from Europe

The first confirmed case of the disease in NC occurred last summer in Chatham County. Agriculture agent, Debbie Roos, did a great post on the diseases with wonderful pictures last August.

The disease has been found in several locations in WNC this year. This is surprising because we have been so hot and dry and downy mildew pathogens tend to like cooler temperatures and wet and humid conditions. Apparently, basil downy mildew is the exception - it likes moderate to warm temperatures!

Basil downy mildew is so devastating because the discoloration of the leaves makes the basil unmarketable. Eventually the leaves will become black or necrotic and die completely.

Slight yellowing of basil plants that are infected with Peronospora behlbarii, causal agent of basil downy mildew.

Discoloration of basil leaf caused by downy mildew.

The basil downy mildew pathogen, Peronospora belbahrii, is spread by contaminated seed, marketing infected plants or leaves and wind-borne spores. This pathogen also infects other plants of the Lamiacea family which inculdes basil, sages, and mints. The spores of the pathogen are abundant on the underside of the leaf.

Discoloration of basil leaf caused by downy mildew, P. behlbarii.

Notice the fuzzy growth on the underside of the leaf. This was the first tip that what we were dealing with was downy mildew. Also, note the black dying tissue.

Dark sporulation of the pathogen P. behlbarii that causes downy mildew on basil on the underside of the leaf.

This disease can often be mistaken for sunburn on basil leaves, which I have also noted this year.

Sunburned basil leaves.

There is a similar downy mildew pathogen that infects coleus, but this pathogen has been shown to be genetically different than the pathogen on basil and the ornamental hosts are no longer considered alternative hosts to the basil disease. To read more about the specifics of basil downy mildew, visit Cornell Plant Pathologist Dr. Meg McGrath's on-line factsheet Basil Downy Mildew - A New Disease to Prepare For.

Management of basil downy mildew requires an integrated approach. Using seed that is not contaminated with the pathogen, selecting less susceptible varieties and applying fungicides are the primary practices. Management should also include increasing airflow through the plant canopy in order to minimize leaf wetness and humidity in order to suppress the disease. This is especially true in greenhouses. Dr. McGrath outlines management steps, including variety trials from New Jersey, in her factsheet.

Fungicides used for downy mildew on basil include OMRI-approved Actinovate and OxiDate, which are labeled for use. Many other conventional fungicides do not yet have the label required to control this disease on basil, with the exception of K-Phite and ProPhyte. It is expected that this will change in the near future. Fungicides should be applied frequently and prior to first symptoms if possible in order to control downy mildew on basil effectively. This is of course tricky, especially in areas without history of the disease.

If you suspect that you have downy mildew on your basil, visit your local NC Cooperative Extension office for help identifying and reporting the disease.