I have grown beautiful basil for many years, but in the past few years the leaves turn brown and the plants die. What am I doing wrong?
You are not doing anything wrong. Most likely the plants have been infected by a fungal disease called basil downy mildew. This disease was first identified in the U.S. in 2008. Since then, it has been annoying growers of the popular herb.
One of the first symptoms of BDM is yellowing on the upper surface of the leaves. The corresponding lower leaf surface will develop purplish to gray spores. If you see yellowing check the undersides of the leaves, spores will be most visible in the early morning. Keep in mind that spores can be present without yellowing. If you love your basil, check the undersides of the leaves frequently.
Affected leaves can be removed to slow down the spread, but the disease develops quickly and releases copious amounts of spores that can infect other basil plants. If a plant is infected, it is best to remove it from the garden and expect that the disease will probably show up on other basil plants.
The best way to prevent BDM is to plant seeds that have resistance to the disease. A variety called “Prospera” is available from many seed companies. Rutgers University has also bred several varieties that are resistant.
Resistant does not mean immune! Plants can still become infected, but often to a lesser degree. To further ensure a happy basil crop, grow the plants in a full sun location with good air circulation. BDM needs wet leaves or 85% humidity in the plant canopy to infect basil leaves. Allow wide spacing between plants. Water early in the day at the soil level without wetting the leaves. At the end of the season, remove all dead basil plants from the garden or till them under to prevent any disease from spreading the following year. These measures should provide for an excellent supply of pesto next season.
For more information on control measures, refer to Cornell’s factsheet on the topic, located at: wdt.me/basil_mildew.
Question answered by Sue Gwise, Consumer Horticulture Specialist, firstname.lastname@example.org .