Basil Downy Mildew Brings Big Trouble To Gardens

Basil downy mildew is infecting basil crops in dozens of states. Find out what you can do to combat this pathogen in your garden.

by Jessica Walliser
PHOTO: Jessica Walliser

Basil downy mildew is a pathogen discovered in North America in the early 2000s, and it’s wreaking havoc on basil crops in dozens of states. Thought to have originated in Africa, this disease is problematic because it spreads extremely fast. Spores can be carried for miles on the air, in plant tissue, or even in seeds. When infected plants are moved from one location to another, the pathogen spreads with alarming rapidity.

What Basil Downy Mildew Looks Like

Basil downy mildew can be difficult to diagnose because early symptoms look a lot like a spider mite infestation or a nutritional deficiency. Early in the infection, the leaves of basil plants develop pale yellow mottling between the leaf veins on the upper leaf surfaces. If you flip the leaves over, you’ll spy tiny, purplish-gray spores scattered across the bottom surface. Eventually, the leaves develop brown splotches on top and powdery, dark spore clusters underneath. Once the disease progresses to its later stages, complete defoliation occurs followed by plant death.

Varieties Resistant To Basil Downy Mildew

The organism that causes basil downy mildew affects only basil plants, but some basil types are more resistant to the pathogen than others. If you’ve faced basil downy mildew before or you want to avoid it, include as many naturally resistant varieties as possible. Unfortunately, all sweet basil varieties are extremely susceptible, but Thai, Lemon, Red Rubin, Red Leaf and Spice basils are among the most resistant. Breeders are working on developing a sweet basil variety that’s resistant to basil downy mildew. In the meantime, here are a few measures you can take to help keep this disease out of your garden.

How To Control Basil Downy Mildew

  • Space plants properly. Good air circulation enables plants to dry off after rain. When watering, try to keep the foliage dry and always water in the morning to give the plants plenty of time to dry before night arrives.
  • Immediately remove plants with signs of infection. You want to get plants out of your garden before the disease can spread. Dispose of them by burying them deeply or tossing them in the garbage.
  • Try organic fungicides. They’re best used as a preventative, rather than as a cure. Potassium bicarbonate fungicides and those based on Bacillus subtilis are effective when applied to upper and lower leaf surfaces early in the season and on a frequent basis. Follow all label instructions carefully. Basil downy mildew does not live in the soil, so there’s no need to take any measures to sterilize the soil or drench it with a fungicide; this does nothing to control the pathogen.
  • Buy local basil plants. Basil downy mildew does not overwinter in cold northern climates. In these areas, new infections often start when infected plants are brought in from southern regions or infected seeds are purchased and grown. For this reason, gardeners should always buy basil plants from a local source or start their own plants from disease-free seeds.

Basil downy mildew is host specific and will not affect any other plants, so there’s no need to worry about it spreading to other vegetables, flowers or herbs.

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