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Downy mildew attacks basil

Downy mildew has taken a toll on basil the last few years. One symptom is fuzzy spores on the undersides of leaves. Photo courtesy Purdue Plant and Pest Diagnostic Lab

Downy mildew has taken a toll on basil the last few years. One symptom: fuzzy spores on the undersides of leaves. Photo courtesy Purdue Plant and Pest Diagnostic Lab

Basil has taken a huge hit over the last few years from a downy mildew, which can quickly decimate a crop in the farmers’ fields, growers’ greenhouses and our gardens.

Downy mildew is the common name for this disease, even though the fungus that causes it may be different, depending on the plant. It gets the name from the symptoms, downy-like fuzz on the leaves.

On basil, the culprit is Peronospora balbahrii. Initially, the leaves turn yellow, and then black spots appear. That’s followed by fuzzy spores on the undersides of leaves. The problem starts at the base of the plant and moves up. The fungus is air borne or can be splashed on plants from infested soil. Plants and seeds can be infected, according to the University of Minnesota Extension, which got its first reports of the disease in 2012.

Here in Indiana, Purdue University has seen only a handful of samples since 2009. “A couple each year,” said Tom Creswell, director of Purdue’s Plant and Pest Diagnostic Lab.

“Most of our reports from the home garden have been from the home garden of Tom Creswell,” he joked. “It seems that if this is showing up extensively in the home garden, then either no one is paying attention or it comes on so late in the season that they’ve already made pesto, or they don’t know they can send samples to the lab for help.”

What can we do?

  • Remove any infested leaves. It’s all right to eat healthy leaves from infested plants.
  • Plant resistant varieties. Sweet basil (Ocimum basilicum) is the most susceptible, which includes Genovese, Nufar, Italian Large Leaf, Queenette, Superbo, Poppy Joe’s and others, according to Minnesota extension. Red Rubin and Red Leaf (O. basilicum purpurescens) and lemon basils (O. citriodorum) are considered moderately susceptible. Blue Spice, Spice, and Blue Spice F1 (O. americanum) are less susceptible.
  • Don’t plant basil in the same spot year after year. If you grow basil in a pot, dump the soil each year and scrub the pot with a solution of one part bleach and nine parts water. Use fresh potting mix each year.
  • Give plants plenty of air circulation.
  • Closely examine basil plants at garden centers to make sure they are symptom free.
  • Consider growing basil from seed. It’s easy and plants comes up quickly.

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