Just when you’re all ready to harvest big bunches of basil to make pesto, basil jam, a salad, a fruity basil drink or margherita pizza, you notice that your basil isn’t looking so good. What happened?
Basil is generally easy to grow and has few problems, but there is a new basil disease, downy mildew, now found in the southern part of Wisconsin that you should be on the lookout for.
Symptoms develop first on lower leaves and move up the plant. You’ll first see leaf curling and yellowing and a gray-purple-brown fuzz that looks like a fine layer of dirt on the underside of leaves. Leaves eventually turn brown.
Of the many types of basil, sweet basil is especially susceptible to downy mildew. Purple leafed varieties, Thai basil, lemon basil and spice basils are less susceptible than the green-leafed sweet basils.
Humid, warm conditions accelerate the spread of the disease. It can be introduced to the garden from contaminated seed, infected transplants or through wind-borne spores from your neighbor’s basil plants. Once it is in your garden, it can spread further by wind, rain or watering splash, or hands, clothing and gardening tools that come in contact with an infected plant.
There is currently no cure for basil downy mildew. Once it is discovered in your garden, you should pull up all the basil plants, both healthy and infected. Use the healthy plants immediately for whatever reason you grew the basil. If you have no immediate plans for use, it can be chopped and frozen in olive oil in ice cube trays or blanched in boiling water for 2 seconds before being plunged into an ice bath. Then dry the leaves and spread them on a cookie sheet. Place it in the freezer immediately. When frozen, store the leaves in plastic freezer bags.
As for the infected plants, bag them in plastic and put them in your garbage.
The easiest way to prevent basil downy mildew is to choose basils other than sweet basil. A few varieties have been bred for downy mildew resistance. They include ‘Devotion’, ‘Obsession’, ‘Passion’, ‘Thunderstruck’, ‘Prospera‘, ‘Eleanora’, ‘Emma’ and ‘Everleaf’.
If you grow basil from transplants, look carefully at the undersides of the leaves before buying. If you grow from seed, attempt to determine if the seed has been steam-treated to kill the downy mildew pathogen. This is a relatively new technique so it may be difficult to find steam-treated seed.
No matter which basil you plant, choose a sunny spot and allow ample space between plants so air can circulate. Try to keep the plants dry. If you need to supplement rainfall, use a drip or soaker hose rather than overhead watering. These practices also help prevent other diseases that may affect basil.
You may be tempted to use a fungicide to kill the mildew but those available to home gardeners just don’t work. You would be wasting your time and money.