Purple blotch is a fungal disease of chives, garlic, leeks, and onions. Caused by the Alternaria porri fungus, purple blotch occurs throughout North America. It is responsible for losses of up to 50%.
Fungi enter mature leaves through wounds and dead tissue caused by chemical overspray, mechanical damage, pollution damage, thrips, and water stress. Germinating Botrytis spores, the fungi responsible for gray mold and chocolate spot can also create points of entry.
Purple blotch symptoms
Once fungal spores attach themselves to leaves, small, water-soaked spots develop. These spots turn brown as plant cells die, creating oval-shaped lesions that get bigger and eventually turn purple, surrounded by a yellow halo. Look closely, and you may see black or brown masses of fungal spores. As the disease spreads, leaves turn yellow and then brown. Lesions may also develop on stalks and flowers.
Purple blotch management
Since the Alternaria porri fungus is nearly always present in the soil, gardeners should monitor plants for conditions conducive to disease. Mid-season warm, wet weather, typically ranging from 77°F to 85°F, is all purple blotch needs to activate. Cycles of low and high relative humidity can also cue spore growth. Temperatures below 55°F slow these fungi to a halt. It doesn't kill them. They lie in wait for warmer weather.
Use these tips to prevent purple blotch from cutting into your onion family harvest:
Growing onions, garlic, leeks, and chives can be very rewarding. Keep as much of your harvest as possible by preventing and controlling purple blotch.
You can grow a surprising amount of food in your own yard. Ask me how!
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