Purple blotch is a fungal disease of chives, garlic, leeks, and onions. Caused by the Alternaria porri fungus, purple blotch is found throughout North America and is responsible for losses ranging from 25 to 50 percent.
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 grey mold and chocolate spot, can also create points of entry.
Purple blotch symptoms
As fungal spores attach themselves to leaves, small, water-soaked spots can be seen. These spots turn brown as plant cells die, creating oval-shaped lesions that get bigger and eventually turn purple, surrounded by a yellow halo. If you look closely, you may be able to 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 need to monitor plants for conditions conducive to disease. In the case of purple blotch, mid-season warm, wet weather, typically ranging from 77°F to 85°F, is all that’s needed to trigger these pathogens into action. Cycles of low and high relative humidity can also cue spore growth. Temperatures below 55°F slow these fungi to a halt, but they simply wait for things to warm up.
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 an controlling purple blotch.
You can grow a surprising amount of food in your own yard. Ask me how!
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