Purple Blotch

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%.

Purple blotch on onion leaves (Ollie Martin) CC BY-SA 3.0

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.

Purple blotch on onion bulb (Ollie Martin) CC BY-SA 3.0

Use these tips to prevent purple blotch from cutting into your onion family harvest:

• Use only certified disease-free seeds and seedlings.
• Space plants for good airflow.
• Avoid applying excessive nitrogen, especially later in the growing season.
• Disinfect garden tools regularly with a household cleaner.
• Avoid overhead watering.
• Control thrips populations.
• Do not work around plants while they are wet.
• Remove weeds and onion family debris from the area.
• Stop watering two weeks before you expect to harvest your crop.
• Dispose of infected plants in the trash. 
• Employ a 3- to 4-year crop rotation.
• Ensure that harvested bulbs are thoroughly dry before putting them into storage.

Growing onions, garlic, leeks, and chives can be very rewarding. Keep as much of your harvest as possible by preventing and controlling purple blotch.