Purple Blotch

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.

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 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.

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 far enough apart to allow for good airflow
• avoid applying excessive nitrogen, especially later in the growing season
• clean garden tools with soap and water or disinfectant after working each area
• 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 2 weeks before you expect to harvest the majority of your crop
• dispose of infected plants in the trash (healthy plants can be composted)
• employ a 3- to 4-year crop rotation
• ensure that harvested bulbs are thoroughly dried before putting into storage

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