Purple Blotch On Onions: Dealing With Purple Blotch In Onion Crops

Purple Blotch Disease On Onion Crops
purple blotch
(Image credit: Howard F. Schwartz, Colorado State University, Bugwood.org)

Have you ever seen purple blotches on your onions? This is actually a disease called ‘purple blotch.’ What is onion purple blotch? Is it a disease, pest infestation, or an environmental causal? The following article discusses purple blotch on onions, including what causes it and how to manage it.

What is Onion Purple Blotch?

Purple blotch in onion is caused by the fungus Alternaria porri. A fairly common disease of onions, it first manifests as small, water-soaked lesions that rapidly develop white centers. As the lesions progress, they turn from brown to purple with a halo of yellow. Often the lesions merge and girdle the leaf, resulting in tip dieback. Less commonly, the bulb becomes infected through the neck or from wounds. Fungal growth of spores of A. porri is fostered by temperatures of 43 to 93 degrees F. (6-34 C.) with the most optimal temperature of 77 degrees F. (25 C.). Cycles of high and low relative humidity encourage spore growth, which can form after 15 hours of relative humidity greater than or equal to 90%. These spores are then spread by wind, rain, and/or irrigation. Both young and mature leaves affected by thrip feeding are more susceptible to purple blotch in onions. Onions with purple blotch present symptoms one to four days after infection. Onions infected with purple blotch become defoliated prematurely which compromises bulb quality, and lead to storage rot caused by secondary bacterial pathogens.

Managing Purple Blotch in Onion

When possible, use pathogen free seeds/sets. Ensure that plants are properly spaced and keep the area around the onions weed free to increase circulation, which will allow the plants to dry from dew or irrigation more rapidly. Avoid fertilizing with food that is high in nitrogen. Control onion thrips, whose feeding makes the plants more susceptible to infection. Purple blotch can overwinter as mycelium (fungal threads) in onion debris, so it is important to remove any debris prior to planting in successive years. Also, remove any volunteer onions which may be infected. Rotate your onion crops for at least three years. Harvest onions when conditions are dry to avoid neck injury, which may act as a vector for infection. Let the onions cure before removing the leaves. Store the onions at 34 to 38 degrees F. (1-3 C.) with a humidity of 65 to 70% in a well aerated, cool, dry area. If need be, apply a fungicide according to the manufacturer’s instructions. Your local extension office may be of assistance steering you to the correct fungicide for use controlling purple blotch in onion crops.

Amy Grant

Amy Grant has been gardening for 30 years and writing for 15. A professional chef and caterer, Amy's area of expertise is culinary gardening.