Bacterial blight of onion is a fairly common disease of onion plants – depending on where you live – that can cause minor losses to a complete loss of onion crop, depending on environmental conditions. While mostly seed borne, onion bacterial blight can be spread by debris and infected volunteer onion plants.
About Xanthomonas Leaf Blight
Onion bacterial blight was first reported in the U.S. in Colorado but has now also been found in Hawaii, Texas, California, and Georgia. It also affects onions in South America, the Caribbean, South Africa, and parts of Asia. The disease is a bacterial infection caused by Xanthomonas axonopodis. The conditions favorable to an infection include moderately warm temperatures and high moisture or humidity. Plants with leaf wounds are more susceptible to infection.
Outbreaks of the bacterial blight are more likely to occur after a period of wet, humid weather. After a storm is a time when onion plants may be especially susceptible because of the moisture and any wounds in the leaves caused by high winds. Overhead irrigation can also make onion plants vulnerable to infection.
Onions with xanthomonas blight will show signs of the disease on leaves first. You may see white spots and then elongated, yellow streaks. Eventually, entire leaves may turn tan or brown. Older leaves are affected first, and affected leaves ultimately die. You won’t see rot in the bulbs, but they may not develop and your yield may decrease significantly.
Managing Xanthomonas Blight in Onions
To prevent this infection in the first place, it is important to start with clean seeds. However, once in the garden, onion bacterial blight can spread in other ways. It may survive in debris or in volunteer plants. Pull out and dispose of any volunteers to avoid infecting your other onions, and clean up debris at the end of each growing season.
If you have an infection crop up in your onions this year, rotate your garden and put in a vegetable that is not susceptible to xanthomonas before you plant onions in that spot again. If your onions get damaged after a storm, use a nitrogen fertilizer to promote healthy leaves. Keep your onions well spaced to avoid moisture between plants and to allow for airflow.
If you take these steps, you should be able to avoid or manage an onion blight infection. If you choose to, there are copper-based bactericides that can be applied to kill the bacteria causing the infection.