It is critical to destroy these infected fruits and other infected plant parts. Be careful not to leave the infected residue in the garden. The fungus will continue to sporulate and cause new infections. Gardeners with a love of onions have seen a common disease developing over the past week or two. Purple blotch, a disease caused by an Alternaria fungus, has been seen causing dieback of the onion tops.
Purple blotch can also affect garlic and leeks.
The disease is expressed as dark spots on the leaves that may show a target pattern. Sometimes, if the day and night temperatures are not far apart, the ring pattern may not be as obvious. As the spots grow larger, they develop a brownish to purple color. The leaves will yellow above the spots and as the spots girdle the leaves, the leaf will die back above that point . Purple blotch is a common disease of onions in South Dakota during periods of high temperatures and very high humidity.