Joe Nunez

UCCE Farm Advisor, Kern County

Vegetable Crops - Plant Pathology

December 10, 1999

Recognizing and Managing Purple Spot of Asparagus

Because asparagus is so adaptable to may soil types and climates, it has become a favorite vegetable for many home gardeners as well as commercial growers. Currently asparagus is a major commercial vegetable crop in California with the state leading the nation in total production. Whether you are growing several plants or several hundred acres of asparagus, one problem to be aware of is purple spot of asparagus. Purple spot of asparagus can become a problem whenever there are cool wet conditions coinciding with the emerging spears in spring. Although purple spot is not necessarily a very destructive disease, it can easily cause enough cosmetic damage that the spears become unmarketable or the very least down graded in value.

Purple spot occurs during the period that the new spears are emerging and before harvest. It causes small lesions on the spears that are longer than it is wide (0.03-0.06 by 0.125 inches). New lesions are reddish to purple in coloration, but become brown with a tan center as the lesions age and enlarge. The lesions themselves are very superficial and do not extend into the spears. Often the lesions will be more numerous on one side of the spears, usually on the side of the prevailing winds.

Cool, wet weather is need for purple spot to occur. Purple spot is rarely a problem during years with dry springs. The fungus Stemphylium vesicarium is the cause of purple spot which survives on crop debris from the previous year. During cool, wet conditions the fungus forms spores, which are then blown onto the newly emerging spears. Although purple spot can occur anytime that conditions are right, it's during periods that harvestable spears are emerging that losses can occur.

Control of this problem is a matter of removing the end of the season's fern growth from the field or garden. Commercial growers need to chop and incorporate the old ferns into the ground so that they breakdown before the new spears emerge in the spring. Burning the crop debris at season's end may be an option along with chopping and hauling the debris out of the field. Home gardeners and small farm growers may remove the dead ferns at the end of the season and compost them or dispose of debris by some other method. Chemical treatments are not allowed for this pest problem in California and with the proper cultural control steps it should not be needed. Removing the previous season's trash also helps to control asparagus rust and the European asparagus aphid, both of which also overwinter in asparagus debris. Although these pests are not a problem in the Southern San Joaquin Valley, they are pests in other parts of the state.