Because asparagus is so adaptable to many soil types and climates, it has become a favorite vegetable for many home gardeners as well as commercial growers.
Asparagus is a major commercial vegetable crop in California with the state being one of the leading areas in the nation in total production. Whether 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.
Although the damage occurs during the period that the new spears are emerging in spring control measures begin in the fall. Purple spot causes small lesions on the spears that are longer than it is wide (0.03-0.06 inches 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 common on one side of the spears, usually on the side of the prevailing winds.
Cool, wet weather is needed 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. It survives on the 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 favorable, it's during the period that harvestable spears are emerging that losses can occur.
Control of this problem is a matter of removing fern growth at the end of the season. Growers need to chop and incorporate the old ferns into the ground so that they break down 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.