Purple spot and Phytophthora diseases of asparagus
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Purple spot on asparagus spears, caused by the fungus Stemphylium vesicarium, is present in many fields this year. During epidemic years spotting can occur on 60 to 90 percent of the spears and may result in rejection of the crop, especially for fresh-market sales. Spots also occur on the asparagus ferns, affecting the main stem, secondary branches and needles (cladophylls). Severe infection of the fern can result in premature defoliation of the plant. Increase in the severity of purple spot disease is associated with extended periods of rainfall, fog or dew.
Purple spot has become the common name of this disease because of the numerous, slightly sunken, purplish spots that occur on the spears. An increase in disease is associated with periods of heavy rainfall, especially when minimum and maximum temperatures are between 32° and 68°F. Periods of rain result in the release of spores from the overwintering fruiting structures on asparagus debris close to the growing spears. Spores may be carried by wind and impacted on the windward side of the spears. Infections are more numerous and occur with shorter wetting durations when spears are sand-blasted or otherwise wounded. The only control measure available is to reduce the amount of overwintering fruiting structures that contain spores that infect the spear. One method to reduce the inoculum in the spring is to control purple spot on the fern during the growing season. Research has determined that the TOM-CAST disease forecaster is a promising alternative to calendar-based spraying of fern in commercial asparagus fields. TOM-CAST alerts growers when the environmental conditions are favorable for purple spot disease development (extended dew or rainy periods accompanied by warm temperatures). Effective fungicides applied according to the TOM-CAST disease forecaster allow growers to manage purple spot disease of asparagus, while saving money and preserving the environment.
Phytophthora crown and spear rot is caused by the soilborne organism Phytophthora megasperma and is beginning to show up in asparagus fields. Symptoms include a “hooking” of the spear and may be accompanied by water-soaked lesions and/or shriveling. Lesions on the spear may enlarge rapidly and turn light brown. However, it is important to note that the “hooking” of the spears can occur without any apparent lesions on the spears. The dormant spore of this pathogen (oospore) can survive in soils for prolonged periods of time, and their germination is favored by excessive rain and warm temperatures. Phytophthora megasperma also produces swimming spores (zoospores) that are easily spread through water, particularly during very wet years. If spear rot becomes a significant issue, growers may want to consider application of the fungicide mefenoxam (Ridomil Gold EC or Ultra Flourish), which can be applied just prior to harvest (pre-harvest interval is one day). However, I do not believe that this application will prevent the “hooking” of the spears unless there is a means to get the fungicide down to the crown. It is likely that asparagus fields showing Phytophthora symptoms on the spears will decline in productivity as the infection progresses in the asparagus crown.