OPTIMAL USE OF FUNGICIDES TO MANAGE PURPLE SPOT AND RUST ON ASPARAGUS FERNS

Purple spot on asparagus (Asparagus officinalis) spears and ferns is caused by the fungus Stemphylium vesicarium, and has become a significant problem for asparagus production in Michigan. Rust is another foliar disease and is caused by the fungus Puccinia asparagi. Both pathogens commonly develop on the asparagus ferns, affecting the main stem, secondary branches, and cladophylls, and may result in premature defoliation. The objective of this research was to develop a viable and economical management program using fungicides. Several studies were conducted in established asparagus fields in cooperation with commercial asparagus growers. Registered fungicides (chlorothalonil, mancozeb and myclobutanil) were compared to reduced-risk products (azoxystrobin, pyraclostrobin, trifloxystrobin, fenhexamid, cyprodinil+fludioxonil), biopesticides (Bacillus subtilis, harpin protein, polyoxin D zinc salt), and other unregistered fungicides (captan, tebuconazole, boscalid, propiconazole). When applied preventively, tebuconazole in alternation with chlorothalonil was especially effective in controlling rust, even under severe disease pressure. Furthermore, when the rust pathogen was newly established, as determined through scouting, applications of tebuconazole alternated with chlorothalonil were effective in limiting further disease development. Disease forecasters can play an important role in integrated pest management systems for asparagus by alerting growers when weather conditions are favorable for disease development and fungicide sprays are needed. A tomato disease forecasting system (TOM-CAST) has been adapted to successfully predict purple spot outbreaks in asparagus. By timing fungicide applications for optimum efficacy, growers typically apply them less frequently. Chlorothalonil, applied on a calendar-based interval or according to TOM-CAST, was consistently effective in limiting purple spot when initiated preventively. Timing applications of chlorothalonil according to TOM-CAST can reduce the number of applications required for purple spot control up to 50% without compromising fern health. Field implementation of TOM-CAST was achieved via extension agents, consultants, an independent scouting service, a website, a code-a-phone system, and commercial demonstration plots.