Issue link: http://winesandvines.uberflip.com/i/279499
p r a c t i c a l w i n e r y & v i n e ya r d a p r i l 2 0 1 4 59 g r a p e g r o w i n g indicated that some portion of the over- wintering cleistothecia was not mature enough for ascospore release to occur and suggested that we did not under- stand the biology of E. necator overwin- tering. This lack of understanding led to a closer examination of the development of cleistothecia as it relates to timing of asco- spore release. In order to address this question, a group under the leadership of Gary Grove (Washington State University) and Walter Mahafee (USDA, Oregon) began developing molecular techniques to monitor pathogen dispersion of grape and hop powdery mildew. Over several years, we demonstrated that these techniques were specific and sensitive (detect 1– 2 spores of E. necator) and were useful in monitoring for the presence of airborne inoculum early in the season. We also showed that delaying the initial fungicide application until air- borne E. necator is detected resulted in saving an average of 2.3 applications dur- Walter Mahaffee, USDA, Oregon; Seth Schwebs, Coastal Viticultural Consultants; Francesca Hand, Ohio State University, Doug Gubler, University of California, Davis; Brian Baily and Rob Stoll, University of Utah BY Improving management of grape powdery mildew with new tools and knowledge M a ny v i n e y a r d m a n a g e r s observed in 2013 that grape powdery mildew (caused by Erysiphe necator) could explode from seemingly being nonexistent to look- ing like a 3-year-old child was playing with a bag of flour. This explosion is in part due to the pathogen's reproductive potential, and that the disease is extremely hard to detect below 1% leaf incidence without very expensive (e.g. 1,000 leaves per acre) weekly scouting. Since E. necator can have a generation of five days and produce more than 100,000 spores per day from a colony, a single colony can become 10 million colonies within a month, even if only one in 10,000 spores successfully infects grape tissue. Couple this with developing shoots and laterals rapidly producing susceptible tis- sue between fungicide applications, and it is easy to see why grape powdery mil- dew (GPM) epidemics are so difficult to control. In a previous PWV article (spring 2011) we presented research on the use of GPM inoculum detection to initiate fungicide applications and the rationale behind using this approach. Basically, 17 years of research 1,2 has shown that significant ascospore release can occur when the temperature is above 45° F and the bark is wet. In numerous regions, these condi- tions occur when there is no susceptible host tissue available and the spores do not survive once released in absence of susceptible host tissue. This effectively reduces the amount of overwintering cle- istothecia that can cause infection in the spring. We have been studying the delay since 1997. 3 We first observed that there was a delay in the release of ascospores from the remaining overwintering cleistothe- cia such that release did not occur even though conditions were suitable until a period of warm weather occurred. This ANNUA L C Y C LE Vegetation Growth SPRING Emergence Cleistothecium (ascocarp) or Mycelium Véraison Harvest Senescence Crop thinning Ascospore Discharge/ Conidium Overwintering Overwintering Outside Inoculum Source GRAPE POWDERY MILDEW AND HOST CYCLE FALL Flowering Fruit Development SUMMER Canopy Manipulation WINTER Continuous throughout the growing season up to 35 generations Dispersion Deposition Infection and Colonization Spore Release Conidiation Fungicide Application The relationship of the annual cycle of the grapevine (green and orange boxes) and pathogen (blue boxes). Both the pathogen and host are dormant in the winter and emerge in the spring when conditions are favorable. The powdery mildew epidemic begins when either ascospores or conidia from flagshoots are dispersed from the bark or infected buds, respectively, and infect grape tissue. The infections develop and begin producing conidia within as few as five days. The conidia are released and begin another cycle. There can be up to 35 generations during one growing season, with each colony producing spores for up to 35 days. In the fall, the pathogen forms cleistothecia, which overwinter on grapevine bark in most regions. Sometimes the pathogen can infect grape buds before véraison and overwintering in the buds. These infected buds become flagshoots in the spring. (B) (SA) (SP) (THD) Spore trap with solar panel (SP) and adjacent temperature/humidity datalogger (THD) in a California vineyard. The aluminum sampling arm (SA) is placed at the top of the canopy in the turbulent mixing layer where spore movement is the greatest. A sealed lead acid battery (B) provides power to sampling arm. Photo BY seth schWeBs