Grape Powdery Mildew Management—A Fungicide Timing and Selection Conundrum

Walt Mahaffee, USDA, and Brent Warneke, Oregon State University, are researching how to improve the efficiency of grape production by optimizing fungicide selection and timing to manage powdery mildew of grape berries.  This research will determine the most effective fungicide application timing with relation to grape inflorescence development/phenological stage and how fungicide mobility impacts disease development.

Fungicide mobility and efficacy was examined in lab and field experiments for grape powdery mildew management. Small-plot field trials were conducted at the Botany and Plant Pathology Farm in Corvallis during the 2015 and 2016 growing seasons. The experiment was 5×3 factorial of fungicide by growth stage in a randomized complete block with four replications. All blocks were treated with sulfur until 14 days prior to one of three growth stages [end of inflorescence elongation, 50% bloom and set] then treated with one of five fungicides [Quintec (quinoxyfen), Toledo (tebuconazole), Luna Privilege (fluopyram), Flint (trifloxystrobin)* or Microthiol (sulfur)].  In addition, there were two additional treatments; sulfur calendar program (applied every 14 days) and an untreated control. All fungicide×growth stage treatments significantly reduced leaf and cluster disease development compared to the untreated control in both years.

The bloom and set application timings significantly reduced disease compared to the sulfur calendar control and the elongation application timing. Luna Privilege, Quintec and Flint applied at 50% bloom or set were consistently effective at reducing powdery mildew incidence among both the berry and leaf data in 2015 and 2016. These data indicate that early in fruit development may be the most opportune time to apply mobile fungicides to prevent fruit infection.

To test the extent of fungicide mobility on mildew infection of inflorescences in the field, inflorescences were covered with a plastic bag prior to each treatment application and removed immediately after the application was completed.  The bagging prevented direct contact of treatment fungicide to the inflorescence. Both Luna Privilege and Flint significantly reduced fruit disease on the bagged clusters, indicating that some fungicide movement could have occurred.

The xylem, translaminar and vapor movement of the fungicides were examined in the lab. All fungicides had some ability to redistribute although the magnitude varied among products. Rally had the most vapor movement while Luna Privilege had the most translaminar movement. Continuing research is examining the effect of surfactants on fungicide motility.

*Please note: resistance to Flint and Rally have been found widespread throughout Oregon.