GROWERS CAN REDUCE SPRAYS DURING CONTROL OF BOTRYOSPHAERIA PANICLE AND SHOOT BLIGHT OF PISTACHIO BY USING A LEAF WETNESS MODEL

Abstract

Botryosphaeria panicle and shoot blight, caused at least by Botryosphaeria dothidea, is a devastating disease of pistachio. Although the disease was discovered in 1984 causing tremendous damage in one of the oldest pistachio orchards in Butte Co., it took about 12 years for the disease to become widespread in the pistachio industry and reach epidemic levels in the late 1990s. Initially there were no fungicides registered in pistachio and the disease caused 40 to 100% yield losses in some of the orchards. In fact, in some cases, the disease became so severe that growers removed pistachio orchards and planted other crops that seemed not to be susceptible to Botryosphaeriaceae fungi. Over the years, research supported by the California Pistachio Industry discovered a large number of fungicides, particularly anilinopyrimidines, strobilurins, carboxamides, and mixtures of carboxamides and strobilurins that showed good to excellent efficacy in controlling the disease. However, because of the magnitude of damage this disease causes, growers started applying multiple sprays that in some cases seemed not to be necessary. Data collected for 15 years for leaf wetness duration, temperature, and disease levels enabled us develop a model that separated rain events into high, medium, and low risk for Botryosphaeria panicle and shoot blight in California pistachio orchards. Disease risk can also be determined by the BUDMON (bud monitoring) technique. Briefly, 100 buds are collected randomly from a field, surface sterilized and plated on APDA plates (10 buds per 9-cm plate). The plates are incubated at 27°C for about 4 to 6 days and the colonies of Botryosphaeria are recorded. Incidence of the pathogen in the buds correlates with disease in fruit clusters and shoots at harvest. The Leaf Wetness Model (LWM) was very successful in determining the risk of infection by B. dothidea and optimizing application of fungicides. In 2010, the two sprays, predicated when the LWM thresholds were exceeded ,essentially controlled the disease and resulted in control that was comparable to control achieved by eight commercial sprays. Pre-mix fungicides Merivon® (fluxapyroxad + pyraclostrobin) and Pristine® (boscalid + pyraclostrobin) were the two most effective fungicides for controlling Botryosphaeria panicle and shoot blight. Other effective fungicides included Luna PrivilegeTM (fluopyram), Quadris TopTM (difenoconazole + azoxystrobin), Luna SensationTM (fluopyram + trifloxystrobin) alternated with Scala™ (pyrimethanil), and AdamentTM (tebuconazole + trifloxystrobin) alternated with ScalaTM. The LWM was also tested in the Butte Co. orchard in 2011 and it was very effective in discriminating between the highest and moderate risk infection events from rain events of low risk. MerivonTM and Pristine® were the most effective fungicides for controlling Botryosphaeria panicle and shoot blight. Other effective fungicides included Luna SensationTM, Luna ExperienceTM (fluopyram + tebuconazole), Quilt XcelTM (azoxystrobin + propiconazole) and Quadris TopTM. Also effective were Ph-DTM (polyoxin D), ScalaTM, and Quash174; (metaconazole).