Assessment of Erysiphe necator ascospore release models for use in the Mediterranean climate of western Oregon

Authors: THIESSEN, LINDSEY - North Carolina State University; Neill, Tara; Mahaffee, Walter - Walt

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2018
Publication Date: 6/22/2018
Citation: Thiessen, L., Neill, T.M., Mahaffee, W.F. 2018. Assessment of Erysiphe necator ascospore release models for use in the Mediterranean climate of western Oregon. Plant Disease. 102(8):1500-1508. https://doi.org/10.1094/PDIS-10-17-1686-RE.
DOI: https://doi.org/10.1094/PDIS-10-17-1686-RE

Interpretive Summary: We examined the utility of several Erysiphe necator ascospore release models developed in other grape production regions for their utility in managing grape powdery mildew in western Oregon. Cleistothecia (syn. chasmothecia) were collected prior to leaf drop and placed onto natural and artificial grape trunk segments that then overwintered outside. Ascospore release was monitored using custom impaction spore traps from leaf drop until the onset of the disease epidemic in the following growing season. Airborne inoculum was also monitored in a naturally infested research vineyard. Weather and ascospore release data were used to assess previously developed models, and correlate local environmental conditions to ascospore release. All models predicted ascospore release prior to bud break and those that predicted magnitude of release predicted exhaustion of inoculum prior to bud break. These predictions did not agree with trapping data. The magnitude of ascospore release could not be correlated to environmental conditions, and a binary ascospore release model was developed where ascospore release is a function of the collective occurrence of the following factors within a 24-hour period: > 6 hours of cumulative leaf wetness during temperatures > 4 °C, precipitation > 2.5 mm, and relative humidity > 80%. An improved understanding of the environmental factors influencing onset of the grape powdery mildew epidemic will help growers improve their disease management program.

Technical Abstract: Predictive models have been developed in several major grape growing regions to correlate environmental conditions to Erysiphe necator ascospore release; however, these models may not accurately predict ascospore release in other viticulture regions with differing climatic conditions. To assess ascospore release in maritime regions of the northwest, cleistothecia (syn. chasmothecia) were collected prior to leaf drop and placed onto natural and artificial grape trunk segments that then overwintered outside. Ascospore release was monitored using custom impaction spore traps from leaf drop (BBCH 97) until the onset of the disease epidemic in the following growing season. Airborne inoculum was also monitored in a naturally infested research vineyard. Weather and ascospore release data were used to validate previously developed models, and correlate local environmental conditions to ascospore release. Ascospore release was predicted by all models prior to bud break (BBCH 08), and was observed from the first rain event following the start of inoculum monitoring until monitoring ceased. Models developed in other regions over-predicted ascospore release in the Willamette Valley, and predicted exhaustion of inoculum prior to bud break. The magnitude of ascospore release could not be correlated to environmental conditions, and a binary ascospore release model was developed where ascospore release is a function of the collective occurrence of the following factors within a 24-hour period: > 6 hours of cumulative leaf wetness during temperatures > 4 °C, precipitation > 2.5 mm, and relative humidity > 80%. Extant methods for estimating ascospore release are not sufficiently accurate in this wet, maritime climatic region.