A single out-of-season fungicide application reduces the grape powdery mildew inoculum

Abstract

Strategies for the control of grape powdery mildew, caused by Erysiphe necator, Schw., require consideration of the development of fungicide resistance. We investigated the effect of inoculum reduction by fungicides with low resistance risk (copper oxychloride, potassium bicarbonate, meptyldinocap, wettable sulphur) in the laboratory and as individual post-harvest and bud break treatments. Ascospore viability and release were analysed after a post-harvest treatment and disease incidence and severity of grapes were determined following a bud break treatment. In the laboratory all fungicides reduced the viability (spring 2018 p = 0.008; autumn 2018 p = 0.021; spring 2019 p = 0.010; autumn 2019 p = 0.484) and the release (spring 2018 p = 0.007; autumn 2018 < 0.001; spring 2019 < 0.001) of ascospores, although the decrease was more pronounced in spring compared to autumn. At Krems, the post-harvest field application of copper oxychloride, potassium bicarbonate, meptyldinocap and wettable sulphur in autumn 2017 reduced the percentage of viable chasmothecia significantly (p < 0.001) by 56–74%. In autumn 2018, the tested fungicides had no effect on the percentage of viable chasmothecia (p = 0.129). At Andau, the percentage of viable chasmothecia was only reduced by copper oxychloride (p = 0.029). The amount of viable ascospores was significantly (p = 0.002) lowered after a post-harvest field treatment with the fungicides by 46–70%, whereas for meptyldinocap only a trend could be determined. Disease incidence and severity were not influenced by sulphur applications at bud break (p = 0.689, p = 0.224) and by different levels of chasmothecia (p = 0.104, p = 0.353). A trend in one year, that a high amount of chasmothecia increased the disease incidence, was related to the weather conditions and the duration from the time of first ascosporic infection until flowering. The data indicates the importance of an effective control with fungicides during the pre-flowering stage in years with a long time from primary infection until flowering. Furthermore, this study shows that post-harvest sprays with low risk fungicides can help to reduce the number of fungicide treatments needed for powdery mildew control at the end of the in-season spray program. This measure could be integrated into an environmentally friendly pesticide reduction program for controlling powdery mildew in viticulture.