Effects of humidity on the development of grapevine powdery mildew

The effects of humidity on powdery mildew development on grape seedlings and the germination of Uncinula necator conidia in vitro were examined. Studies were conducted at an optimum temperature of 25 2degreesC. Disease on foliage was markedly affected by humidity levels in the test range of 39 to 98% relative humidity (RH), corresponding to vapor pressure deficits (VPD) of 1.914 to 61 Pa. Incidence and severity increased with increasing humidity to an optimum near 85% RH, and then appeared to plateau or decrease marginally at higher values. Conidial density and chain length also were proportional to humidity, but were influenced less strongly. There was a strong, positive linear relationship between humidity level and frequency of conidium germination with RH treatments of less than or equal to84%. However, germination frequency fell sharply at RH levels above a mean of 87%. All measures of humidity were equally accurate in predicting germination responses; however, VPD was slightly more effective than RH in accounting for effects on disease development and pathogen sporulation, and both were more effective than absolute humidity. Humidity appears to play a significant role in grapevine powdery mildew epidemiology, confirming the benefits of management practices to avoid and mitigate high humidity in the vineyard canopy.