An evaluation of biological and abiotic controls for grapevine powdery mildew. 2. Vineyard trials

Copyright © 2008 Australian Society of Viticulture and Oenology Inc.Grapevine powdery mildew (Erysiphe necator) affects grape yield and fruit quality worldwide. Managers of conventional vineyards rely mainly on synthetic fungicides and sulfur to control powdery mildew, while in organic vineyards sulfur is the main control agent, often in rotation with canola-based oils, bicarbonates and biological control agents. The efficacy of those materials has not been evaluated critically under field conditions in Australia. Accordingly, a range of materials showing most promise in previous greenhouse trials (Crisp et al. 2006 Australian Journal of Grape and Wine Research 12, pp. 192-202) were assessed via field trials in commercial vineyards. Applications of either milk or whey (alone, or mixed with a canola oil-based product), as well as applications of potassium bicarbonate (commercial formulation), all reduced the severity of powdery mildew compared with untreated vines. Eight applications of a 1:10 dilution of milk, 45 g/L whey powder or programs comprising rotations of potassium bicarbonate plus oil and whey, applied at 10-14 day intervals, reduced the severity of powdery mildew to levels not significantly different from that on vines sprayed with sulfur (wettable powder, 3-6 g/L). However, the relative control of powdery mildew by the test materials in field trials was dependent on the susceptibility of the grapevine cultivar and the extent of spray coverage achieved. In vineyards where highly susceptible cultivars were planted, and spray coverage was compromised, the resultant control of powdery mildew was reduced; and sometimes to commercially unacceptable levels.P. Crisp, T.J. Wicks, D. Bruer and E.S. Scot

Maturation of cleistothecia of Uncinula necator (powdery mildew) and release of ascospores in southern Australia

We paired genetically characterised isolates of U. necator under controlled conditions and raised the resultant cleistothecia to maturity. Progeny were recovered from cleistothecia formed in the glasshouse and growth room 97-110 days after cleistothecial initials were observed. Cleistothecia collected from naturally infected leaves from vineyards in the Adelaide Plains and Langhorne Creek, South Australia, in March and April (autumn) 1998 and stored in the same vineyards, were induced to release ascospores in the laboratory between March 1998 and February 1999. This paper is the first report of cleistothecia of U. necator releasing ascospores during summer, autumn and winter in southern Australia. Cleistothecia did not require a period of over-wintering in order to release ascospores that infected detached grapevine leaves in vitro. Furthermore, cleistothecia that over-wintered in the field were triggered to release ascospores in the laboratory in spring. Release of ascospores in spring was coincident with observations of ascospore-derived infections in the Waite Campus vineyard, confirming that cleistothecia provide an additional source of primary inoculum for powdery mildew in South Australia. Autumn release of ascospores suggests the possibility that U. necator may complete two sexual generations per year in Mediterranean climates, with consequent increased risks of development of fungicide resistance and changes in pathogenicity.Gee, L. M. ; Stummer, B. E. ; Gadoury, D. M. ; Biggins, L. T. ; Scott, E. S