First Report of Grapevine Downy Mildew (Plasmopara viticola) in Commercial Viticulture in Western Australia

Despite the suitability of climate, Western Australia was one of the few grape (Vitis vinifera L.) growing areas free of grapevine downy mildew (Plasmopara viticola (Berk. & M. A. Curtis) Berl. & De Toni in Sacc.). Area freedom had been maintained by restricting the movement of host material and machinery from outside the state and fungicide use in Western Australia vineyards had been considerably less. P. viticola was detected in 1997 in 14 of 15 vines growing at Kalumburu, a remote community in the semi-arid tropics of Western Australia, and was eradicated. In October 1998, grape leaves with oilspots typical of downy mildew were received from a grower in the Swan Valley near Perth, one of the main production areas of Western Australia. Sporangia were hyaline and ellipsoid (14 × 11 μm), were borne on treelike sporangiophores, and were consistent with those described for P. viticola (1). This is the first record of P. viticola in commercial viticulture in Western Australia. A response plan for exotic diseases was activated and after 2 weeks of surveillance the disease was found in 45 of 70 vineyards surveyed of the 280 vineyards in the Swan Valley. Given the extent of spread, eradication of downy mildew was not considered possible. Weather data for August to October 1998 indicated the likelihood of several infection periods from budburst to flowering when the disease was first detected. Crop loss will be considerable in many vineyards. P. viticola was also found in bench-grafted cuttings in pots in leaf consigned from the Swan Valley to several other areas in August 1998. Downy mildew was found in other areas only in association with these consigned vines. An industry code of practice, including hygiene, is being developed to slow the rate of spread of P. viticola in Western Australia. Reference: (1) Anon. C.M.I. Descriptions of Pathogenic Fungi and Bacteria No. 980, 1989.