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IDENTIFICATION OF RACES OF PLASMOPARA HALSTEDII ON SUNFLOWER IN SOUTH AFRICA
IDENTIFICATION OF RACES OF PLASMOPARA HALSTEDII ON SUNFLOWER IN SOUTH AFRICA A VILJOEN 1 and TJ GULYA2 1ARC-Grain Crops Institute, Private Bag X1251, Potchefstroom 2520, South Africa; 2USDA Northern Crop Science Laboratory, P.O. Box 5677, Fargo, ND 58105, USA Background and objectives Downy mildew, caused by Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc., is an economically important disease of sunflower (Helianthus annuus L.) in Europe and the United States [1]. The disease in South Africa was first noticed in a commercial field near Standerton (Mpumulanga Province) and in a seed production field near Kroonstad (Free State Province) during the 1993/94 planting season [2]. During the 1995/1996 season, downy mildew was found in experimental hybrids near Heilbron (Free State Province), and in commercial fields near Heilbron, Marikana and Potchefstroom (both Northwest Province), and Delmas (Mpumulanga Province). At Heilbron, five hybrids were infected with P. halstedii, whereas three others showed symptoms of the disease at Potchefstroom, Delmas and Marikana [2]. Outbreaks of downy mildew in both seasons could be attributed to the cool, wet, climatic conditions that prevailed. Disease incidence in all the fields was low, with less than 1% of the plants affected. Downy mildew was again found in Delmas, Potchefstroom and Marikana during the 1996/97 season, and was newly reported in the dry, warm Northern Province districts of Groblersdal, Settlers and Warmbad. The objectives of this study were to identify races of P. halstedii that occur in South Africa, to evaluate local hybrids for resistance, and to identify germplasm with resistance to South African races. Materials and Methods Leaf samples infected with the downy mildew fungus were collected during the 1995/96 and 1996/97 growing seasons and sent to the USA for race identification. A total of 41 samples were recovered from Heilbron (ten), Potchefstroom (ten), Marikana (ten), Delmas (seven) and Warmbad (four). These isolates were inoculated onto a set of 12 differential lines and identified to race. Several isolates were single-spored and then characterized to determine the diversity within field isolates. Twenty-five commercial hybrids were evaluated for resistance to some of the most prevalent South African races. Results and Conclusions A total of ten races were identified on a set of 12 publically available differential lines, with five virulence formulas (1000, 1003, 3000, 7003 and 7323) about equally comprising 82% of the isolates. The Warmbad collection site yielded the fewest races (three) and also the least virulent, while Heilbron samples had the greatest diversity with five races. The evaluation of twenty-five commercial hybrids for resistance to indigenous races of P. halstedii revealed that about half were resistant to weakly virulent races, such as race 1000 and race 3000, while only one hybrid (NX 12244) was resistant to all races tested. This lack of resistance in current sunflower hybrids, coupled with the absence of metalaxyl seed treatment suggest that downy mildew is a potentially dangerous disease of sunflower in South Africa. A limited number of public germplasm is available with resistance to all races present in South Africa, namely the USDA lines HA 335 to HA 339 and Rha 340. Until multi-race resistance is incorporated into local hybrids, seed companies should consider seed treatment with low rates of metalaxyl to manage downy mildew. References 1. Miller JF, Gulya TJ, 1987. Crop Science 27, 210-12. 2. Viljoen A, Van Wyk PS, Nowell DC, Gulya TJ, 1997. Plant Disease 81, 111.