A New Race, 704, of Plasmopara helianthi Pathogen of Sunflower Downy Mildew in Italy.

Abstract

The recent changes of the European Common Agricultural Policy and the market needs of oleaginous crops for energy purposes caused a renewed increase of sunflower (Helianthus annuus L.) cultivation in Italy. During 2006, surveys on approximately 92 ha of Umbrian (central Italy) sunflower fields were carried out for monitoring distribution and race variability of the pathogen. Twelve fields planted with commercial hybrids were surveyed. Downy mildew was only observed in five fields, with 2 to 3% of disease incidence. Systemic mildewed plants showed stunting, leaf chlorosis, and sporulation on the underside of leaf surface. Pathogen inocula were directly recovered from infected leaves by brushing the fungal structures or after infected leaves were incubated in a humid chamber at 18 to 20°C in the dark for 24 to 48 h. After increasing initial inocula on the suscpetible cv. Ala, race identification of four isolates was determined by the reaction of three standard sets of nine differential sunflower lines using a triplet code (3). Thirty to forty pregerminated seeds for each differential line (three replicates per line) were inoculated by the whole-seedling immersion technique (1). After 12 days, plants were maintained at 20°C and 100% relative humidity for 24 to 48 h to enhance pathogen sporulation and evaluate for susceptible (sporulation on cotyledons and/or first true leaves) or resistance (absence of sporulation or weak sporulation only on cotyledons) reactions. Inoculation tests were performed twice. The isolates were also evaluated for their sensitivity to metalaxyl-M (Apron XL 31.8%) used at the Italian registered rate (1.05 g of a.i. per kg of seed). Treated and untreated seeds of cv. Ala (50 seeds per pot with three replicates) were sown into pots filled with a sterilized sandy-loam mixture (1:1, vol/vol). Five days after sowing, soil drench inoculation was performed by spreading over the pots (80 ml per pot) a zoosporangia suspension (1 to 2 × 104 zoosporangia per ml) of each isolate. Disease incidence (DI) was determined by counting the number of uninfected and infected plants (sporulation on cotyledons and/or true leaves). Hypocotyls of plants that seemed uninfected were cut into sections (2 to 3 cm long) and placed in a humid chamber to allow pathogen sporulation. The experiments were performed twice. Three isolates were characterized as race 700 and one as a mixture of races 700 and 704. Race 700 is more widespread in Italy, whereas race 704 was reported in France in 2002 (2). All Plasmopara helianthi isolates produced 84 to 89% infection on plants from untreated seeds, whereas DI from fungicide treated seeds was significantly lower (8 to 17%). To our knowledge, this is the first report of race 704 in Italy. All field isolates were also metalaxyl-M sensitive as already reported in other European countries (4).