Sunflower (Helianthus annuus L.) plants showing systemic downy mildew (Plasmopara helianthi Novot.) symptoms (stunting and leaf chlorosis) were collected during May and June 2001-2003 in fields located in four regions of central Italy (Umbria, Tuscany, The Marches, and Emilia-Romagna). Generally restricted, a low disease incidence (2 to 12%) was observed in fields planted with commercial hybrids. In the surveyed areas, higher field incidences (20 to 30%) of downy mildew were only observed where suitable climatic conditions occurred. To determine pathogen race variability of P. helianthi, 18 isolates (one for each field) were recovered directly from infected leaves showing sporulation or after 24 h of incubation in a humid chamber at room temperature. After increasing initial inocula on the susceptible cv. Ala, race identification of all isolates was determined by the whole-seedling immersion technique (2) on three sets of nine differential sunflower lines (three lines per each set): HA-304, RHA 265, RHA 274; PMI-3, PMI-17, 803-1; and HAR-4, QHP-1, HA 335 using triplet coding (1). All differential lines were tested twice with 40 seeds per replicate (three replicates per line). Twelve days after inoculation, plants were placed in a chamber maintained at 20°C and 100% relative humidity for 24 h and then evaluated for a susceptible (sporulation on cotyledons and/or first true leaves) or resistance (absence of sporulation) reaction. Races 700 and 703 were identified during the 3-year survey. Race 700 was recovered frequently (56% of isolates) from the Italian regions except Emilia-Romagna. Race 703 was not detected in Tuscany. Races 700 and 703 have been previously isolated in other European countries (3,4), but to our knowledge, this is the first report of the occurrence of either race in Italy. Our results confirm previous investigations and suggest that the restricted presence of downy mildew should be attributed to several factors: (i) lack of any source of resistance to both races in commercial hybrids; (ii) insufficient seed dressing with metalaxyl as observed on remnants of commercial hybrid treated seeds tested in laboratory analyses; and (iii) short rotation of cereals (often only 2- to 3-year interval) with sunflower crops. Although greenhouse tests showed most P. helianthi isolates were not controlled by metalaxyl seed treatment at the registered rate (2.1 g kg-1) loss of fungicide efficiency in fields has not yet been observed indicating that appropriate phytosanitary measures and proper seed treatment can provide control of sunflower downy mildew.
References: (1) T. J. Gulya. Pages 76--78 in: Proc. 17th Sunflower Res. Workshop, Fargo, ND, 1995. (2) T. J. Gulya et al. Helia 14:11, 1991. (3) M. L. Molinero-Ruiz et al. Plant Dis. 86:736, 2002. (4) F. Virany and T. J. Gulya. Plant Pathol. 44:619, 1995.