In May 1999, marshelder (Iva xanthifolia Nutt.) plants with chlorotic upper leaves and abundant white sporulation on abaxial surfaces were observed in roadside ditches near Fargo, ND. Ovoid to elliptical zoosporangia (20 to 30 μm × 15 to 20 μm) that were borne on sporangiophores 400 to 500 μm long and branched monopodially at right angles were recovered from infected marshelder leaves. Dimensions of the zoosporangia and sporangiophores fall within those reported for Plasmopara halstedii (Farl.) Berl. & de Toni (1). Zoosporangia rinsed from marshelder plants were used to inoculate 3-day-old sunflower seedlings. The seedlings were immersed in a suspension of 2 × 104 zoosporangia per ml for 3 h, and planted in a greenhouse maintained at 18 to 24°C with a 16-h photoperiod. Systemic chlorosis, stunting, and sporulation were observed on sunflower plants 12 days after inoculation. Isolates from two individual marshelder plants were inoculated on a standard set of nine sunflower downy mildew differential lines, and were identified as race 3 (virulence pattern 700) and race 4 (virulence pattern 730). The marshelder isolates were evaluated for metalaxyl sensitivity by a soil drench inoculation method. Seed of an oilseed sunflower hybrid commercially treated with metalaxyl (138 g a.i./100 kg of seed) were planted in flats filled with sand and perlite (1:1, vol/vol). Three days after planting, the flats were drenched with a zoosporangial suspension (2 × 104 zoosporangia per ml) for four consecutive days. Both isolates produced 100% infection on plants grown from metalaxyl-treated seed, indicating the isolates were metalaxyl-insensitive. While over 80 species within 35 Compositae genera are reported to be hosts for P. halstedii, this is the fourth report of pathogenicity on sunflower by zoospores originating from other genera, the other three being Ambrosia (ragweed) (4), Dimorphotheca (cape marigold) (2) and Xanthium (cocklebur) (3). The cross-infectivity of the P. halstedii from Ambrosia, Iva, and Xanthium indicates that Compositae weeds may serve as a reservoir of P. halstedii to infect cultivated sunflower, and these weeds may help to perpetuate the metalaxyl-insensitive strain of P. halstedii.
References: (1) G. Hall, Mycopathologia 106:205, 1989. (2) E. E. Leppik, Plant Dis. Rep. 49:940, 1965. (3) F. Viranyi, Helia 7:35, 1985. (4) I. Walcz et al. Helia 33:19, 2000.
