CHARACTERIZATION OF RESISTANCE OF SPINACH TO WHITE RUST (ALBUGO-OCCIDENTALIS) AND DOWNY MILDEW (PERONOSPORA-FARINOSA F SP SPINACIAE)

Resistance of spinach to white rust (Albugo occidentalis) and races 3 and 4 of downy mildew (Peronospora farinosa f. sp. spinaciae) was quantified on several cultivars and breeding lines in separate field inoculation experiments. Several cultivars and breeding lines from the Arkansas breeding program had undergone repeated field selections for white rust resistance (Fall Green, Ozarka II, FA88-310, FA88-354, and FA86-70) while others had not (Grandstand, St. Helens, and Hybrid 424). Resistance to both diseases was quantified by measuring disease incidence and severity at regular intervals 7-32 days after inoculation. The latent periods and the area under the disease progress curve (AUDPC) also were compared. Overall, the cultivars and breeding lines that had undergone selection for white rust resistance had significantly higher levels of field resistance to both white rust and races 3 and 4 of downy mildew relative to the other cultivars tested. Two cultivars (Fall Green and Ozarka II) and three breeding lines (FA88-354 and FA88-310 or FA86-70) had significantly lower disease incidences and severity ratings to white rust and race 4 of downy mildew on most sampling dates than did St. Helens, Hybrid 424, and Grandstand. The AUDPC for Fall Green, Ozarka II, FA88-354, and FA88-310 or FA86-70 also was significantly lower than that of St. Helens, Hybrid 424, and Grandstand for white rust and race 4 of downy mildew. Fall Green, Ozarka II, and FA88-354 had significantly lower disease incidences and severity ratings than did Grandstand in the race 3 downy mildew tests. The AUDPC for Fall Green, Ozarka II, and FA88-354 also was significantly lower than that of Grandstand in the race 3 downy mildew tests. The estimated latent periods were generally 1-4 days longer for Fall Green, Ozarka II, FA88-354, and FA88-310 or FA86-70 in the white rust and race 3 tests compared with the other cultivars tested. The latent periods for the race 4 tests could not be accurately estimated. Several of the cultivars and breeding lines have measurable levels of field resistance to both white rust and races 3 and 4 of downy mildew and can be used to help manage these economically important diseases of spinach.