Virulence of Pythium Species Isolated from Wheat Fields in Eastern Washington.

Although Pythium root rot in wheat (Triticum aestivum) is well documented, limited information is available concerning which species of Pythium are most responsible for disease damage. The objective of this study was to examine the variation in virulence on wheat among isolates of Pythium collected from cereal grain fields in eastern Washington. Isolates of nine Pythium species were tested for virulence on spring wheat cultivars Chinese Spring and Spillman. Cultivars were planted in pasteurized soil infested with Pythium isolates and placed in a growth chamber maintained at a constant 16°C and ambient humidity. Plant height, length of the first true leaf, and number of seminal roots were recorded, and roots were digitally scanned to create computer files that were analyzed using WinRhizo software. Pythium isolates caused a significant reduction (P < 0.05) in the number of root tips, root length, and length of the first leaf. Differences in virulence were detected among species and among isolates within species. Isolate Pythium debaryanum 90136 and P. ultimum 90038 were the most virulent and may prove useful in future disease screening assays of Triticum germ plasm.