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The effectiveness of one cycle (C) per year of phenotypic recurrent selection was evaluated for improving tolerance of peas (Pisum sativum L.) to aphanomyces root rot (Aphanomyces euteiches Drech.). Each cycle included field-screening a population of F 2 lines (summer), diallel intermating among lines selected from the field (fall green- house), and one generation of selfing F 1 plants (spring greenhouse) to produce F2 lines for the next cycle. The schedule is repeated for each cycle. A blocks-within- replicates design was employed in the field screening of C1 and C2 to improve within-block homogeneity. Selection intensities were 12.4%, 11.1%, and 10.6% for C0, C1, and C2, re- spectively. Using the performance of a tolerant control line, Mn 108, as a basis of comparison, the realized gain in dry seed yield and survival was 32% and 68% from C0 to C1 and 22% and 115% from C1 to C2, respectively. Aphanomyces root rot of pea, also called common root rot, occurs in most pea-growing areas of North America and in northern Europe, Australia, New Zealand, and Japan (Hage- dorn, 1984). The fungus Aphanomyces euteiches was first iden- tified as the causal organism in 1925 (Jones and Drechsler, 1925). Losses in Wisconsin average ≈10% annually (Pfender, 1984), but under conditions of severe disease development, most pea cultivars are killed even before flowering, and entire fields may be lost. Since viable oospores of the organism can exist in the soil for more than 10 years (Pfender, 1984), Gritton et al. (1984) recommended that peas not be grown on the same field more often than once every 4 or 5 years. Although several methods of control have been considered, including the use of cruciferous soil amendments (Muehlchen et al., 1990; Papavi- zas, 1967), trifuralin and dinitramine herbicides (Harvey et al., 1975), and bacterial seed treatments (Parke et al., 1991), the use of resistant cultivars would also be desirable. To our know- ledge, there are currently no cultivars available with high levels of resistance. Moderate resistance or tolerance has been found in certain wild-type lines, but these are associated with an un- desirably tall growth habit and colored flowers, which are un- desirable because they are often associated with undesired pigment in the shelled pea. This association was attributed to linkage, rather than pleiotropy; many lines found to be susceptible pos- sessed the' same set of dominant alleles (Marx et al., 1972). Breeding lines approaching commercial type and having some resistance to aphanomyces root rot have been released by Wash- ington State Univ. and the U.S. Dept. of Agriculture, Agricul-

Use of One Cycle of Recurrent Selection per Year for Increasing Resistance to Aphanomyces Root Rot in Peas