Criteria for Identifying Pathogenic Races ofPhytophthora fragariaeon Selected Strawberry Genotypes

A quantitative classification method for assessing resistance to Phytophthora fragariae var. fragariae in strawberry

“…Fifteen classifications are in agreement with the literature, and three are divergent from at least one earlier report (`Md683'-A2, `Md683'-A6, 'Yaquina A'-Al0) (Converse, 1970;Converse & Scott 1962 ;Milholland et al ., 1989 ;Montgomerie, 1966 ;Nickerson & Murray, 1993 ;Pepin & Daubeny, 1966 ;Scott et al ., 1975) . These divergences can be explained by differences in the applied disease thresholds and experimental parameters .…”

“…The flexibility of this threshold contrasts with the fixed thresholds of earlier classification methods, which classified a cultivar as resistant either if it never showed any infection (Montgomerie, 1966 ;Pepin & Daubeny, 1964, 1966, if invasion was confined to the root tips (Converse & Scott, 1962 ;Converse, 1967Converse, , 1970, if invasion was limited to 3% or less of the total root length (Nickerson & Murray, 1993) or if it remained below a certain level of a disease severity index (Milholland et al ., 1989) . These thresholds are more or less equivalent to fixed PIRTs of 0 .01%, 3%, 3%, and 5%-10%, respectively .…”

“…'Md683' originated from the USDA strawberry breeding program at Beltsville, MD (Scott et al ., 1950) . 'Blakemore' is highly susceptible to all these isolates while the other genotypes are resistant to some of them (Converse, 1970, Milholland et al ., 1989 . Stock plants of 'Blakemore', 'Del Norte' and 'Md683' were kindly supplied by R.D .…”

“…fragariae (Wilcox et al ., 1993), is a soilborne fungus which causes red core (red stele) root rot in strawberry (Hickman, 1940) . Many cultivars have been tested for race specific resistance (Converse, 1970 ;Kennedy et al ., 1986 ;Maas et al ., 1989 ;McKeen, 1958 ;Milholland et al ., 1989 ;Montgomerie, 1966 ;Nickerson & Murray, 1993 ;Pepin & Daubeny, 1964,1966Scott et al ., 1950Scott et al ., , 1975, but investigators often reached different conclusions . These inconsistencies hamper differentiation of cultivars and isolates, elucidation of the genetics of resistance, and breeding for resistance (Van de Weg, 1989) .…”

Genotype-dependence in the interaction betweenGlomus fistulosum, Phytophthora fragariae and the wild strawberry (Fragaria vesca)

“…The number of oospores were counted per root segment and an average number calculated. Disease severity indices were calculated using the following formula, mean number of oospores per root segment x % root segments infected / 100 where DSI < 1 = resistant cultivar and DSI > 1 = susceptible cultivar (Milholland et al, 1989). Ten inoculated and 10 control plants were assessed; 5 plants were taken, at random, from each treatment for mycorrhizal assessment and all ten were scored for DSI.…”