RACE‐SPECIFIC RESISTANCE IN GROUNDSEL (SENECIO VULGARIS) TO THE POWDERY MILDEW ERYSIPHE FISCHERI

Abstract

SUMMARYTwo-hundredrand-fifty lines of groundsel {Senecio vulgaris L.) were tested for their reactions to five different isolates of the powdery mildew fungus Erysiphe fischeri Blumer. Fifty-one of the lines were also tested for their reactions to three further isolates. All the lines were tested using detached leaves but a number of them were also tested using seedlings. The seedling tests gave esentially the same results as the detached leaf tests. Over 50 % of the host lines were highly resistant (gave low infection types) to one or more of the mildew isolates and this resistance was isolate-or race-specific. Eight specific resistance factors were differentiated by the eight mildew isolates used in the tests. Some of the resistance factors appear to be present in species of Senecio other than S vulgaris. All eight mildew isolates were complex races with virulence for all but one of the specific resistance factors, the isolates differing from each other in the possession of avirulence for a unique specific resistance factor. The interactions between the host lines and mildew isolates are indicative of a gene-for-gene interaction but the genetic analysis required to establish this was not attempted. The majority of lines which were susceptible to each isolate developed the highest or most susceptible infection type. It is considered that tolerance ot the parasite may operate together with race-specific resistance in an overall survival strategy which has evolved to enable groundsel to cope with mildew attack.

Polymorphism for disease resistance in the annual legume Amphicarpaea bracteata

“…Recent studies of plant-pathogen interactions in natural communities have revealed that variation in plant disease resistance is widespread (Wahi, 1970;Zimmer and Rehder, 1976;Burdon, 1980;Segal et a!., 1980;Burdon and Marshall, 1981; Burdon et a!., 1983;Alexander et a!., 1984;Dinoor and Eshed, 1984;Parker, 1985;Harry and Clarke, 1986). This extensive variability implies that natural plant populations have the capacity to undergo significant adaptive evolution in response to pathogen attack.…”

THE GENETICS OF RACE‐SPECIFIC RESISTANCE IN GROUNDSEL (SENECIO VULGARIS L.) TO THE POWDERY MILDEW FUNGUS, ERYSIPHE FISCHERI BLUMER

“…We have found repeatedly that it is very easy to isolate additional pathotypes from the local mildew population which differentiate additional specific resistance factors (Harry, 1980;Clarke, Bevan & Crute, 1987). For example, out of 12 isolates selected at random from the mildew population around Glasgow in 1984, nine were of different pathotypes (Clarke et al, 1987), a id none of them was the same as any of the eight described previously by Harry & Clarke (1986). Since the local population of mildew is composed of large numbers of pathotypes, it follows that line lOj will carry large numbers of matching specific resistance genes.…”

“…These were selected from among the 250 lines whose reactions to different mildew pathotypes were reported previously (Harry & Clarke, 1986). Table 1 lists their reactions to each of the five mildew pathotypes used.…”

“…The mildew pathotypes, each originally derived from a single conidial chain, were maintained on detached leaves of the appropriate susceptible line as described previously (Harry & Clarke, 1986). The detached leaves were incubated on benzimidazole agar (30 mg 1~^) in Petri dishes under lights (20 W m~^) in a growth cabinet at 19 °C with a 16 h photoperiod.…”

“…Reactions of the parent lines and the Fj and Fg generations were determined on detached leaves as described previously (Harry & Clarke, 1986), except that, because at least four, and sometimes all five, mildew pathotypes were tested on each plant, only two leaves could be used per pathotype and it was not possible to confirm resistance reactions in this study. Levels of infection were assessed, as before, on a scale of 0 to 4, with leaves showing infection types 0 and 1 being recorded as resistant, and those developing infection types 2 to 4 as susceptible.…”