Frequency and geographical distribution of specific virulence factors in Bremia lactucae populations in England from 1973 to 1975

Abstract

SUMMARYSamples of lettuces infected with Bremia lactucae were collected from seven areas of England during 1973‐5. These were tested under controlled environmental conditions on a range of differentially resistant cultivars to determine the frequency of virulence (V) factors in the pathogen population. In most cases the results could be accommodated by the relationship advanced by Crate & Johnson (1976). A virulence phenotype was determined for each isolate and individual virulences within isolates were identified as present at high or low frequencies. The most common V factors were V3, V4, V6 and V8 which reflected the resistance (R) factor complement of lettuce cultivars currently in commercial production. There were few gross differences in the geographical distribution of virulence factors between the seven areas. In the West Central region, however, which encompasses the Evesham and Lancashire production areas, results indicated that the use of cultivars containing resistance (R) factors 2 and 8 respectively would have provided protection from the disease during 1974. There were differences in virulence combinations at the holding or locality level within each large area, which could be exploited by growers providing testing was done in their particular locality.Universally susceptible cultivars and those with only limited resistance factors were often found to be infected with B. lactucae populations which contained large numbers of V‐factors. It might have been expected that these pathogen populations would have been restricted to cultivars with large numbers of R‐factors. This finding conflicts with the suggestion that pathogen populations with complex virulence patterns are ‘unfit’ as compared with those with more simple virulence components.

The inheritance of specific virulence in Bremia lactucae (downy mildew) to match resistance factors 1, 2, 4, 6 and 11 in Lactuca sativa (lettuce)

“…Tliese results show that se.xual recombination may play an important role in generating new virulence phenotypes in B. lactucae (Crute & Norwood, 1980;Michelmore & Ingram, 198]). Field isolates corriraonly carr)' many viruience determinants which appear to be unnecessary, since the matching R-factors are not in cultivars grown commercially (Crute & Dixon, ]978; Dixon & Wright, 1978) or have only been found in foreign wild germplassTi (Norwood et a!., 1981). The flexibility in the pathogen population to allow this is likely to arise from random outbreeding and hence the maintenance of a high frequency of heterozygotes.…”

The location and characteristics of novel sources of resistance to Bremia Lactucae Regel (downy mildew) in wild Lactuca L. Species

“…Recent surveys of B. Zactucae populations in the UK (CRUTE & DIXON, 1978 ;DIXON & WRIGHT, 1978) Sweden (WELLVING & CRUTE, 1978) and Czechoslovakia (LEBEDA, 1979a, b;1980a, b), showed that no single resistance factor was completely effective since the virulence factors necessary to overcome all 11 J. hf. NORWOOD,L R. CRUTE AND A. LEBEDA R-factors are frequent in the pathogen populations.…”

Variety is the spice of life

“…de Bary with virulence factors 1, 2 and 3 occurring more frequently. But, unexpectedly, races with virulence factors 5 to 11 have also occurred even though hosts with R-genes 5 to 11 have not been used commercially (Malcolmson, 1969;Shattock, Janssen, Whitbread & Shaw, 1977)-a state of affairs supported by Dixon & Wright (1978) who found that some universally susceptible lettuce cultivars were infected with populations of downy mildew, Bremia lactucae (Regel), containing large numbers of specific virulence factors. These instances seem to be at variance with van der Plank's (1 968) contention that natural selection acts against races of pathogens with unnecessary specific virulence factors, a generalisation possibly applicable to plants with tolerance factors enabling them to withstand different pollutants.…”

“…These instances seem to be at variance with van der Plank's (1 968) contention that natural selection acts against races of pathogens with unnecessary specific virulence factors, a generalisation possibly applicable to plants with tolerance factors enabling them to withstand different pollutants. The observations made by Malcolmson (1969) and by Dixon & Wright (1978) could have a profound effect on the strategy Table 9. Development of mycorrhizas when direrent isolates of Amanita muscaria were inoculated to different seedlots of Betula pubescens (P. A .…”