FIELD EFFICIENCY OF BRASSICA-NAPUS SPECIFIC RESISTANCE CORRELATES WITH LEPTOSPHAERIA-MACULANS POPULATION-STRUCTURE

Field experiments were conducted in Versailles, France, to assess blac kleg resistance of Brassica napus cultivars Quinta and Glacier under n atural infection conditions. Blackleg disease severity was assessed tw ice during growth of B. napus. Quinta resistance was highly expressed as only 13% to 18% of the plants exhibited leaf symptoms in December, whereas Glacier and other cultivars displayed more than 80% of infecte d plants. In June (harvest), 70% (first year) to 41.5% (second year) o f Quinta plants were canker-free. In contrast, Glacier was as infected as the susceptible control cultivars, with more than 88% of plants di splaying canker. The Leptosphaeria maculans population structure was e xamined in parallel. Based on soluble protein patterns, 9% of the 299 fungal isolates collected were characterized as Tox(0) species, and be longed to the NA1 sub-group. All but two of Tox(0) isolates were isola ted from atypical dark necrotic leaf lesions, mainly occurring on Quin ta. In contrast, the Tox(+) isolates were recovered from typical leaf lesions. Following a cotyledon inoculation test on the differential se t Westar, Quinta and Glacier, 92 to 95% of Tox(+) isolates collected o n susceptible cultivars were characterized as PG3 isolates, i.e. aviru lent on Quinta. The remaining Tox(+) isolates belong to PG4, i.e. viru lent on the three cultivars. No PG2 isolate, i.e. avirulent on both Qu inta and Glacier, was identified in the sampling. The present study su ggests that specific resistance expressed at the cotyledon level can b e efficient under field conditions where the corresponding avirulent r aces of the pathogen are prevalent.