Relationship between Brassica napus seedling and adult plant responses to Leptosphaeria maculans is determined by plant growth stage at inoculation and temperature regime

Abstract

Contradictory reports exist in the literature on the relationship of seedling and adult plant resistance to blackleg disease in Brassica napus (oilseed rape, canola) caused by Leptosphaeria maculans. Two repeat experiments were carried out to determine whether adult plant responses in certain cultivars of B. napus to a specific strain of L. maculans are evident in the response of the plants at the seedling stage and to evaluate whether the relationship between seedling and adult plant responses to L. maculans is determined by the growth stage of the plant at inoculation and the temperature of incubation prior to field planting. Point inoculations were made with pycnidiospores (conidia) on to cotyledons or youngest leaves of four B. napus cultivars (Westar, Surpass 400, Dunkeld and Grouse) at different growth stages under two temperature regimes (18/24 and 11/18 °C) which were rated for cotyledon or leaf lesions before they were planted out in the field and the severity of subsequent crown cankers assessed. Results across the four cultivars showed there was a significant effect of plant growth stage at inoculation ( P < 0.001), and cultivars ( P < 0.001) and a significant interaction between growth stage and cultivars ( P < 0.001) in relation to cotyledon lesions, leaf lesions, and crown cankers, with both temperature regimes. Overall, the cultivars differed in resistance to the strain tested in relation to cotyledon, leaf and crown canker symptoms, with cv. Dunkeld more resistant than Grouse > Surpass 400 > Westar. Different temperature regimes during inoculation, and for a minimum of 4 weeks post-inoculation, had significant effects on the growth rate of the plant as well as the development of disease symptoms. There was a high degree of positive correlation between the severity of cotyledon or leaf lesions with subsequent crown canker that developed for inoculations at nearly all growth stages at the 18/24 °C regime and the early growth stages (particularly up to and including growth stage 1,01, the first true leaf exposed) at the 11/18 °C regime. Overall, plant growth stage at inoculation clearly determined the expression of seedling and/or adult plant resistance. For all cultivars tested, at the higher temperature regime, expression of resistance between seedlings and adult plants, for nearly all growth stages at inoculation, could not be distinguished. In contrast, at the lower temperature regime, this relationship did not exist for inoculations carried out after growth stage 1,05 (the fifth true leaf exposed). These findings may explain the variable results obtained in previous studies attempting to define ‘adult plant resistance’ to blackleg and also indicates the need for standardizing screening processes for identification of host resistance.