Pathogenicity studies on isolates of Leptosphaeria maculans from brassica seed production crops in south‐east England

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Abstract
SUMMARY: Out of 117 isolates of Leptosphaeria maculans collected from a range of brassica seed crops in south‐east England, 26 were virulent when tested on cabbage cv. January King. Of these, 24 were isolated from oil‐seed rape (Brassica napus) and the remainder from a swede (B. napus) and a cabbage (B. oleracea) seed crop. Virulent strains were derived from infected stems and from single ascospores of the sexual state present on diseased stubble. In glasshouse tests virulent isolates were not host‐specific and caused severe cankers on cabbage, kale, swede, turnip and oil‐seed rape; under field conditions, ascospores originating from diseased oil‐seed rape stubble produced severe cankers and high levels of seed‐borne infection in eight horticultural and vegetable brassica hosts (B. napus, B. campestris and B. oleracea). Virulent and non‐virulent types could be distinguished by certain cultural characteristics. On nutrient agar virulent types grew slowly, irregularly and quickly staled whereas non‐virulent types grew rapidly, regularly and did not stale. The latter also produced a yellow‐brown pigment in liquid culture. The potential for cross‐infection of virulent strains originating from oil‐seed rape to other brassica seed crops has serious implications for disease incidence on forage and vegetable brassica seed crops.