Effects of temperature on ascospore germination and penetration of oilseed rape (Brassica napus) leaves by A‐ or B‐groupLeptosphaeria maculans(phoma stem canker)

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Abstract
Ascospores of both A‐group and B‐groupLeptosphaeria maculansgerminated at temperatures from 5 to 20°C on leaves of oilseed rape. Germination of ascospores of both groups started 2 h after inoculation and percentage germination reached its maximum about 14 h after inoculation at all temperatures. Both the percentage of A‐/B‐group ascospores that had germinated after 24 h incubation and germ tube length increased with increasing temperature from 5 to 20°C. Germ tubes from B‐group ascospores were longer than those from A‐group ascospores at all temperatures, with the greatest difference at 20°C. Hyphae from ascospores of both groups penetrated the leaves predominantly through stomata, at temperatures from 5 to 20°C. A‐group ascospores produced highly branched hyphae that grew tortuously, whereas B‐group ascospores produced long, straight hyphae. The percentage of germinated ascospores that penetrated stomata increased with increasing temperature from 5 to 20°C and was greater for A‐group than for B‐groupL. maculansafter 40 h incubation.