Colonization of winter oilseed rape tissues by A/Tox(+) and B/Tox(0) Leptosphaeria maculans (phoma stem canker) in France and England

Abstract

The colonization of winter oilseed rape plants and epidemiology of phoma stem canker differed between A/Tox(+) and B /Tox(0) Leptosphaeria maculans. In France and England, where plant colonization was investigated during two and three growing seasons, respectively, there was a difference in timing of leaf infection; A/Tox(+) L. maculans was predominant on leaves in the autumn (October/ November) but there was an increase in the incidence of B/Tox(0) in the winter (January/ February). In May, June and July both species could be isolated from all external parts of the plant (root to the upper stem) and all crown (stem base) tissues, although they differed in their distribution. At the root and crown, A/Tox(+) L. maculans was predominant and was located throughout the cortex, wood and pith tissues, but the rarer B/Tox(0) was located mainly in the cortex. Approximately equal numbers of A/Tox(+) and B/Tox(0) isolates were obtained from the upper stem - there was a greater proportion of B/Tox(0) isolates than at the crown. In England, after harvest in 1999 and 2000, pseudothecia on the lignified tap root and crown tissues produced predominantly A/Tox(+) ascospores (94%), while pseudothecia higher up the stem produced more B/Tox(0) ascospores (60%) than A/Tox(+) ascospores (40%). The timing of the onset of leaf spotting, earlier in the season for A/Tox(+) than B/Tox(0) L. maculans, and the predominance of mycelium of A/Tox(+) at the crown are consistent with the assumption that A/Tox(+) is more likely to cause the most damaging stem cankers than B/Tox(0) L. maculans. Identification as A/Tox(+) or B/Tox(0) by cultural characteristics differed only slightly (2.3%) from identification by molecular techniques.