Aggressivity

Several strains of Phoma lingam can be isolated from oilseed rape which differ in phenotypic and genotypic traits. For practical reasons the difference of aggressiveness on oilseed rape is of major interest.
Already 1946 a New Zealand study reported that different isolates can be characterised, which also differ in their impact on oilseed rape (Cunningham, 1946). Similar results have been shown for German oilseed rape growing regions by Kuswinanti et al. (1999). A collection of isolates was established from single ascospores wich were discharged from pseudothecia under in vitro conditions.

Much more effort is needed to check aggressiveness on true leafs or stems. However, the outcome of those tests is very simmilar. A-type isolates produce typical leaf lesions also observed under field conditions as can be seen in the middle-left figure. Leaf lesions of B-type isolates are small, necrotic and very restricted on plants grown and inoculated in the greenhouse (fig. middle-right). These kind of lesions can be also observed in the field but are not strongly correlated with B-type infections due to other fungi which produce similar symptoms.

On stems aggressiveness of A- and B-type isolates differ also significantly. Although the quality of the symptom is not significantly different the lesion size is considerably bigger after infection with A-type isolates. Bottom-figures display these differences. After inoculation of oilseed rape plants at the 3-leaf-stage which have been punctured at the stem and punctures covered with either a mycelium plug or a spore solution, show considerable differences of lesion sizes after 49 dpi. The left figure shows symptoms after inoculation with A-type wheras symptoms of B-type isolates are shown in the bottom- right figure. Wheras lesions of A-types are considerable larger, stem embracing and deep, those of B-types are even smaller and superficial. B-type lesions will also disappear sometimes due to wound reactions of the host plant.

Differences in aggressiveness and the observation that the proportion of A-type isolates is rather higher at the stem basis (stem basis rot) indicate that economic importance is mainly related to A-type infections. B-type isolate seem to be more prominent at higher stem segments. Its economic impotance is still unclear but it may be concluded that it is circumstantial.

Note: In the international literature different terms were or are even still in use to describe these pathotypes. Beside others terms like "aggressive/non-aggressive", "highly virulent/weakly-virulent" or "virulent/avirulent" are in use. All these terms rely on the observation that isoaltes fall into two different groups of aggressiveness on the host plant Brassica napus. These terms do not include aggressiveness on other host plants and therefore are oversimplifing. Therefore, A- and B-type are more neutral terms and do not implicate properties which are not true if other host plants than oilseed rape are considered. Reclassification of the teleomorph taxonomy (please refer to this chapter) may also help to circumvent those problems.