Comparative Morphology and Behaviour of A1 and A2 Isolates of Phytophthora cinnamomi

Abstract

Studies of morphology, physiological behaviour and pathogenicity indicated that some significant differences existed between isolates of the A1 and A2 compatibility types of Phytophthora cinnamomi from native plant communities in Australia.

The majority of A1 isolates produced zoosporangia more rapidly and in greater numbers than A2 isolates, and zoosporangia were produced evenly on mycelial discs of A1 isolates whereas they occurred mostly at the perimeter of similar discs in the case of A2 isolates. Chlamydospores were produced more rapidly by A1 than A2 isolates, and in the former tended to be of an even size and were frequently single, whereas in A2 isolates the size was extremely variable and production was usually in clusters.

Both compatibility types showed similar temperature-growth optima, but the mean growth rate of A1 isolates, over the range 15-30°C, was less than that of A2 isolates. Both types responded similarly to high temperature treatments and to a range of osmotic potentials and both showed similar pathogenicity to lupin and Pinus radiata seedlings. While both compatibility types showed similar reactions to a number of inhibitors, the growth of A2 isolates was inhibited more by rose bengal and pyronin G than that of A1 isolates.

Thus, although significant differences were recorded, there was a general similarity of behaviour of isolates of both compatibility types and there were no striking differences from the recorded behaviour of the fungus in other countries.