Cultural characters, protein patterns and unusual mating behaviour of Phytophthora gonapodyides isolates from Britain and North America

Isolates of Phytophthora gonapodyides associated with roots of woody hosts or from aquatic habitats in Britain (10 isolates) and North America (Alaska to California, 12 isolates) were compared. They showed similar characteristics including similar sporangial dimensions, colonies with often distinctive petaloid patterns and a silvery appearance, slow growth rates at 20, 25 and 30 °C, and a maximum temperature for growth of about 35°. Most isolates exhibited a similar protein banding pattern distinct from that of P. cryptogea or P. drechsleri, and including a characteristic band designated the ‘PG band’. Three isolates deviated from this pattern and two of these also produced chlamydospores and chains of swellings in culture. These isolates might be hybrids related to P. gonapodyides or a different taxon, in which case ‘P. gonapodyides’ is polyphyletic.

All the isolates were self-sterile. When paired directly or indirectly via polycarbonate membranes with A2 sexual compatibility types of heterothallic species such as P. cambivora, P. megakarya or P. meadii, gametangia were produced which were morphologically characteristic of these species. The range of heterothallic species responding was different from that responding sexually to Trichoderma volatiles. P. gonapodyides isolates may be sterile A1 compatibility types able to produce a compatibility substance which induces selfing in the heterothallic A2s.

Three other self-sterile Phytophthora isolates associated with woody hosts in Britain were a distinct group, with a different colony pattern, a distinct 30° growth optimum and an ability to grow at 37°. Their protein banding pattern was different from that of either P. gonapodyides or P. cryptogea/P. drechsleri, but showed similarities to these species. One isolate induced gametangial formation in P. drechsleri A1 types, the two others were sexually neutral. These three isolates might also be species hybrids, possibly with P. gonapodyides and P. cryptogea as parents.