Phosphonic acid was more effective in inhibiting the production of sporangia of Phytophthora clandestina in sterile pond water (ED50= 1·4 p.p.m.) than it was in inhibiting the growth of mycelium of the fungus on either corn meal agar (ED50= 13·8p.p.m.) or lima bean agar (ED50= 236 p.p.m.). Experiments under controlled environmental conditions showed that better control of tap root rot of subterranean clover caused by P. clandestina was achieved by application of potassium phosphonate to a pasteurized soil mixture than by a spray on the foliage alone. However, in a soil from a pasture, foliar sprays were more effective than soil treatments. Application of superphosphate at a rate of 250 kg/ha or higher to the soil reduced the effectiveness of sprays. In three experiments in irrigated pastures a spray of potassium phosphonate at 300 or 313 ml/ha, applied to cotyledons of subterranean clover and to soil, reduced severity of Phytophthora root rot and increased the annual production of dry matter of the legume by 1·96 to 5·11 t/ha in comparison with untreated controls.

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The reaction of 10 commercial potato cultivars to infection by Spongospora subterranea was evaluated in the field and in the greenhouse. Micropropagated plants were used in the greenhouse and seed pieces were used in the field. The relative susceptibilities of the cultivars when measured in the greenhouse and in the field were comparable. The overall severity of scab on tubers was 8 times higher in the greenhouse than the field. The relative susceptibilities of 3 standard cultivars (Kennebec, Katahdin and Russet Burbank) were consistent over 3 greenhouse trials. When comparing planting material in the greenhouse, the incidence and severity of scab on the progeny of micropropagated plants did not, in general, differ significantly from that on the progeny of seed potatoes. Furthermore, the incidence and severity of scab on tubers of the highly susceptible cv. Kennebec grown in the greenhouse were not affected significantly by type of potting mix (well-drained, compared with poorly drained which inhibited plant growth). Screening potato cultivars for resistance to powdery scab on a small scale in the greenhouse, using micropropagated plants, is suggested as a preliminary to the evaluation of cultivars in the field.