Chemical control of Phytophthora cinnamomi in potted Leucospermum plants

The efficacy of fosetyl-Al (soil drench), metalaxyl-M (soil drench), fluazi nam (soil drench) and potassium phosphonate (foliar spray) in controlling P hytophthora cinnamomi root infections of Leucospermum cvv. High Gold, Sunri se and Spider was compared. The Leucospermum plants were potted, placed in a fibreglass tunnel and inoculated. Fungicide treatments began 4 weeks afte r inoculation. Stem lesion length was measured and the degree of leaf chlor osis as well as root rot were rated at the conclusion of the experiment. Th e pathogen was re-isolated from the root and crown of each plant, and soil baiting and dilutions were done to monitor soil populations of P. cinnamomi . During the 1997 season, metalaxyl-M gave the best control compared with t he other treatments. The combination of metalaxyl-M with cv. Spider complet ely suppressed symptom development. However, during the 1998 season when cl imatic conditions were favourable for disease development, no differences w ere observed between the treatments and the control. Cv. Spider had higher tolerance against the fungus even with the most aggressive isolate and duri ng the warmer 1998 season, indicating the importance of combining chemical control with plant resistance. None of the treatments showed any phytotoxic ity.