Surveys in 1976 and 1977 showed that false rust, caused by Synchytrium psophocarpi (Rac) Gaumann occurred widely throughout Papua New Guinea (P.N.G.) on winged bean (Psophocarpus tetragonolobus (L.) DC.). During this period disease severity was slight in most localities. Descriptions of the pathogen based on material collected in P.N.G., and the Philippines are given. Sporangia of S. psophocarpi germinated over a temperature range of 5 - 30°C in the presence of free moisture. When exposed to air sporangia lost viability in 4 days. Zoospores became stationary 30 min after emergence and encysted 2 - 3 h later. A minimum period of 12 h of leaf surface wetness was required for infection to occur. Following inoculation disease symptoms were recorded after 7 days, mature sori rupturing after 22 days. Attempts to infect winged bean seeds and other legume species were unsuccessful. Resting spores were not found. Sporangia were dispersed by wind or water splash. Spore trap counts of airborne sporangia showed a diurnal periodicity. Maximum catches occurred between 16.00 and 18.00 h, lowest counts being recorded between 06.00 and 07.00 h. Concentration of airborne sporangia increased during the wet season and subsequently declined two months after the onset of the dry season. Trapping sporangia from an infection source revealed a concentration gradient, with the highest concentration recorded at lm and the lowest at 20m. Observations of disease development in the field showed that infection occurred at random. Of the fungicides evaluated in vitro tridemorph plus maneb was the most effective in inhibiting germination of sporangia. Fentinacetate plus maneb gave the best control in a preliminary field trial. All 125 lines of winged bean screened from the P.N.G., germplasm collection were susceptible to S. psophocarpi. In regional trials of 10 of these lines, however, variation in susceptibility was apparent. Two lines from Thailand showed resistance following artificial inoculation.
|Date of Award||1981|