VALIDATION OF DOWNCAST IN THE PREDICTION OF SPORULATION-INFECTION PERIODS OF PERONOSPORA DESTRUCTOR IN THE LOCKYER VALLEY

The disease forecasting system 'Downcast' was developed in Canada to i ndicate sporulation-infection periods of Peronospora destructor (Berk. ) Caspary in onions. During 1989, observations were made of downy mild ew development in trap plants and of disease progress in an unsprayed plot of onions in the Lockyer Valley, Queensland, and related back to predicted sporulation-infection periods using the forecaster. Over 113 days, there were 20 days when sporulation and infection were predicte d, and of these 14 occurred during 4 weeks in July and August. Of 16 g roups of trap plants exposed in the field, each for 7 days, 7 were sub jected to predicted sporulation-infection periods. Two predicted sporu lation-infection periods early in the season did not result in infecti on of trap plants or of plants in the field plot, probably due to abse nce of inoculum. Subsequently, groups of trap plants exposed in the fi eld during predicted sporulation-infection periods became infected whi le those exposed when weather was unsuitable remained healthy. 'Downca st' shows promise as a technique for more effective management of dise ase caused by Peronospora destructor.