USE OF PETAL INFESTATION TO FORECAST SCLEROTINIA STEM ROT OF CANOLA - THE IMPACT OF DIURNAL AND WEATHER-RELATED INOCULUM FLUCTUATIONS

Seven experiments were conducted in commercial canola (Brassica napus) crops in 1987 and 1988. In each experiment, petal samples were collected three times a day at approximately 0800 (morning), 1300 (early afternoon), and 1800 hours (late afternoon). Mean percentage of petals infested (MPPI) with Sclerotinia sclerotiorum was assessed using an agar plate test. A general pattern of increasing MPPI from morning to afternoon was observed in most crops. Analysis of variance indicated that MPPI was significantly higher during the afternoon than the morning in four of seven crops. The increases in specific crops ranged from 0.3 to 46% MPPI; reductions ranging from 0.5 to 14% MPPI were also observed on a few days. The observed pattern in MPPI probably resulted from periodicity in ascospore discharge related to several environmental factors. Changes of up to 92% MPPI occurred during the flowering period in some crops. Such changes may be more important than diurnal variations to a forecasting system for sclerotinia stem rot based on petal infestation. However, because relatively large changes from morning to afternoon occasionally occur, it is suggested that, as a precaution against underestimating MPPI, petals should be sampled in the afternoon.