Relationship between environmental factors, effects of fungicide applications, and premature defoliation of black locust trees (Robinia pseudoacacia L.)

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From 1989 through to and including 1994 extensive premature defoliation of black locust ( Robinia pseudoacacia L.) was observed in a progeny test plantation in Greene County, in the Georgia Piedmont. During July 1991 and 1992, all observed trees suffered greater than 50% defoliation. Leaflets of black locust showing symptoms of leaf spotting began abscising as early as mid-June following wet springs. Onset and progress of defoliation was followed on 50 trees in two plots in 1993 and 1994. In both years, sample trees were cut at the base and allowed to regrow from sprouts. A protectant fungicide, chlorothalonil (Daconil WDG 1.5 g/l water) was applied to run-off weekly to one-half of the growing sprouts beginning 17 April through 22 August 1994 by backpack sprayer. Air temperature, relative humidity, rainfall, and leaf wetness were monitored. Abscission of unprotected leaflets in 1993, a dry, hot year, began in mid-July and averaged 36% by late August; abscission of unprotected leaflets in 1994, a wet, cool year, began in mid-June and averaged 80% by late August; whereas, abscission of protected leaflets in 1994 began in late June and averaged 70% by late August. The Gompertz model described the increase of leaflet abscission with time. The estimated rate of leaflet abscission for unprotected leaflets in 1993 was significantly slower ( k = 0.056/wk) compared to the rate of abscission of unprotected leaflets in 1994 ( k =0.070/wk). Estimated rates of leaflet abscission were not significantly different between protected ( k = 0.063/wk) and unprotected trees ( k =0.070/wk) in 1994. Cumulative hours of leaf wetness of 600 h were associated with initial leaf spotting and defoliation in 1993 and in 1994. Delay in leaflet abscission of protected trees indicates that fungal infection may have contributed to premature defoliation of black locust with onset related to moisture conditions favourable for infection.