RELATIONSHIP BETWEEN ENVIRONMENTAL-FACTORS, EFFECTS OF FUNGICIDE APPLICATIONS, AND PREMATURE DEFOLIATION OF BLACK LOCUST TREES (ROBINIA-PSEUDOACACIA L.)
Ec. Whiting et al., RELATIONSHIP BETWEEN ENVIRONMENTAL-FACTORS, EFFECTS OF FUNGICIDE APPLICATIONS, AND PREMATURE DEFOLIATION OF BLACK LOCUST TREES (ROBINIA-PSEUDOACACIA L.), International journal of pest management, 43(3), 1997, pp. 213-219
From 1989 through to and including 1994 extensive premature defoliatio
n 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% defoliati
on. Leaflets of black locust showing symptoms of leaf spotting began a
bscising as early as mid-June following wet springs. Onset and progres
s of defoliation was followed on 50 trees in two plots in 1993 and 199
4. In both years, sample trees were cut at the base and allowed to reg
row from sprouts. A protectant fungicide, chlorothalonil (Daconil WDG
1.5 g/l water) was applied to run-off weekly to one-half of the growin
g sprouts beginning 17 April through 22 August 1994 by backpack spraye
r. Air temperature, relative humidity, rainfall, and leaf wetness were
monitored. Abscission of unprotected leaflets in 1993, a dry, hot yea
r, began in mid-July and averaged 36% by late August; abscission of un
protected leaflets in 1994, a wet, cool year, began in mid-June and av
eraged 80% by late August; whereas, abscission of protected leaflets i
n 1994 began in late June and averaged 70% by late August. The Gompert
z model described the increase of leaflet abscission with time. The es
timated rate of leaflet abscission for unprotected leaflets in 1993 wa
s significantly slower (k=0.056/wk) compared to the rate of abscission
of unprotected leaflets in 1994 (k=0.070/wk). Estimated rates of leaf
let 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 an
d defoliation in 1993 and in 1994. Delay in leaflet abscission of prot
ected trees indicates:that fungal infection may have contributed to pr
emature defoliation of black locust with onset related to moisture con
ditions favourable for infection.