RESPONSE OF SOYBEAN TO LOWER-CANOPY DEFOLIATION DURING SEEDFILL

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Abstract
One of the effects of Septoria brown spot on soybean is accelerated lower-canopy defoliation. Field experiments were conducted in 1986 and 1987 to determine the response of soybean yield and yield components to progressive lower-canopy defoliation during seedfill. Soybean cultivar Williams 82 was machine planted in rows 76 cm wide and hand thinned to a uniform stand of 23 plants m−1. Three treatments were used: controls; defoliated, starting at R5 (beginning seed stage; DEF1), and defoliated, starting 7 d past R5 (DEF2). Defoliation treatments were applied by removing the leaflets and petioles from the lowest three or four leaf-bearing nodes four times at 4-d intervals so that only three or four nodes at the top of the plant remained leaf-bearing when defoliation ceased. Yield and yield components were determined for upper, lower and total nodes. Although the two growing seasons were very different, yield responses to defoliation were similar in both years. On the average, DEF1 reduced seed yield by 18% which is similar to losses that occur when brown spot is induced by inoculation; DEF2 reduced yields by 9% which is similar to yield losses reported for naturally occurring brown spot. Yields of DEF2 exceeded yields of DEF1 because of differences in pod number in 1986 and seeds/pod in 1987. Controls out-yielded DEF2 because of differences in seeds/pod in 1986 and seed size in 1987. The proportion of the total yield contributed by pods found on the upper nodes averaged 76% and was not affected by defoliation.Key words: Glycine max, Septoria glycines, brown spot, leaf loss

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