The Retention and Redistribution of Captan on Apple Foliage

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Abstract
Residue levels of captan were monitored on apple foliage during the 1980 and 1981 growing seasons. The residues did not build up on matured leaves sprayed at 7- or 14-day intervals because the captan deposits decreased rapidly between sprays. When captan was applied to runoff at 1.2 g a.i. (active ingredient)/l, initial deposits on matured leaves were 5-13 .mu.g a.i./cm2. The greatest deposits occurred early in the season when foliage was sparse and penetration of the spray into the 3 canopy was greater. Captan residues rapidly decreased to .apprx. 20% of the initial deposit as apple seedlings were subjected to 0.5-0.8 cm of simulated rain. The remaining 20% of the initial deposit was more tenacious, i.e., the rate of captan loss decreased as rainfall increased. Captan residues decreased exponentially as the duration of simulated rainfall increased in laboratory studies. In the orchard, captan residues decreased linearly as the amount (i.e., duration and/or intensity) of rainfall increased, but > 25 mm of rain was required to reduce the captan deposit to 20% of the initial level. Seven days after captan was applied to runoff at 1.2 g a.i./l, leaves that were mature (fully expanded), immature (expanding), or not emerged at the time of application (emergent) bore captan deposits of 2.0, 1.5, and 1.2 .mu.g/cm2, respectively. A captan concentration threshold of 1-2 .mu.g/cm2 is proposed for protecting apple leaves from infection by Venturia inaequalis [apple scab].

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