Abstract
The initial effect of ‘tonic’ fungicide sprays on Coffea arabica in Kenya is a 2–3 month delay in leaf fall and this causes a marked increase in yield even when the trees are not noticeably diseased. Five field experiments were carried out to investigate leaf abscission responses to 2-chloroethane phosphonic acid (CEPA) of fungicide-sprayed and unsprayed genotypes. Fungicide-sprayed leaves showed a significantly lower abscission response. Fungicides appear to remove a factor, possibly the leaf surface microflora, which causes increased levels of endogenous ethylene. This induces the leaves to senesce and abscise prematurely under natural conditions, or makes them more responsive to exogenous ethylene. There were marked genotype differences in thresholds for leaf abscission, and the locally selected cv SL28 was among the most resistant to CEPA-induced abscission. It was concluded that tonic sprays of fungicide will continue to be effective in Kenya in increasing yields, even after the introduction of new d...
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