Effect of copper fungicide on Colletotrichum gloeosporioides and other microorganisms on avocado leaves and fruit

An investigation into the effect of copper fungicide on non-target microorg anisms on avocado (cvv. Hass and Nabal) showed that populations of filament ous fungi, yeasts, and bacteria on leaves were at least 10-fold less after a single application of copper than on unsprayed leaves. This detrimental e ffect of copper on microorganisms on the phylloplane was confirmed in rando m samples collected from 5 commercial avocado orchards with regular pestici de spray programs but where isolated unsprayed trees were also available. A n intensive 16-month study was carried out on 2 adjacent orchards, 1 that w as sprayed with copper fungicide at monthly intervals from October to April each year, and the other that had not been sprayed for at least 6 years. P opulations of filamentous fungi, yeasts, and bacteria on leaves and fruit w ere 10-100-fold lower in the sprayed orchard than in the unsprayed orchard. However, populations in the sprayed orchard recovered during the winter no n-spray periods to levels comparable to the unsprayed orchard. In 1993 and 1994, fruit were harvested from both orchards, ripened, and assessed for an thracnose caused by Colletotrichum gloeosporioides. In both years, there wa s significantly less disease in unsprayed fruit (mean disease rating 1.13 a nd 0.32, respectively) than in sprayed fruit (mean disease rating 1.83 and 2.18, respectively). These results show that copper fungicide is detrimenta l to phylloplane microorganisms and suggest that those organisms are provid ing some natural suppression of C. gloeosporioides on avocado.