The role of epiphytic microorganisms in the suppression of Colletotrichum Gloeosporioides on avocado

Abstract

Anthracnose, caused by Colletotrichum gloeosporioides, is the most important disease of avocado fruit in Australia. This study aimed to determine whether microorganisms on the avocado leaf and fruit surface played a role in the biological suppression of the disease. The term biological suppression was used in its broadest sense to cover the disease suppressive effect of naturally occurring microorganisms, stimulation of biological control by the creation of an environment that is conducive to antagonism, and the more traditional approach of inundative application of microorganisms that are antagonistic to the pathogen. Observations of the avocado phylloplane showed that a wide range of filamentous fungi, bacteria and yeasts were present on the leaf and fruit surface. Amongst the fungi, species of Cladosporium, Altemaria, Epicoccum, Pestalotiopsis, Colletotrichum and the yeast-like Aureobasidium were predominant. Fluorescent pseudomonads and yellow-orange pigmented bacteria occurred at population densities of 103 - 105 cm'2. Bacillus species were isolated consistently and a range of phenotypic tests including nitrate reduction and starch hydrolysis on 33 isolates showed that there was a ’pumilus group’ and a ’subtilis group’. This conclusion was supported by partial sequencing of 16S rDNA of two typical representatives of each group. Pink and white yeasts (possibly Cryptococcus spp. and Sporobolomyces spp.) were also present but attempts to identify them were inconclusive. There were marked seasonal fluctuations in the population density of these microorganisms on the leaf surface and microbial populations were frequently more numerous inside the canopy than on the outside. Compass direction had a significant effect on microbial populations, the southern aspect being more favourable than the northern aspect, particularly in summer. A study on the effects of pesticides that are commonly used in avocado orchards showed that a single spray of copper fungicide significantly reduced populations of filamentous fungi, yeasts and bacteria on avocado leaves, whereas the insecticide endosulfan had no effect. In an orchard (cv. Hass) where copper fungicide was sprayed regularly from November to May, all categories of microorganisms were 10 to 100-fold less numerous than in an adjacent unsprayed orchard. Samples taken in winter, when pesticides were not applied, showed a recovery of microbial populations in the previously sprayed orchard, so that the numbers of microorganisms were similar in both orchards. Dilution plating of leaf washings onto copper amended agar showed that some yeasts in the sprayed orchard were more tolerant to copper than those in the unsprayed orchard. When fruit were harvested from the two orchards, ripened and assessed for the presence of anthracnose and stem-end rot, there was significantly less diseased fruit in the unsprayed orchard in two successive years.