PREDICTING AVOCADO FRUIT ROTS BY QUANTIFYING INOCU- LUM POTENTIAL IN THE ORCHARD BEFORE HARVEST

The fruit rot pathogens Colletotrichum gloeosporioides and C. acutatum infect avocados following germination of rain dispersed spores to produce quiescent appressoria. Stimulation of appresso-rial formation is by contact with a hard surface such as leaves and fruit. In a preliminary study a number of different methods to quantify inoculum potential pre-harvest without sacrificing valuable fruit were compared with historic fruit rot data. Fruit from several orchards with a consistent his-tory of low and high disease were used. Previous survey work of 23 orchards over three seasons had shown that unless growers changed practice the relative amount of rots in their fruit generally remained constant. Four methods were compared; a. spores were washed from leaf discs, b. appressoria were counted, c. leaf tissue was surface sterilised and placed on fungal growth media, d. isolations were made from dead branches. Following quantification it was shown that appres-soria and washed spore numbers were unrelated to historic fruit rots, but amount of leaf tissue infected with Colletotrichum spp. and Botryosphaeria sp. showed potential as a predictor of rots. Key Words: Colletotrichum acutatum, C. gloeosporioides, Glomerella cingulata, Botryosphaeria spp