ADVANCES IN AERIAL PHOTOGRAPHY Detecting Avocado Phytophthora Root Rot The Use of Multispectral Photographical Techniques

In recent months, intensive investigations into the detection of root rot have been carried out in certain selected avocado orchards in the Eastern Transvaal Lowveld. Multi-spectral techniques, which are superior to conventional infra-red photography, appear as an exciting new development, enabling one to distinguish at a glance between diseased and healthy trees. Not only are differences more clear-cut but it becomes possible to grade the trees into different classes of infection simply by examining the photographs and noting differences in colour intensity of individual trees. Even diseased sectors on individual trees can be detected by this method. THE PROBLEM Avocado root rot caused by the fungus Phytophthora cinnamomi is one of the major obstacles in the production of avocados (Zentmyer, Paulus, Gustafson, Wallace and Burns, 1965). In California it is estimated that over 2,200 hectares of avocadoes have been destroyed by this pathogen. This amounts to approximately 23 % of the total planting (Borst, 1970). In South Africa the infection in individual orchards may vary from 5 % to as high as 80%. The overall root rot infection in South African avocado orchards is thought to be approximately 20%. A survey currently being conducted will enable a more reliable estimate to be made of this problem