5.2.21
FIELD SPRAYS OF BACILLUS SUBTILIS AND FUNGICIDES FOR CONTROL OF PRE-HARVEST FRUIT DISEASES OF AVOCADO

L KORSTEN

Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, Republic of South Africa

Background and objectives
Black spot (BS) and sooty blotch (SB) caused by Pseudocercospora purpurea and an Akaropeltopsis sp., respectively, are the two most important pre-harvest fruit diseases of avocado (Persea americana) in South Africa [1]. Losses from BS of up to 69% have been recorded in untreated orchards on the susceptible cultivar Fuerte [2], while SB results in lower market value of fruit due to unsightly discoloration of the skin. Traditionally, control of BS has been based on pre-harvest benomyl, copper oxychloride or cupric hydroxide sprays [3]. Several disadvantages associated with the repeated use of fungicides on avocados have been noted. These include costs of removing copper spray residues on fruit in the packhouse, and build-up of pathogen resistance to benomyl [2] Possible adverse effects of agrochemicals on human health and the environment necessitate alternative disease control measures such as biological and integrated control. Korsten [1] reported effective control of post-harvest decay from anthracnose, stem-end rot (SE) and Dothiorella/Colletotrichum fruit-rot complex (DCC) when using pre-harvest Bacillus subtilis field sprays. This paper reports on biological and integrated control field experiments done over a 3-year period at three geographically distinct farms in order to obtain data for product registration. It also reports on the commercialization of the biocontrol product and its formulation to improve shelf life.

Results and conclusions
In our study, the most consistently effective treatment for BS of avocado was achieved with an integrated programme of B. subtilis and fungicide sprays. For biocontrol agents to be accepted commercially, it is essential to show consistent control over several years at more than one location. A reduced number of chemical sprays, coupled with promising levels of control achieved with a biological product, makes an integrated approach a promising alternative to existing chemical control. Similarly, the biological treatment on its own proved effective, but only from the second year of spraying. This could indicate that the antagonist first needs to establish itself on the tree, as was subsequently found in antagonist survival studies. Acceptable control of plant diseases through biological or integrated treatments is not always evident in the first season, and therefore requires patience on the part of the grower. During the last 2 years of spraying with the antagonist, control was equal to that achieved with fungicides. According to Swinburne [4], commercial acceptance of biocontrol agents depends on their ability to perform as well as, or better than, commercial fungicides. In preliminary experiments at one farm, application dates were based on general spray guidelines for the whole industry. However, epidemiological data for P. purpurea obtained from that specific farm indicated that spore release by the pathogen occurs earlier than at the other major production areas. Applications of B. subtilis, aimed at establishing the antagonist prior to arrival of P. purpurea inoculum, resulted in sustained control thereafter. Timing of application is thus of crucial importance in biological or integrated control programmes. Inconsistent control of SB and SE were recorded with all spray programmes. More promising results with anthracnose control were achieved with the biological and integrated treatments. The use of an antagonist carrier in the product formulation ensures stable shelf life and high cell concentrations (107 c.f.u./ml) concentrations. Comparative costs of a single commercial antagonist spray application were similar to costs of copper fungicides.

References
1. Korsten L, 1993. PhD thesis, University of Pretoria.
2. Darvas JM, Kotzé JM, Wehner FC, 1987. Phytophylactica 19, 489-493.
3. Lonsdale JH, 1991. South Africa Avocado Growers' Association Yearbook 14, 61-62.
4. Swinburne TR, 1978. Annals of Applied Biology 89, 94-95.