Evaluation of biological control systems for control of mango postharvest diseases

Abstract

Mangifera indica L has become an important export crop. Two of the most important diseases on mangoes of major concern in South Africa are anthracnose (Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. in Penz) and stem-end rot (SR) [Botryosphaeria parva, previously known as Dothiorella dominicana (Petrak and Cif)]. Limited post-harvest control may be achieved with pre-harvest applications of copper-based fungicides. However, reduction in the number of fungicides re-registered due to stricter requirements, pathogen resistance, negative effects on the environment and on human health has left most of the smaller industries, such as the mango producers, in search for alternative control measures. In light of the above, there was increased scientific interest in biological control. The antagonist (Bacillus licheniformis) used in this study was previously isolated from the mango phylloplane and shown to have some potential as a biological control agent. The aim of this study was therefore to use B. licheniformis as a model system to obtain a better understanding of biological control systems in the post-harvest arena. The objectives of this study were; 1) to elucidate the mode of action of B. licheniformis, 2) to evaluate attachment, colonisation and survival of B. licheniformis on the fructoplane and 3) to further assess the antagonist’s performance in terms of consistency of efficacy in a commercial environment, alone or in combination with other products. Results from this study confirmed the in vitro antagonism of B. licheniformis. In addition the antagonist inhibited in vitro growth of C. gloeosporioides and B. parva by means of antibiosis, production of volatiles and competition. A bioactive compound was produced after ten days incubation. Competition was confirmed when the antagonist was able to produce extracellular enzymes, such as chitinase, to hydrolyse complex compounds, while restricting the pathogens ability to hydrolyse these compounds. Iron-uptake from an iron-rich medium was achieved by the antagonist and gave B. licheniformis an in vitro growth advantage. In vivo, the minimum concentration of the antagonist that was effective at inhibiting the pathogens was 107 cells/ml. Antagonist applications integrated with chemicals gave most effective post-harvest control that was equal to or more effective than the commercial chemical used. Scanning electron microscopy studies revealed that the antagonist attaches to the fruit surface. The recovery of the antagonist from treated fruit was high proving increased survival on the fructoplane. Temperature studies in vitro and in vivo revealed that the antagonist was most effective in inhibiting growth of the pathogens under cold storage conditions (10ºC). This was significant as results from commercial trials confirmed those obtained in the laboratory. Calcium carbonate added in vitro and in vivo enhanced the activity of the antagonist. In this study, B. licheniformis was effective against the pathogens with in vitro assays, on the fructoplane and lastly in a commercial set up. As it is feasible to incorporate this commercial formulation into the standard packing line, further work should focus on testing this antagonist efficiency throughout the mango season and on all cultivars. An ideal biological control agent must be compatible with other systems used. It should have combined action, either not be affected by the other organisms on the fructoplane and also be able to function in an integrated system. There should be high efficacy as well as consistency of efficacy. A biological control agent should function over a broad spectrum of organisms. If a biological control agent if efficient over a long period of time, then large scale production should be tested.