Studies in the latent phase of Colletotrichum species causing ripe rots of tropical fruits

Abstract

The factors underlying the development of latent infections by species of Colletotrichum causing ripe rots in tropical fruits were investigated with respect to the four aspects of nutrition, ·enzyme activity, toxins and respiration. Most of the work was carried out with Gloeosporium musarwn on the Cavendish banana.
Failure of G. musarum to develop in green fruit is not due to inability to utilize starch nor is a deficiency of the simple sugars a limiting factor. The addition of nutrient solutions containing highly organized nitrogenous compounds leads to accelerated development in the inoculated green banana.
Species of Colletotrichum app.ear to have a poor capacity for secreting macerating enzymes and pectinesterase compared with P.enicillium digitatwm and Rhizopus nigricans when grown on similar media. This capacity is augmented by increasing the nitrogen content of the media. The addition of "Pectinol" or the solutions from liquid cultures of Colletotrichum and other fungi effects a varying acceleration of G. musarum development. By heating these solutions to destroy enzyme activity it was shown that the action could be a nutritive one rather than enzymatic.
No anti-fungal substance of the phytoalexin type could be demonstrated in the banana nor was a toxic reaction exhibited by cold extracts of the green peel. Hot aqueous extracts of the outer peel, using commencing temperatures of 80°-95° C, were highly toxic. Neither cool nor hot extracts of ripe peel showed toxic properties. It was possible to obtain toxic extracts from other fruit subject to latent infection and some circumstantial evidence was obtained suggesting that a toxic substance may be one factor in determining the state of latency. Confirmation must await the results of more detailed investigations.
The respiratory changes taking place in ripening fruit at the. time of the climacteric were compared with those induced by parasitic attack and the hypothesis put forward that the former provides a metabolic environment assisting fungal development in the host. With respect to this hypothesis, the effect of a number of reagents on the development of G. musarum in bananas was investigated. Those substances known to produce a marked effect on metabolic activity, such as DNP and 2,4-D, were usually associated with accelerated fungal development. The reverse holds for white oil, benzimidazole and kinetin.
The influence of wounds in stimulating Colletotrichum development was attributed to metabolic changes simulating ripening which take place in the adjacent tissue as a result of wound stimulus.
The paper concludes with a suggested interpretation of the development of G. musarum in relation to the environment provided by the green and the ripe banana.