5.2.15
FRUIT DECAY PATHOGENS: FRIENDS OR FOES?

GI JOHNSON

Australian Centre for International Agricultural Research (ACIAR), GPO Box 1571, Canberra, ACT 2601, Australia

Background and objectives
As producers and consumers of tree fruits, we see fruit diseases as undesirable, reducing the quantity and quality of the trees' production. Is the damage caused by fruit pathogens as significant when it comes to tree species survival? Could fruit decay provide some ecological advantage aiding tree survival or seed dissemination? In nature, pathogens, pests and disperser-vertebrates compete for tree fruits. The fruits evolved as nutritive, protective, dispersal devices for tree seeds, and part of this evolution involved a regulation of pathogen/pest invasion/consumption, both of the fruit and the whole tree. Pathogen invasion is often limited by host defences until fruit and seed mature. Quiescent infections initiated in the fruit peel and pedicel stake a claim on the fruit for fungi. In wet years when abundant seed production is less important to the tree, decay may develop early, removing excessive/unnecessary fruit. When fruit are retained until the seed mature, fruit rots aid in the removal of flesh, liberating seed and the discouraging seed predators. Seed transmission of the fungi may also be assured in the process. A balance may evolve which allows plant reproduction but also allows sustenance of microorganisms, insects and vertebrate-dispersers. Modern horticulture by necessity upsets this balance, and the application of pesticides has been a crucial part of crop husbandry. How can reliance on pesticides be reduced? Understanding the basis of the natural balance that regulates disease could be the key to reducing reliance on pesticides.

My research [1] has been concerned with understanding infection processes, modes of infection, the multiplicity and variability of pathogens, and the natural regulation of invasion.

Results and conclusions
We have shown [1] that the stem-end rot pathogens (Botryosphaeria spp., Diaporthe spp.) on mango and other fruits occur as symptomless inhabitants (endophytes) of fruit tree branches, and invade peduncle and pedicel tissue endophytically. They remain quiescent until abscission-zone barriers are breached during ripening, and cause stem-end rots of many fruits. The stem-end rot pathogens are transmitted via seed and soil to the next generation of seedlings. Copper fungicides have favoured their dominance over a more diverse endophytic microflora. Do stem-end rot fungi confer benefits to the hosts that tolerate their presence? Can they be exploited for other purposes? Can an understanding of the factors that regulates the balance between symptomless inhabitant and decay pathogen enable us to reduce fruit disease?

The fungi are not alone in their evolutionary struggle for fruit as a growth substrate. Fruit flies (Bactrocera spp.) oviposit on near-ripe but undecayed fruit, simultaneously inoculating the fruit with bacteria (Klebsiella oxytoca and Enterobacter cloacae) that digest fruit flesh and provide an essential food base for the developing larvae [2]. Our research [3] suggests that the fruit fly-associated bacteria also inhibit fruit-decay pathogens. The bacteria may repress pathogen invasion, ensuring that adequate fruit flesh remains available for the larvae to complete their development. Our findings are a demonstration of 'natural' biological control in action. The opportunities to exploit this knowledge to reduce fruit decay will be discussed.

References
1. Johnson GI, 1996. Acta Horticulturae 455, 575-586.
2. Drew RAI, Lloyd AC, 1987. Annals of the Entomological Society of America 80, 629-636.
3. Johnson GI, Atkin C, Teakle DS, Hayward AC, 1994. Biological Control of Fruit Diseases Workshop, Abstracts, 47.