Glomerella cingulata (anthracnose)

The applicability of control strategies depends as much on the characteristics of the crop on which they are being used as on the diseases at which they are aimed. For perennial cash crops, the long-term investment involved and the value of the produce increases the need for adequate disease control (Waller, 1988, 1992).

The ubiquity of inoculum sources of C. gloeosporioides and its often rapid epidemic development under suitable conditions reduce the effectiveness of many general phytosanitary practices. Although general orchard hygiene has a place in integrated disease control, examples of good field control of Colletotrichum diseases effected solely by measures aimed at reducing inoculum sources are hard to find. Greater knowledge of the specificity of strains of the pathogen may enable effective phytosanitary practices to be developed. Options for integrated control of the disease on mango have been discussed by Arauz (2000).The necessity for wet conditions to coincide with susceptible crop stages for development of Colletotrichum epidemics should offer an opportunity for disease control through manipulation of cropping patterns. Chemical methods can also be used to change growth patterns: application of potassium nitrate sprays can stimulate mango flowering; defoliants have been used to modify the wintering pattern of rubber and avoid secondary leaf fall (Rao, 1972). Application of calcium salts can enhance resistance to bitter rot in apples (Biggs, 1999). Cultural practices such as spacing and pruning can reduce the suitability of environmental conditions for Colletotrichum disease development by assisting more rapid drying of the tree canopy, as well as allowing better penetration of fungicide sprays. The worst effects of Colletotrichum diseases may also be avoided if susceptible crops are grown in drier environments.Because C. gloeosporioides is an opportunistic pathogen, the avoidance of predisposing conditions such as mechanical or physiological damage is particularly significant. Control of other pests and diseases, and avoidance of harvesting damage to fruit, all help to offset secondary infections.

Biological control methods for Colletotrichum diseases are now receiving increasing attention, although the potential of biological control through the effect of phyllosphere antagonists has been realised for some time (Lenné and Parbery, 1976). Jeger and Jeffries (1988) have recently discussed the possibilities of biological control of post-harvest fruit diseases. However, there is still much to achieve before practical methods for field use can be established; given the epidemiological versatility of Colletotrichum diseases on perennial crops, biological control will need to be integrated with other control techniques for maximum effect.Initial experiments with an antagonistic strain of Pseudomonas fluorescens against anthracnose on mango have been encouraging, and control was achieved in unfavourable conditions for the biocontrol agents (Jeffries and Koomen, 1992). While the post-harvest situation offers the greatest potential for success, there are still advantages in considering a pre-harvest approach to the biological control of anthracnose disease. For example, pre-harvest agents can interact directly with the spore or germination hypha rather than with an established appressorium. Modifications to the surface microflora through cultural manipulation of plants in the field may also help to control these pathogens on plant shoots. In some countries, notably the USA, there are now considerable problems in finding suitable effective chemicals that are still permitted for agricultural use, and alternatives are urgently required (Jeffries and Koomen, 1992).

Generally, Colletotrichum diseases can be controlled by a wide range of chemicals such as copper compounds, dithiocarbamates, and benzamidazole and triazole compounds; other fungicides such as chlorothalonil, imazalil and prochloraz are also effective against Colletotrichum. Systemic compounds are particularly effective because of their ability to penetrate host tissues and eradicate latent infections. However, the problem of fungicide tolerance quickly arose when the benzamidazoles were widely used to control Colletotrichum diseases where there is epidemiological continuity or appreciable carry-over of inoculum between seasons. Such problems are less evident when these fungicides are used primarily for post-harvest control, as there is less opportunity for carry-over of inoculum to subsequent crops. In post-harvest fungicide usage, it may be necessary to use hot fungicidal dips to ensure adequate penetration of the fruit cuticle for eradication of quiescent infections (Eckert and Ogawa, 1985; Prakash et al., 2000); there are also further limitations imposed by consumer protection legislation. Current recommendations for post-harvest control of anthracnose of mango in Australia includes a 5-minute heated benomyl dip or 30-second unheated overhead spray of prochloraz on the packing lines. Mangoes harvested in wet weather require careful handling to ensure adequate disease control and to avoid brush damage.Chemical control is more widely used on high-value perennial crops, partly because disease-resistant cultivars have often not been selected. For successful chemical control of many Colletotrichum diseases, timing and placement are of critical significance. Fungicides must be applied to protect the young, expanding crop tissues, whether leaves, blossom or fruit, against infection during wet periods (Griffiths et al., 1971; Fitzell et al., 1984). Timing against blossom blights is particularly critical (Denham and Waller, 1981; Prior and Ryder, 1987). Both rapid expansion of the surfaces of susceptible crop stages and the natural erosion of fungicide by rainfall make adequate fungicide protection difficult to achieve, and repeated applications are often necessary to maintain protection in diseases such as mango anthracnose, but extensive spraying imposes limitations on economic feasibility (Waller, 1992). Stickers such as polyisobutene improve control of mango anthracnose (Prior and Ryder, 1987). Systemic fungicides such as benzimidazole compounds are very effective against the disease and are widely used, usually alternating with protectants to reduce the risk of resistance, but resistance has been detected in Florida (Spalding, 1982) and also in Malaysia (Jeffries et al., 1990).Copper fungicides have been shown to reduce phylloplane micro-organisms on avocado that naturally suppress the disease (Stirling et al., 1999).On tree crops, application of fungicide to the right target may require the use of high-pressure hydraulic hand lances which can reach the susceptible young tissues at the top of tree canopies. Fogging has been used against secondary leaf fall of rubber, and air-assisted sprayers may be effective on smaller tree crops. The water-borne nature of Colletotrichum spore dispersal can facilitate the movement of fungicides to the correct target. Most fungicide deposits are redistributed to some extent by rainwater, and by ensuring an adequate reservoir of fungicide at the tops of tree canopies, distribution of inoculum during rainfall will also be accompanied by distribution of fungicide (Waller, 1992). A reduction in the number, and therefore the cost of spray applications can only be achieved by applying the spray to the target more effectively, or by improving the residual activity of the fungicide after deposition. Research on mangoes and other crops suggests three ways in which immediate improvements might be achieved: (a) massive single-dose application; (b) reduced spray volume with smaller nozzles; (c) stickers to extend the activity of spray deposits (Bailey and Jeger, 1992).

Many crop varieties exhibit varying degrees of resistance to Colletotrichum diseases, and in many cases this has often been shown to be of a polygenic and durable nature. Resistance is more frequently used in annual crops because of more rapid breeding methods, and the easier change of cultivars. Cultivars of tobacco and cotton with resistance to Colletotrichum diseases have been bred or selected and are being used for control in the field. Yam varieties also differ in resistance. Cuticular waxes have been implicated in resistance of pepper fruits to infection (Ou et al., 1999).Differences in susceptibility to Colletotrichum diseases have been demonstrated between varieties of most perennial crops, but resistance is often partial and is greatly influenced by other host and environmental factors. Its effectiveness in commercial cultivars is often greatly limited by this and by the requirement to combine a range of polygenically determined factors affecting quality, yield, agronomic characters and disease resistance in a breeding programme. An understanding of the mechanisms involved in resistance at the tissue level may well assist in the development of new techniques for using resistance against Colletotrichum, and new knowledge at this level is being rapidly gained (Waller, 1992). However, there are several examples of the use of resistance to Colletotrichum diseases in perennial crops. In rubber, clones resistant to secondary leaf fall have been widely planted, but there is some evidence for the emergence of new pathotypes of Colletotrichum which are eroding the effectiveness of resistance (Chee, 1990).