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Disease Management

Quarantine
For more than a century, strict quarantine measures kept coffee rust from invading the Americas. Some plant pathologists have speculated that urediniospores spread from Africa to Brazil on the wind, but it is far more likely that the rust was carried on coffee seedlings or perhaps that urediniospores clung to the surfaces of other plants imported into coffee-growing areas. Once the barrier of the oceans had been breached, wind dispersal came into play. Following the initial introduction into Brazil in 1970, an 80 km (50 mile)-wide "safety zone" was established by eradicating coffee in the zone, but within 18 months, coffee rust had jumped the gap in the direction of the prevailing winds.

In Central America, the new infections were eradicated by killing the infected coffee plants plus the symptomless plants in a 30-meter (yard) radius by spraying them with an herbicide mixed with diesel fuel. Initially the eradication effort seemed to be effective, but eventually, again probably because of wind dispersal, the coffee rust became too well established. There are now very few coffee-growing regions of the world where coffee rust has not yet invaded.

Cultural management
Coffee rust must be managed as a continuous epidemic on a perennial crop, and therefore, any factor that can reduce sporulation, spore dispersal, or infection, even a small amount, can mitigate the epidemic. Good cultural management is paramount, but there are no simple rules to follow. The varieties grown, the character of the soils, the amount and distribution of rainfall, and numerous other factors all interact to dictate what is required.

Figure 11

One of the key cultural management decisions is whether to produce the coffee in full sun or with some degree of shade (Figure 11). This often is more a sociopolitical decision than an agronomic one. Some say that rust is easier to control on properly spaced plants in full sun, since they dry faster and, therefore, have shorter periods of leaf wetness. (They also are easier to spray with fungicides.) Others argue that shade-grown coffee has less rust because the closed canopy of shade trees prevents dew formation on the coffee leaves and therefore reduces infection. However, dew usually does not remain long enough to support infection. It is more likely that shade reduces the susceptibility of the plant to rust because yields are reduced; production of a heavy crop depletes the tree of nutrients and makes it more susceptible to infection. In general, sun-grown coffee is produced on large, well-capitalized farms that can afford to control the rust with fungicides, the cost of which is offset by the higher yields. The small, "low-tech" producers tend to favor shade-grown coffee, which, despite its lower yields, requires less external input in the form of pesticides and fertilizers. (For a discussion of the environmental impacts of sun- and shade-grown coffee, see http://www.audubon.org/local/cn/97december/sgsoca.html

Since the susceptibility of the plant is markedly affected by its nutritional status, the depletion of nutrients by a high yield in a given season can increase the severity of rust not only in that season but in subsequent years as well, unless appropriate adjustments in nutrients are made. Fertilization with nitrogen (N) and phosphorus (P) tends to reduce the susceptibility to rust, but excessive potassium (K) increases susceptibility. In general, application of micronutrients reduces susceptibility. Since their effects can be felt over several seasons, such adjustments must be made carefully.

Proper pruning and training of the coffee plant help to prevent overcropping and maintain the vigor of the plant, thereby reducing its susceptibility to rust. These also help to improve air circulation to promote rapid drying of the foliage, and it facilitates spraying by opening up the canopy. Wider row spacing to reduce the planting density also improves air circulation and spray coverage.

Coffee is very sensitive to weed competition, so good weed control is important in maintaining plant vigor and thereby reducing susceptibility to rust. Good weed control also helps to facilitate air circulation and rapid drying of the canopy.

Fungicides
On susceptible varieties and in environments favorable for the fungus, fungicides are important tools in the management of coffee rust epidemics. In deciding when and what to spray, any given fungicide application has to be considered a long-term investment, with effects not only in the current season but in future seasons as well. Keeping inoculum levels low toward the end of one rainy season will have a major impact on reducing the level of infection at the start of the next rainy season. Preventing defoliation this season will prevent yield losses next season and maintain plant vigor well into the future.

Copper-containing fungicides are very effective in controlling coffee rust, and copper has a "tonic effect" on coffee plants, that is, it increases yields independent of its effect in rust control. One disadvantage of using copper-containing fungicides is that they must be present on the leaves before infection occurs. Another disadvantage, aside from cost, is that copper accumulates in the soil, particularly in the organic matter, and it can reach levels toxic to plants and to other organisms in the environment. To reduce the amount of copper used, copper-containing fungicides can be alternated with systemic organic fungicides, or one or two copper sprays can be applied early in the season, followed by one or occasionally two sprays of a systemic later in the season to arrest developing rust lesions.

The dithiocarbamate (organic, protective) fungicides are effective for the control of coffee rust and also sometimes have a tonic effect, but their residues do not adhere as well as those of the copper-containing, protective fungicides or the systemic fungicides under the heavy rains of many of the coffee-producing regions.

The timing of the applications and the coverage are important. As a general rule, the intervals between sprays should be less than 21 days to be sure to keep new growth covered. Forecast models exist for timing of fungicide applications according to temperature and rainfall. Since infection occurs on the undersides of the leaves, the sprays should be directed upward to cover the lower leaf surfaces.

Resistant cultivars
At the time that coffee rust first appeared in Brazil, virtually all the coffee in the Americas, and indeed nearly all the coffee in commercial production, could trace its lineage to a single tree planted in the conservatory of King Louis XIV in 1713. The resulting genetic uniformity of commercial coffee production posed (and continues to pose) an enormous potential risk of devastating epidemics.

The existence of resistance to coffee rust in wild Coffea species has been known for some time. So far, nine genes for resistance have been identified, mostly derived from C. canephora and C. liberica (Figure 12). One challenge to the breeders is to combine rust resistance with good agronomic characteristics and good quality coffee. The next challenge is to deploy these resistance genes in such a way that they are not immediately overcome by new races of H. vastatrix. So far, more that 40 different races of H. vastatrix have been identified, with some new ones able to attack previously resistant hybrids. New rust races continue to appear. To reduce the rate of selection of virulent races, the breeders of Cenicafé, a national coffee research center in Colombia, have created a composite cultivar with uniform agronomic characteristics and coffee quality, but with a mixture of genes for rust resistance.

Figure 12

One distressing development in coffee breeding has been the dramatic loss of genetic diversity among the wild Coffea species. There is very little genetic diversity in coffee outside the tropical forests in southwestern Ethiopia, where Coffea evolved. Because of logging, fuelwood harvest, and expanded cultivation driven by a growing human population, these forests have been reduced to less than one-tenth their original size. Ethiopia's Institute of Biodiversity Conservation and Research is struggling to hang on to what is left, and the Ethiopian government has prohibited the export of coffee plants and coffee seed from the country.

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