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Disease Cycle and Epidemiology

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The disease cycle is a simple one. Urediniospores initiate infections that develop into lesions that produce more urediniospores (figure 6). Since coffee is a perennial plant on which the leaves remain green throughout the year, the epidemic is continuous, with some fluctuation from season to season, depending on rainfall.

Figure 6

Survival
Hemileia vastatrix survives primarily as mycelium in the living tissues of the host, and since infected leaves drop prematurely (figures 7 and 13), this effectively removes a huge amount of potential inoculum from the epidemic. But a few green leaves always persist through the dry season, and dry urediniospores can survive about 6 weeks, so there is always some viable inoculum to infect the newly formed leaves at the start of the next rainy season.


Figure 7	Figure 13	Figure 5

Spore dispersal
The urediniospores can be dispersed by both wind and rain (figure 5). By observing patterns of infection on individual leaves and among leaves within the canopy, it is clear that splashing rain is an important means of local dispersal. The patterns of infection on a regional scale, particularly in those areas where the fungus was newly introduced, have shown that the long-range dispersal is primarily by wind. A small, perhaps epidemiologically insignificant amount of urediniospore dispersal is by thrips, flies, wasps, and other insects. Movement across oceans, deserts, and mountain ranges has very likely been caused by human intervention.

Infection
Urediniospores germinate only in the presence of free water (rain or heavy dew); high humidity alone is not enough. The whole process of infection requires about 24 to 48 hours of continuous free moisture, so while heavy dew is enough to stimulate urediniospore germination, infection usually occurs only during the rainy season. The seasonal variation in disease incidence is primarily due to variation in rainfall. Where there are two rainy seasons per year, there are two peaks in severity of coffee rust. Infection occurs over a wide range of temperatures (minimum 15°C/ 59°F, optimum 22°C/ 72° F, and maximum 28°C/ 82°F). Infection only occurs through stomata on the underside of the leaf.

Sporulation
It takes 10-14 days from infection for new uredinia to develop and urediniospores to be formed (figure 4). The rust lesions continue to enlarge over a period of 2 to 3 weeks. A single lesion will produce four to six crops of spores, releasing about 300,000 urediniospores over a period of 3 to 5 months. Secondary cycles of infection occur continuously during favorable weather, and the potential for explosive epidemics is enormous.

Figure 4

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