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Pathogen Biology

The "coffee leaf disease" was first reported by an English explorer on wild Coffea species in the Lake Victoria region of East Africa in 1861. In 1869, the Reverend H. J. Berkeley and his assistant, Mr. Broome, reporting in the Gardeners' Chronicle, described the fungus they found associated with the disease on some dried coffee leaves sent from Ceylon (now Sri Lanka). They gave the name Hemileia vastatrix to the devastating fungus with half-smooth spores (Figure 8). Urediniospores of other rust fungi are typically round to oval, not kidney-shaped, and have fine spines over their entire surface. It belongs to the class Basidiomycetes, the order Uredinales, and the family Pucciniaceae.

Figure 8

Hemileia vastatrix exists primarily as dikaryotic (having pairs of haploid nuclei that divide in tandem), nutrient-absorbing mycelium ramifying intercellularly within the leaves of its coffee host. Clusters of short pedicels bearing dikaryotic urediniospores protrude through the stomata on the undersides of the leaves (Figure 9). Occasionally under cool, dry conditions toward the end of the season, teliospores are produced among the urediniospores on older, attached leaves. Following karyogamy and meiosis, the teliospores germinate to produce basidia, each of which forms four haploid basidiospores (Figure 10).


Figure 9	Figure 10

The basidiospores will germinate in vitro, but it is not known what plant, if any, they can infect. It is clear that they do not infect coffee. It is not known whether the basidospores are functional or are simply remnants of an ancestral, long-cycled (up to five different spore stages) rust fungus. No alternate host is necessary; H. vastatrix can survive and reproduce quite nicely by urediniospores alone.

Often a hyperparasitic fungus, Verticillium hemileiae, will colonize the coffee rust lesions. Hyperparasites are parasites that parasitize other parasites and are sometimes used as biological control agents. With coffee rust, this hyperparasitism reduces the viability of the urediniospores, but it has very little impact on overall rust development.

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