The coffee leaf rust pathogen Hemileia vastatrix: one and a half centuries around the tropics

Abstract

Taxonomy and history: Hemileia vastatrix Berk. and Broome (Basidiomycota, Pucciniales) was described in 1869 in eastern Africa and Ceylon as the agent of coffee leaf rust and has spread to all coffee cultivation areas worldwide. Major disease outbreaks in Asia, Africa and America caused and continue to cause severe yield losses, making this the most important disease of Arabica coffee, a cash crop for many tropical and sub-tropical countries.

Life cycle and disease symptoms: Hemileia vastatrix is a hemicyclic fungus with the urediniosporic life cycle as its most important (if not only) source of inoculum. Chlorotic spots are the first macroscopic symptoms, preceding the differentiation of suprastomatal, bouquet-shaped, orange-coloured uredinia. The disease can cause yield losses of up to 35% and have a polyetic epidemiological impact on subsequent years.

Disease control: Although the use of fungicides is one of the preferred immediate control measures, the use of resistant cultivars is considered to be the most effective and durable disease control strategy. The discovery of 'Híbrido de Timor' provided sources of resistance that have been used in several breeding programmes and that have been proven to be effective and durable, as some have been in use for more than 30 years.

Genetic diversity and molecular pathogenicity: Although exhibiting limited genetic polymorphism, the very large genome of H. vastatrix (c. 797 Mbp) conceals great pathological diversity, with more than 50 physiological races. Gene expression studies have revealed a very precocious activation of signalling pathways and production of putative effectors, suggesting that the plant-fungus dialogue starts as early as at the germ tube stage, and have provided clues for the identification of avr genes.

Keywords: Hemileia vastatrix; Pucciniales; coffea; coffee leaf rust; obligate biotrophy.

Figure 1

Coffee leaf rust symptoms and signs. (A) Chlorotic spots and urediniosporic sori on the lower leaf surface. (B) Severe defoliation in plants at the front as a result of disease, contrasting with resistant plants elsewhere in the field.

Figure 2

Hemileia vastatrix infection process. (A) Urediniospore (u), scanning electron microscopy (SEM). (B) Germinated urediniospore (u) with germ tube (gt) and appressorium (ap) over stomata on the lower surface of the coffee leaf, 17 h after inoculation (hai), SEM. (C) Appressorium (ap) over stomata and penetration hypha (arrow), 24 hai, light microscopy (LM). (D) Appressorium (ap) over stomata and intercellular hypha with an haustorium (h) within a subsidiary cell, 48 hai, LM. (E) Intercellular hyphae (arrows) and haustoria (h) within epidermal and mesophyll cells, 20 days after inoculation (dai), LM. (F) Haustorium (h) within a spongy parenchyma cell, 20 dai, SEM. (G) Intercellular hyphae (arrows) in the spongy parenchyma, 20 dai, SEM. (H) Urediniosporic sorum protruding through the stomata in a bouquet shape, 21 dai, LM. (I) Urediniosporic sorum, 21 dai, SEM.

Figure 3

Hemileia vastatrix urediniospore germination (Rijo and Sargent, 1983). (A) Beginning of germination, with cytoplasm passing through the germ pore with two nuclei (n) and one evident nucleolus (bar, 1 µm). (B) Transverse section of the germ tube showing different organelles (bar, 0.5 µm).