Coffee Rust (Hemileia vastatrix) by Shawn Steiman

"One of the most feared pathogens to coffee growers is Hemileia vastatrix Berk. and Br. (Uredinales), or the coffee rust fungus.  The first documentation of this agent appeared in1861.  In 1869, the fungus appeared in Ceylon (now Sri Lanka) and within ten years it devastated the entire coffee industry.  In the years since, H. vastatrix has appeared in every coffee producing region except Hawaii.  This fungus is largely responsible for the modernization of coffee plantations in South America. 

Coffee rust is characterized by yellow-orange powdery lesions on the abaxial surface of leaves where it attacks through stomata; it rarely occurs on stems or fruit.  All Coffea genotypes are susceptible to some degree, though cultivars such as Timor and Icatu exhibit a high resistance (Ferreira and Boley, 1991).  Additional Rubiaceae can also be infected (Coutinho et al. 1995).  Impaired photosynthesis, premature defoliation, and reduced floral initiation constitute most of the damage (Brown et al. 1995).  This reduced photosynthetic capacity and the heavy carbohydrate sink created by fruits limit the amount the growth of woody tissue that gives rise to the next season's crop.  Therefore, the following season's crop is reduced.  In fact, losses due to coffee leaf rust can reach 70%, although 15-20% is more typical (Ferreira and Boley, 1991; Brown et al. 1995).

Coutinho et al. (1994), like Waller (1982), discovered that older leaves are relatively resistant to the pathogen.  Specifically, they noted fewer appressoria and a lower uredosorus density on older leaves.  Temperature also influences H. vastatrix to the extent that normal uredospore germination and other infection processes only occur between 15-30  C (Brown et al. 1995).  Brown et al. (1995) also showed that the mean minimum temperature is directly related to the severity of infection.  Furthermore, as the number of days below 15  C increases, the severity of the infection goes down.

The life cycle of H. vastatrix resembles that of other rusts, although it passes through fewer stages than the familiar Puccinia graminis.  The pycnial and aecial stages have not been observed.  Germination occurs within 5 hours of inoculation between 21-29  C in the presence of free water (Mabbett, 1998).  Contrary to expectations based on its tropical habitat, H. vastatrix produces teliospores.  Dry, cold conditions appear to be requisite for teliospore formation and peak urediosorus production (Coutinho et al. 1995).  For a description of the teliospores, see Coutinho et al. (1995).  The alternate host for this heteroecious organism remains unknown.

Coutinho et al. (1993) studied appressorium formation.  They discovered that H. vastatrix exhibits several unique properties.  First, unlike many leaf pathogens, directional growth is absent and adhesion to the epidermis is weak.  Second, the germ tube is relatively long compared to other rusts.  Third, following appressorium formation, a structure dubbed an appressorium wedge develops.  The authors speculate that this organ may serve as an attachment device in the absence of other adhesive forces or that it may anchor the appressoria from within the stomatal cavity.  Hemileia vastatrix exhibits thigmotropism to subsidiary and guard cells like many other rusts.  Leaf contours and the collisions of germ tubes led the authors to conclude that ridges of 4 µm or higher induce appressorium formation.  In addition, appressoria develop most rapidly between 13-16  C and assume shapes that are either torpedo-like/roundish (13-19  C) or irregular (>19  C).  Finally, more appressoria formed beyond stomata in younger leaves (Coutinho et al., 1994).

Controlling H. vastatrix is a daunting task; chemicals such as propiconazole, tridimenol, tridemfon and copper oxychloride are just partially effective.  Amongst them, copper containing fungicides like copper oxychloride are the most effective and widely used.  High solubility, variability in the target, the inability of pests to evolve resistance, high adhesiveness to leaves (allowing for fewer applications) and the ability to serve as a nutritional supplements among other properties account for the exceptional utility of this metal complex  (Mabbett, 1998).  Non-chemical control consists of pruning infected leaves and reliance on resistant cultivars (Hillocks et al., 1999).

Though eradicating this ubiquitous fungus is impossible, avoiding catastrophe is not.  Early intervention usually can prevent the spread of this disease.  Also, shade culture may be useful in avoiding epidemics because monocultures of all crops tend to promote outbreaks.  Finally, a better understanding of the life cycle may lead to further advances in the control of Hemileia vastatrix."