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).
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).
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.
Brown, J; Kenny,
M; Whan, J and Merriman P. The effect of temperature on the
development of epidemics of coffee leaf rust in Papua New
Guinea. Crop Protection. 14(8): 671-676.
Brown, J; Whan, J;
Kenny, M and Merriman P. 1995. The effect of coffee leaf rust
on foliation and yield of coffee in Papua New Guinea. Crop
Protection. 14(7): 589-592.
Coutinho, T; Rijkenberg,
F and Van Asch, M. 1993. Appressorium formation by Hemileia
vastatrix. Mycological Research. 97(8): 951-956.
Coutinho, T; Rijkenberg,
F and Van Asch, M. 1994. The effect of leaf age on infection
of Coffea genotypes by Hemileia vastatrix.
Plant Pathology. 43: 97-103.
Coutinho, T; Rijkenberg,
F and Van Asch, M. 1995. Teliospores of Hemileia vastatrix.
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Hillocks, R; Phiri,
N and Overfield D. 1999. Coffee pest and disease management
options for smallholders in Malawi. Crop Protection. 18: 199-206.
Ferreira, S and Boley,
R. "Hemileia vastatrix". <http://www.extento.hawaii.edu/kbase/crop/Type/h_vasta.htm>
Nov 1991. (Accessed 12/2/99)
Mabbett, T. 1998.
Rust continues to corrode coffee yields. International Pest
Waller, J. 1982.
Coffee rust epidemiology and control. Crop Protection. 1: