Introduction to Fungi, Third Edition

632 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
bud of the alternate host. Between late spring
and autumn, infected pear leaves show bright
orange-coloured lesions up to 1 cm in diameter
(Plate 12e). These give rise to spermogonia at the
upper leaf surface, and later to aecia at the lower
surface. The aecia are greatly swollen and elongated.
Eventually, they rupture at their sides,
leaving a cap joined to the aecium by trellis-like
threads (Plate 12f). The characteristic tube-like
aecia of Gymnosporangium are referred to as
roestelioid, after the generic name Roestelia
previously given to such aecial stages. Aeciospores
infect susceptible Juniperus spp. in
the autumn (Borno & van der Kamp, 1975).
Gymnosporangium fuscum can cause serious
damage to pear trees because heavy infections
reduce the photosynthetic capacity of the trees,
and yields and ultimately the trees themselves
decline after severe successive infections. This
species is very much on the increase in Europe
and elsewhere, especially because of the planting
of ornamental Juniperus spp. (e.g. Chinese juniper)
and pear trees side-by-side in gardens. Whereas
the pathogen needs to infect the alternate
host afresh each year, it can survive almost
indefinitely in its principal host once this has
become infected. Several communities and
countries have therefore launched eradication
schemes to remove infected Juniperus trees.
Since the basidiospores do not normally travel
distances longer than about a mile, this approach
can reduce the infection pressure to acceptable
limits. Chemical control of G. fuscum on pear
trees is possible but obviously not desirable
(Ormrod et al., 1984).
Several related species of Gymnosporangium are
also commonly encountered, e.g. G. clavariiforme,
which alternates between Juniperus communis
(a plant not infected by G. fuscum) and Crataegus
spp., or G. cornutum on J. communis and Sorbus
aucuparia (mountain ash, rowan). Gymnosporangium
juniperi-virginianae is the cause of North
American cedar-apple rust, alternating between
the ‘Eastern red cedar’ (Juniperus virginiana) and
apple (Malus spp.). On the principal host, telia
often develop from gall-like deformations called
‘cedar-apples’. This species causes considerable
economic damage in North America where it is
native. It does not seem to be present in Europe as
yet (Smith et al., 1988). It can be controlled by
fungicide applications similar to those effective
against apple scab caused by Venturia inaequalis
(see p. 478).
22.6.3 Hemileia vastatrix
The phylogenetic position of H. vastatrix, the
cause of coffee leaf rust, has not yet been
resolved. It appears to be an ancient rust lineage
possibly pre-dating the separation between
Pucciniaceae and Melampsoraceae sensu Dietel
(Wingfield et al., 2004). We assume that it belongs
to the Pucciniaceae sensu Dietel because its
teliospores are stalked. Coffee is one of the
most valuable commodities on the world
market, and H. vastatrix causes the most important
disease of it. So far, only urediniospore,
teliospore and basidiospore stages have been
reported, and an alternate host has not been
found. Therefore, it is likely that the disease is
spread exclusively by urediniospores. Hemileia
was so named because its urediniospores are
unusual in being half smooth, with the other
half of their surface spiny like the urediniospores
of other rusts. Urediniospores of H. vastatrix
infect coffee leaves in the typical rust fashion,
i.e. they require free water for germination and
form appressoria over stomata. Resistance is
expressed as a hypersensitive response after the
formation of the first haustorium (Silva et al.,
2002). Unusual features include details of appressorium
formation (Coutinho et al., 1993), the
fact that infection occurs exclusively through
stomata located on the lower (abaxial) leaf
surface, and the formation of urediniospores
and teliospores through stomata (Ward, 1882),
i.e. typical rust pustules rupturing the epidermis
are not formed. Another remarkable feature is
that removal of the coffee berries, which are not
themselves infected by H. vastatrix, drastically
reduces the severity of the disease (Monaco,
1977).
The story of coffee rust is fascinating and its
first part has been told eloquently by Large
(1940). The disease was discovered on cultivated
coffee in Ceylon (now Sri Lanka) in about 1869.
The origin of H. vastatrix seems obscure. Ceylon
was then the major coffee-growing region for the