Introduction to Fungi, Third Edition

634 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
resistance genes from wild coffee species can give
rise to cultivars with stable partial resistance.
Altogether 9 major resistance genes and about 30
relatively stable races of H. vastatrix are known
(Monaco, 1977; Kushalappa & Eskes, 1989).
22.7 Melampsoraceae
22.7.1 Cronartium
About 20 species of Cronartium are known. They
are macrocyclic and heteroecious, alternating
between various dicotyledonous plants as principal
hosts and Pinus spp. as alternate hosts. The
one-celled teliospores of Cronartium are produced
in long columns projecting out of the telia. The
teliospores are attached to each other but not to
their host tissue, and they are unstalked. In
general, the mycelium on the alternate host is
perennial whereas the principal host needs to be
re-infected afresh each growing season. Reduced
forms with aecia on Pinus spp. are named
Peridermium (or Endocronartium). Their ability to
re-infect the Pinus host by means of aeciospores
produced from aecia indicates that these are
aecidioid uredinia (Ono, 2002). Cronartium and
Peridermium spp. form a well-resolved monophyletic
group of rust species also clustering
together with their telial hosts (Vogler & Bruns,
1998). Economic damage is generally caused
mainly by the infection of Pinus spp. The
symptoms are similar between the various
species considered below.
Cronartium ribicola causes white pine blister
rust, which alternates between Ribes spp.
(currants, gooseberry etc.) and those Pinus spp.
which produce their needles in groups of five
(e.g. P. strobus). Infection of the alternate host
proceeds by basidiospores germinating on the
needles of Pinus spp. in autumn. During the
following spring, infection spreads to the needle
base and ultimately to the branches or main stem
where lesions become visible as cankers. These
produce spermogonia and aecia (Smith et al.,
1988). Large cankers can girdle the entire trunk
of trees, causing host death and widespread
economic and ecological damage to Pinus forests
especially in North America and Europe. In
summer, aeciospores infect Ribes spp., where
uredinial and telial lesions arise on the leaves.
Severe infections can cause premature leaf
abscission, but this stage of the disease is not
economically as serious as that on Pinus. Itisat
present impossible to control C. ribicola. Since
its basidiospores cannot travel long distances, a
massive attempt at eradicating the principal host
was launched in North America. However, this
campaign failed because Ribes spp. are too
abundant and re-grow easily from roots or
seeds remaining in the soil (Maloy, 1997).
Fungicide treatment of infected trees was
attempted by aerial sprays with cycloheximide
(actidione), but this was ineffective (Dimond,
1966; Maloy, 1997). More recently, attention
has turned towards the breeding of resistant
cultivars. Although C. ribicola is probably of
Asian origin and was introduced into Europe
in the eighteenth century and to North America
around 1900, major gene resistance does exist
among individual plants of Pinus spp. previously
unexposed to the rust (Kinloch & Dupper, 2002).
Resistance is often based on a gene-for-gene
interaction whereby incompatibility commonly
occurs as a hypersensitive response at the stage
of needle infection (Jurgens et al., 2003). Resistance
breeding holds promise because the populations
of C. ribicola in Europe and North America
are genetically quite uniform. The alternative to
resistance breeding is to give up P. strobus as a
forestry tree, but the ecological damage to forests
which regenerate by natural rejuvenation
remains immense (Kinloch, 2003).
Cronartium quercuum is the cause of fusiform
rust especially on Pinus taeda and P. elliottii. These
species were planted in the South-Eastern USA
where C. quercuum originated on native, relatively
resistant Pinus spp. The principal hosts are
Quercus spp. The life cycle is similar to that of
C. ribicola, as is the economic damage, with the
exception that this species is still confined to the
USA. The disease is now kept under control by
the planting of rust-resistant cultivars (Powers &
Kuhlman, 1997; Schmidt, 2003).
Cronartium flaccidum causes resin top disease
on Scots pine (Pinus sylvestris), P. nigra, P. pinaster
and other species in Europe (Smith et al., 1988).
The principal hosts are various herbaceous