WIND-DISPERSED DISEASES - The Epidemiology of Plant Diseases - page 404

rarely for P. graminis f.sp . tritici and P. triticina , or not at all for P. striiformis f.sp .
tritici . The asexual reproduction (by urediniospores or conidia) decreases and stops
as sexual reproduction begins (with teliospores and cleistothecia), which is triggered
by rising temperature or by host senescence.
The epidemic strategy of B. graminis , P. graminis f.sp . tritici and P. triticina is
mainly based on a fast infection cycle, a large sporulation capacity and a high
infection efficiency. Puccinia striiformis f.sp . tritici differs by a slower infection
cycle compensated for by a systemic growth of the lesion within the leaf. In general,
the increase in the disease severity for mildew and stripe rust is due not only to new
infections but, more importantly, to lesion growth. For leaf rust and stem rust,
colony growth can be neglected.
Blumeria graminis differs from the rust pathogens by its ability to withstand dry
conditions, while high humidity and even wetness are necessary for Puccinia spp.
infection. The strategy of the rusts seems to be adapted to prevailing climates. The
cereal rusts adapted to an unstable temperate environment exploit the available
resources during short periods; other tropical rusts, adapted to wet and warm
conditions in a relatively stable environment with a nearly permanent availability of
susceptible host tissue, have a longer infectious period, a later sporulation peak and
a lower infection efficiency than some temperate rusts (Sache and de Vallavieille-
Pope, 1995). The consequences for the strategy of breeding for resistant cultivars
against these temperate pathogens are to recommend selection for genotypes with
increased latent periods so delaying epidemics. Partial resistance associated with the
frequent failure of haustorium formation is also a source of potentially durable
resistance (Niks and Rubiales, 2002). Necrotrophs such as Septoria can be controlled
mainly by reducing lesion growth and consequently sporulation capacity.
The early stages of infection by these obligate biotrophs are governed by a strict
morphogenetic programme and climatic constraints. Their life cycles are
synchronized with those of their hosts for both food supply and reproduction. For
example, for the asexual stage of Puccinia spp., penetration coincides with night
conditions, and colonisation of the host tissue coincides with higher temperatures
occurring during daytime. These fungi appear to be less flexible than most
necrotrophs. The tight morphogenetic control of infection structures (appressoria,
haustoria) may also determine the narrow range of hosts that these fungi are able to
parasitize and can be seen as a means of preserving the exclusiveness of habitat.
The speed of epidemics of polycyclic diseases depends on the actual values
of monocyclic parameters, influenced by meteorological conditions and host
susceptibility. A greater understanding of airborne diseases has been achieved by the
quantification of these monocyclic parameters. Because of the great number and
complexity of the influencing variables, no single technique can assess these
epidemiological parameters (de Vallavieille-Pope et al ., 2000). For example, the
development of standardised and simplified methods enables the quantification of
latent periods. Wall sterol content can provide an objective tool to assess pathogen
biomass. The quantification of effects of influencing variables forms the basis
for the prediction of epidemic progress. Such studies will have more and more
useful applications because of the increasing number of weather stations measuring