Agriculture Reference
In-Depth Information
Brändle et al. (1997), respectively. Considering the significant contribution of the
ascospores to the infection of winter wheat, Felsenstein (1996) concluded that the
distribution of the pathogen by wind is not so important in this period and the effects
on neighbouring populations are restricted.
On artificially inoculated barley field plots, the overall ranking of B. graminis
f.sp . hordei pathotype frequencies was the same before and after summer (Bousset
and de Vallavieille-Pope, 2003b). However, the variability in pathotype frequencies
on volunteers was neither correlated with the frequencies in the populations of
conidia on the crop before summer nor with the expected frequencies in the
ascospore population. Chance events such as bottlenecks might have a large
influence on the pathotype frequencies during summer survival. A pathotype
dominant at the end of the summer might possibly have oversummered through
asexual reproduction, or alternatively it may have originated from sexual
reproduction when the frequency of this pathotype was high and both mating types
were represented in the population of conidia before summer (Bousset and de
Vallavieille-Pope, 2003a).
15.3.2 Overwintering
The mildew population grows during autumn on the winter crop until the lower
cardinal temperature is reached, restricting further development. Mildew can survive
the cold period in winter in the vegetative stage on overwintering green plants. The
size of the population can be drastically reduced in strong winters, causing severe
wilting of leaves.
Puccinia triticina can survive as sporulating or dormant mycelium and/or as
viable urediniospores in uredinia on dead leaves (Eversmeyer et al ., 1988) under
quite severe winter conditions, the limiting factor being the survival of host tissue
for re-infection that can occur at relatively low temperatures. Studies in controlled
environment chambers indicated that a few hours of freezing are extremely
detrimental to the survival of single, unprotected urediniospores of P. triticina and
P. graminis f.sp. tritici (Eversmeyer and Kramer, 1995). However, exposure to a
wide range of temperatures above 0°C does not significantly reduce viability.
Puccinia triticina can survive during the sexual stage on the alternate hosts
Thalictrum spp. (the most common); Isopyrum spp. in Siberia, Anchusa spp. in
Portugal and Clematis spp. in Italy. In most parts of the world, however, P. triticina
rarely infects the alternate host and is highly successful in its survival by the
uredinial stage alone.
Puccinia graminis f.sp. tritici can also survive in the sexual stage on the alternate
hosts, for instance, the barberry bush ( Berberis vulgaris ) (the most common) and
Mahonia spp . After the sexual stage, the spread of stem rust from barberry bushes is
local, though there is no reason to suspect that aeciospores are not transported as far
as urediniospores (Roelfs, 1985a). Since the widespread eradication of Berberis ,
which, however, can never be complete (Peterson et al. , 2005), the occurrence of the
aecidial stage is rare and the pathogen survives without the sexual stage in most
areas of the world.
Search WWH ::




Custom Search