WIND-DISPERSED DISEASES - The Epidemiology of Plant Diseases - page 393

asexual life cycle. For mildew, for instance, the infection efficiency is drastically
reduced (e.g. Wright and Heale, 1984), the incubation (e.g. Hau, 1985) and latent
periods (Jones, 1978) are prolonged, the lesion size is smaller (e.g. Eckhardt et al .,
1984a) and spore production lower (e.g. Russell et al ., 1976) on the upper leaves.
These components result in reduced disease severities on the upper leaves (Shaner,
1973; Russell et al ., 1976).
The expression of adult plant resistance can be influenced by the environment.
A barley plant well supplied with water will produce upper leaves with higher
susceptibility (Aust and von Hoyningen-Huene, 1986). Adult plant resistance to
stripe rust in several winter wheats increases at high post-inoculation temperatures
and decreases when subsequently transferred to a lower temperature (Qayoum and
Line, 1985; Schultz and Line, 1992). However, high-temperature adult plant
resistance expressed under US Pacific northwest conditions may not be adequately
expressed under cool temperatures, as under UK conditions (Johnson, 1992). Minor
resistance genes to wheat stripe rust are a function of the thermal conditions
affecting the plant during the post-inoculation or pre-inoculation phase, which can
modify the infection type (Brown and Sharp, 1969).
15.2.6 Spore dispersal
The aerial dispersal of plant pathogens by wind is mainly a passive movement
involving three events: liberation, transport and deposition.
(a) Spore liberation
The conidia of B. graminis are attached to each other in a chain, whereas the rust
urediniospores, produced singly on pedicels, are detached and accumulate in sori
erupting through the plant cuticle before dispersal.
To liberate spores, the wind speed must be higher than a certain threshold, which
decreases with the duration of calm conditions. For B. graminis , the wind speed
must exceed 1 m/s after 2 h, and 0.2 m/s after 12 h in calm air. For P. striiformis
f.sp. tritici , 0.6 m/s is necessary after a quiet air period (Rapilly, 1991). The
cumulative number of liberated spores is related to the wind speed by an S-shaped
curve for both pathogens. However, rapid increase of the wind speed is more
important than continuous high wind velocities (e.g. Aylor, 1990).
The maximum spore removal was observed at 2.3 and 2.8 m/s for P. triticina and
P. striiformis f.sp. tritici , respectively, in experimental conditions (Geagea et al .,
1997). Forces necessary to remove the rust spores are two to three orders of
magnitude larger than those required to remove B. graminis conidia (Bainbridge and
Legg, 1976; Geagea et al. , 1997). The difference may be related to the more external
position of the conidia of B. graminis and the size of the spores.
High relative humidity can cause free water on the leaf surface, which prevents
spore liberation (Pauvert, 1984). In the experiments of Adams et al. (1986), conidia
of B . graminis were liberated in darkness when an abrupt change, especially