WIND-DISPERSED DISEASES - The Epidemiology of Plant Diseases - page 391

et al., 1975). After the tillering stage of the host, the growth of the lesion is restricted
by leaf veins, causing the characteristic stripe lesions.
15.2.4 Spore production
When the latent period is complete, spore production starts and continues during the
infectious period. The spore production of mildews and rusts increases rapidly to a
maximum and then decreases gradually to zero, with the typical asymmetrical bell-
shaped sporulation curves (e.g. Aust, 1981). To withdraw nutrients from the host
over an extended time requires maintenance of integrity of the infected host cells
and tissues, which is reflected in the green islands commonly observed around
lesions.
Temperature influences the duration of the infectious period. For B. graminis, for
instance, the infectious period decreases from 23 days at 10°C to 13 days at 26°C
(Aust, 1981). On average, the infectious period tends to be longer for P. striiformis
f.sp. tritici than for the other three fungi (Table 15.1). An exception is P. triticina , in
which sporulation continued for up to two months under optimum conditions and
low lesion density (Mehta and Zadoks, 1970). This long sporulation period is
interpreted as a survival mechanism.
The sporulation intensity of the pathogens depends also on temperature. For
example, barley mildew sporulation intensity increases with temperature up to the
optimum at about 18-22°C (e.g. Pauvert, 1976), but decreases for higher
temperatures, so that at 27°C only 40% of the maximum value can be reached
(Stephan, 1980). On average, the sporulation intensity per lesion of P. triticina is
one order of magnitude less than that of the three other fungi (Table 15.1).
The number of spores produced is reduced by rain droplets. Heavy precipitation
with more than 1 mm of rain caused B. graminis sporulation to decline by more than
70%. Friedrich (1995a) showed that sporulation also decreases with high vapour
pressure deficit that characterizes dry conditions.
For the rusts, sporulation is a function of relative humidity. For P. striiformis f.sp.
tritici, for instance, the relative humidity must exceed 50% to initiate sporulation. The
number of spores formed per unit of time is exponentially related to the increase in
relative humidity. However, liquid water stops sporulation (Rapilly, 1979). The rate
of lesion development and spore production is curtailed by low light intensity for
P. triticina (Clifford and Harris, 1981). This is probably an indirect effect of light on
host nutritional status, which governs the growth of the biotrophic rust fungi.
Host nitrogen content affects spore production. In leaves with a low nitrogen
content, spore production per lesion was reduced compared to medium and high
nitrogen content, but lesion size of wheat leaf rust was unaffected (Robert et al.,
2004). Nitrogen regime also affects yellow rust through changes of nitrogeneous
substrates in wheat leaves, resulting in significant effects on the upper asymptote of
P. striiformis f.sp. tritici epidemics in winter wheat (Neumann et al. , 2004).
Infection efficiency and sporulation rate can also be modulated by induced
resistance, as shown by Calonnec et al. (1996) for P. striiformis f.sp. tritici .