Agriculture Reference
In-Depth Information
the formation of the flag-2 leaf in spring barley (Stephan, 1984). It is generally
assumed (Aust and von Hoyningen-Huene, 1986) that powdery mildews are
favoured by relatively dry atmospheric conditions and moderate temperatures if
susceptible host tissue is available. A combination of wet and dry conditions, such
as rain interrupted by sunny intervals, may favour powdery mildew epidemics (Aust
and von Hoyningen-Huene, 1986). No general conclusions can be drawn concerning
the environmental factors promoting powdery mildew epidemics, either directly or
through the host plant, but it is clear that the apparent infection rates of the disease
progress curves are strongly correlated with the initial disease levels on wheat
(Rouse
et al
., 1981) and barley (Scholze, 1985).
Although temperature affects every stage of the disease cycle, viable inoculum,
susceptible hosts and duration of free moisture are all factors influencing rust
development. Stem rust develops at warmer temperatures (30°C optimum) than do
the other wheat rust diseases; thus it is most frequently a disease caused by the
reproductive portion of the life cycle (Roelfs, 1985a). Stripe rust is favoured by cool
temperatures and, unlike the other cereal rusts, can even function at relatively low
temperatures, in particular under the snow. Higher winter temperatures and lower
spring temperatures contribute to epidemics. Development of stripe rust epidemics
after booting is positively correlated with the mean temperature for the range of
12.9-16.2°C. From 16.2-20.3°C the epidemic rate is negatively correlated with
temperature (Ellison and Murray, 1992). Constant or mean temperatures above 22 or
25°C inhibit and may even eliminate the stripe rust fungus. Years with severe
epidemics had warmer than normal temperatures in January allowing the survival of
fall-infected wheat foliage (Coakley
et al
., 1988). The precipitation frequency for
the month of June was significantly correlated with stripe rust development.
In general, severe stripe rust epidemics are most likely when late summer and
fall infections occur, when abundant mycelium survives the winter and when cool
nights, warm days and heavy dews occur during spring and summer growing
seasons (Coakley
et al
., 1988).
Puccinia striiformis
f.sp.
tritici
urediniospores
produced between 5 and 10°C germinated best, whereas spores produced at 30°C
and above were unable to germinate (Rapilly, 1979). This observation may help to
explain the correlations observed between high temperatures and the decrease in
epidemics. The viability of urediniospores of the stripe rust fungus decreases more
rapidly in sunlight than the viability of urediniospores of other cereal rust fungi
(Maddison and Manners, 1972).
Stripe rust foci can appear during autumn or winter, when the relative humidity
is often very high, which allows only a short distance spread of urediniospores.
A generalized epidemic, which usually occurs only in spring, can take place if
numerous foci that are difficult to distinguish are present. It seems that endemic
multiple foci corresponding to a few infected plants, or even a few sori, initiate
epidemics rather than long-distance transport.
For leaf rust, a temperature variable (either the minimum temperature or the
hourly temperature equivalence function) was the most important factor explaining
variation in overwintering (Eversmeyer and Kramer, 1996). The inclusion of a
precipitation variable for summer and autumn indicates the importance of moisture
in establishment on volunteer wheat. Re-infection periods during winter and early
