Agriculture Reference
In-Depth Information
is apparent from Table 5.5 that the different diseases have many common
features in their life cycles and in measures for their control.
A sequence of events occurs in the development of disease. First, there
must be a primary source of infection from which spores of the pathogen arrive
on the crop leaves. If environmental conditions are conducive, spores can
germinate, penetrate and infect leaves. There follows the 'latent period' during
which infection spreads within the leaf. At the end of the latent period the
pathogen has the capacity to sporulate when environmental conditions are
suitable. When the infected leaf produces spores, a second cycle of infection and
disease is initiated. Depending very much on weather conditions, as well as on
the influence of protectant fungicides that may have been applied, and possibly
on antagonism from other leaf surface fungi, the spores from the first cycle may
infect a larger leaf area and progress towards an epidemic. Total leaf
devastation in three or four cycles can occur in weather conditions conducive
to disease spread. Alternatively, the disease may peter out if the infected leaf
area in the second cycle is less than the first and continues to decline with each
cycle.
Most leaf-infecting fungal pathogens require periods of high humidity
along with temperatures within a certain range to sporulate. They often
require darkness for sporulation and hence spores form at night. Spore release
is frequently triggered by light and a decrease in humidity the following
morning. The quantity of spores formed can increase the higher the humidity
and peak around a temperature optimum (see Fig. 5.13). Once released the
spores have a limited life - for example, downy mildew conidia remain viable
for just 3 days (Jesperson and Sutton, 1987). Once they have landed on a leaf,
the spores typically require a minimum duration of leaf wetness - along with
temperatures within a specific range - to germinate and penetrate the leaf and
infect it. The duration of leaf wetness needed may be shorter at optimal
temperatures for infection (see Fig. 5.14).
The latent period tends to be shorter the warmer the temperature,
provided it remains within the range tolerated by the pathogen. The duration
of the latent period is also shortened by increased density of infecting spores,
possibly because this causes rapid depletion of resources available to the
pathogen which, in turn, triggers sporulation. For example, for leek rust the
latent period is shortest at 19-22°C and decreases by about 1.8 days for every
tenfold increase in spore density (Gilles and Kennedy, 2003). Similar trends
were found for onion downy mildew (Hildebrand and Sutton, 1984).
For all these diseases, debris from previous infected crops left on the soil
surface or in waste heaps can act as a primary source of infection for the next
crop. During the growing season spores produced on nearby infected crops can
also introduce infection to a field. Where successive crops overlap in time, for
example with over-wintered and spring-sown onion crops in temperate regions
like maritime western Europe, infection can 'bridge' from a crop to the
following crop, resulting in an uninterrupted series of disease cycles.
