Agriculture Reference
In-Depth Information
development, the severity of the disease becomes more determined by environmental
conditions than by the initial level of inoculum.
Accumulation of debris from successive crops, owing to reduced cultivations, is
probably responsible for increases in tan spot (caused by
Pyrenophora tritici-
repentis
)
in the United States and Australia (Adee and Pfender, 1989; Summerell
and Burgess, 1989). The disease has also become more common and severe in
Europe in recent years, probably as a result of the increasing popularity of minimum
cultivation techniques (Bill Clark, ADAS Boxworth, UK, personal communication).
Similarly, the related net blotch disease of barley (caused by
Pyrenophora teres
)
became a serious disease in Britain in the late 1970s and early 1980s when such
reduced cultivation techniques coincided with widespread popularity of a very
susceptible cultivar.
Infected cereal debris can be effectively reduced by stubble burning, which in
many cases reduces rather than eradicates inoculum. The technique is effective
against rice stem rot (caused by
Sclerotium oryzae
), for example (Webster
et al.,
1981), but this practice is illegal in many western countries and may not, therefore,
be used to reduce inoculum of diseases of field crops. In marked contrast to these
examples, Doupnik and Boosalis (1980) showed that planting sorghum directly into
wheat stubble decreased the incidence of stalk rot (caused by
Fusarium
moniliforme
)
compared with normal cultivations. Moisture stress is an important
component in the epidemiology of fusarium diseases in cereals and it is likely that
increased water conservation owing to the mulch of stubble decreases subsequent
susceptibility of sorghum to fusarium stalk rot.
11.2.4 Living plants as inoculum sources
Obligate pathogens may produce resting structures or survive between crops on
living plants (either growing plants or storage organs) that often provide abundant
inoculum (survival on seeds has already been considered in section 11.2.1). In cereal
production, for example, late developing tillers and volunteer plants provide a 'green
bridge' which enables such pathogens to survive the gap between the harvesting of
one crop and the emergence of the next. Infected weeds can also act as strong
sources of inoculum for nearby crop plants that share their susceptibility to
particular diseases.
Blumeria graminis
(cause of powdery mildew of cereals) provides a good example
of such a pathogen. Cleistothecia can survive on dead leaf debris but, for emerging
autumn-sown crops, these are likely to be less important as a source of inoculum than
are the windborne conidia produced in such generous abundance on self-sown plants.
Disease gradients can be found in crops adjacent to such sources of infection.
Similarly, early season mildew gradients can be found in spring barley crops adjacent
to an overwintering source of inoculum in winter barley (Yarham and Pye, 1969).
The short generation time and prolific sporulation of the fungus ensures such
rapid build-up of mildew that severity across the field soon comes to reflect envi-
ronmental factors such as topography (and hence shelter/exposure) and soil nutrient
status, that influence the susceptibility of the plants (Yarham and Gladders, 1994).
