Agriculture Reference
In-Depth Information
the fungus after infection. The systemic fungicide metalaxyl is widely used as a
systemic fungicide against downy mildew. It is always combined in a mixture
with a protectant fungicide, usually either the dithiocarbamate, mancozeb or
chlorothalonil, a chlorophenyl fungicide. This is to reduce the risk of metalaxyl-
resistant strains of the pathogen evolving. Another strategy to reduce the
selection pressure for fungicide resistance is to alternate the active ingredients
used in successive fungicide treatments. Trials in the UK have found that the
fungicides that are most effective can vary from site to site, depending on the
prevalent diseases. In recent years control of onion downy mildew in the UK by
metalaxyl has not been satisfactory, and a combination of the systemic fungi-
cide, dimethomorph with the protectant, mancozeb has been approved as an
alternative.
Onions, with their waxy, lanceolate leaves, present a difficult surface for
pesticide spray retention. There has been much work carried out to improve
the efficiency of pesticide deposition and to minimize run-off by using improved
spray nozzles aimed and angled appropriately for the crop and by reducing
spray volumes when the crop is small. This, together with more effective
fungicides, has made it possible to reduce the overall fungicide application
quantities in recent years, in keeping with governmental and public pressure to
reduce pesticide usage. Growers still generally routinely spray every 7-10 days
with fungicides while onion crops are growing, the interval between sprays
depending on disease risk. Efforts to reduce the number of sprays by developing
disease forecasting systems have been pursued in several countries.
The emphasis on fungicides effective for the control
B. squamosa
and other
leaf blights that became prevalent in the 1970s is thought to have contributed
to a revival in importance of downy mildew in The Netherlands, the UK and
New York State in the 1990s, as fungicides for
B. squamosa
control were not
effective against downy mildew, which had been problematic in the 1950s but
well controlled with metalaxyl in the 1960s (de Visser, 2005, personal
communication). Increase in the use of onion sets that can carry downy
mildew inoculum over winter and that develop a large leaf canopy in which a
microclimate favourable to downy mildew can occur early in the growing
season is another factor that is thought to have contributed to the increasing
importance of downy mildew in the UK in the 1990s. In addition, a trend
toward larger fields of onions, frequently 150-200 ha in area, may have
contributed to the problem. These examples show how changes in cultural
practices, fungicide use and importance of various diseases interact in an ever-
changing and fluid situation for allium crop disease in a growing region. A
similar interactivity and fluidity applies to most aspects of agronomy, hence the
need for continual monitoring, diagnosis and development work by skilled
scientists and agronomists to maintain crop productivity.
With precise knowledge of the environmental conditions needed for
sporulation and infection, it is possible to develop predictive models for leaf disease.
Information on field microclimate can now be routinely recorded using electronic
