Agriculture Reference
In-Depth Information
and producing denser leaf canopies (hence, higher within-crop humidities), which
favour the development of diseases. At the same time, a growth regulator may,
by shortening and strengthening the straw, reduce the risks of lodging caused by
eyespot.
11.3.3 Nutrient status
Just as the rate of epidemic development of a disease may be determined by the
inherent susceptibility of the plant to infection, so it can be influenced by variation
in susceptibility consequent upon the nutrient status of the host. The rate of build-up
of barley powdery mildew, for example, is likely to be more rapid in a crop
suffering from manganese deficiency, and net blotch is more likely to be severe on
plants deficient in potash. Nitrogen deficiency, on the other hand, tends to retard the
epidemic development of most foliar diseases. Avoidance of synthetic sources of
nitrogen and the subsequently lower levels of N available to organic crops is likely
to be one of the reasons why such crops show lower levels of foliar diseases, by
comparison with conventionally-grown crops where nitrogen is rarely limiting.
Increases in fungal diseases in relation to increases in nitrogen are well docu-
mented. In cereals, for example, mildew is known to be associated with high levels
of nitrogen. Similarly, brown and yellow rust (causal agents Puccinia recondita and
P. striiformis respectively) are both exacerbated by increased nitrogen (Clare et al.,
1990; Nuttall and Stevens, 1991). Daamen et al. (1989) found that while yellow rust
and mildew were increased on wheat in Holland, eyespot and fusarium foot rot were
unaffected. The development of an epidemic may also be influenced by the growth
stage at which nutrients are applied. For example, under high disease pressure as is
often experienced in south-west Britain, applications of nitrogen earlier than are
normally advised increase the risk of severe disease on winter barley (Jordan et al.,
1989).
The increase in use of nitrogen during the 1970s increased both crop output and
the impact of diseases in UK wheat crops. This relationship was initially suggested
as a cause of the epidemics of septoria leaf blotch on winter wheat, although early
studies and observations on nitrogen response experiments proved to be incon-
clusive. Leitch and Jenkins (1995) have indicated that the expansion of green
leaf area associated with increased nitrogen allows greater opportunity for disease
development. The precise mechanisms for this relationship are unclear but it is
likely that increased leaf nitrogen levels and increased canopy owing to the higher
nitrogen will both increase disease. Both effects allow more potential for greater
disease-induced yield losses associated with decreases in green leaf area caused by
damage from pathogens (Bryson et al., 1995). These effects on temperate cereal
crops are also reflected in the effect of nitrogen in tropical crops. Cu et al. (1996)
showed that although high rates of applied nitrogen fertilizer are needed to maintain
yields of rice, they increase sheath blight (caused by Rhizoctonia solani ) . Yield
reductions of between 20 and 42% were recorded at the highest rates of nitrogen (up
- 1
to 200 kg ha ).
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