Managing crop disease through cultural practices - Disease Control in Crops - page 13

mechanisms responsible for those effects have been dealt with comprehensively else-
where (Datnoff et al., 2007a; Walters & Bingham, 2007). The sections below will deal
briefl y with the infl uence of nitrogen, phosphorus, potassium, calcium and silicon on
plant disease. The effects of sulphur on plant disease, in particular the phenomenon of
sulphur-induced resistance, is dealt with in Chapter 11.

2.4.2.1 Nitrogen

Nitrogen fertiliser applied above the recommended rates can result in increased disease
incidence and lesion area. This has been demonstrated for biotrophic fungal pathogens
such as powdery mildews and rusts (e.g. Mascagni et al., 1997; Hoffl and et al., 2000) and
necrotrophic pathogens such as Magnaporthe grisea, the cause of rice blast (Talukder
et al., 2005). It is commonly thought that application of nitrogen fertiliser can increase
disease severity via effects on crop canopy development. Thus, large canopies with
high shoot densities may be more conducive to spore transfer and pathogen infection
than sparse canopies. For example, nitrogen has been shown to increase the severity of
Fusarium head blight in wheat, and it has been suggested that this might be the result of
a nitrogen-induced increase in canopy size, leading to an altered microclimate (Lemmens
et al., 2004). In contrast, work on yellow rust on winter wheat suggested that the impact
of nitrogen on disease was the result of effects of nitrogenous substances in wheat leaves
on pathogen growth, rather than effects on canopy growth and microclimate (Neumann
et al., 2004).
However, nitrogen fertilisation is not always associated with increased disease.
Several studies have reported no effect of nitrogen on disease severity (e.g. Buschbell &
Hoffmann, 1992; Olesen et al., 2000), while Hoffl and et al. (2000) found that the effect
of nitrogen depended on the type of pathogen. Thus, nitrogen increased susceptibility of
tomato to the powdery mildew pathogen Oidium lycopersicum and the bacterial patho-
gen Pseudomonas syringae pv. tomato, while it had no effect on susceptibility to the
vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici (Hoffl and et al., 2000).
In contrast, tomato plants were more susceptible to Botrytis cinerea when grown under
low nitrogen conditions (Hoffl and et al., 1999). These results do not support the view
that nutrient-limited plants are better defended (Bryant et al., 1983; Herms & Mattson,
1992). Indeed, nitrogen limitation has been found to severely compromise the ability of
Arabidopsis thaliana to express induced resistance to pathogen infection (Dietrich et al.,
2004, 2005). It is clear therefore that generalising about the effects of nitrogen on plant
disease is unwise and practically, although manipulation or assessment of crop nitrogen
status might be used as part of disease control strategies, the approach adopted will
depend on the crop and the pathogens from which it is most at risk (Walters & Bingham,
2007).

2.4.2.2 Phosphorus

In an analysis of some 2440 studies of the effects of fertiliser on more than 400 diseases
and pests, Perrenoud (1990) found that, in general, phosphorus fertilisation tended to
improve plant health, with reductions in disease recorded in 65% of cases. Nevertheless,
phosphorus fertilisation increased disease and pest problems in 28% of the cases examined