Agriculture Reference
In-Depth Information
(Perrenoud, 1990). As with nitrogen, the effects of phosphorus on plant disease may be
the result of direct effects on the pathogen, host plant metabolism, leading to effects on
pathogen food supply, and effects on plant defences (Walters & Bingham, 2007). Indeed,
foliar application of phosphate salts has been shown to induce resistance to pathogens
in a range of crop plants, including cucumber (Mucharromah & Kuc, 1991), broad bean
(Walters & Murray, 1992), grapevine (Reuveni & Reuveni, 1995), maize (Reuveni et al.,
1994) and rice (Mandahar et al., 1998).
Clearly, an adequate phosphorus supply is important for crop growth and in turn, may
well help to reduce disease. However, the regime of phosphorus fertiliser used will depend
on a range of factors, including the crop and the pathogens likely to be important. Reuveni
& Reuveni (1998) suggested that foliar-applied phosphate might be used as part of an
integrated disease control programme, although grower adoption of such an approach will
depend on the existence of other effective disease control measures and the economics of
disease control in the particular crop.
2.4.2.3
Potassium
In an analysis of 181 papers reporting effects of potassium on plant disease, Prabhu
et al. (2007) found that 120 (66%) reported reductions in disease, while 49 (27%)
reported an increase in disease. Although this suggests that in many cases, potassium
is associated with disease reductions, Prabhu et al. (2007) point out that inadequate
consideration has been given to the effects of associated anions, nutrient balance and
nutrient status to allow the defi nitive role of potassium to be determined. For example,
it has been suggested that in some cases, the effects of potassium, applied as potassium
chloride fertiliser, might be due to the chloride ion rather than potassium (Fixen et al.,
1986). Further, chloride fertilisation has been shown to suppress disease in cereal crops
(Engel et al., 1994).
As indicated in the section above, there has been much interest in the application of fer-
tilisers to crop foliage, including the effects of foliar fertiliser application for crop disease
control (Reuveni & Reuveni, 1998; Ehret et al., 2002). Foliar-applied potassium chloride
has been shown to control Blumeria graminis and Septoria tritici on wheat in fi eld stud-
ies (Cook et al., 1993; Mann et al., 2004), probably due to osmotic effects on the fungal
pathogens, disrupting pathogen development and subsequent infection (Kettlewell et al.,
2000; Mann et al., 2004).
Application of potassium to defi cient soils usually increases plant resistance to diseases
(Prabhu et al., 2007). This might be partly related to the effect of potassium in increas-
ing epidermal cell wall thickness or disease escape as a result of vigorous crop growth
(Prabhu et al., 2007), although the mechanisms by which potassium affects plant disease
are not well understood.
2.4.2.4
Calcium
There are many reports that application of calcium to soils, foliage and fruit reduces
the incidence and severity of a range of diseases of crops, including cereals, vegetable
crops, legumes, fruit trees, as well as post-harvest diseases of tubers and fruits (Rahman
& Punja, 2007). For example, calcium has been shown to inhibit anthracnose (caused by
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