Managing crop disease through cultural practices - Disease Control in Crops - page 12

humidity is increased and leaves are wet for longer time, all of which provides favour-
able conditions for pathogen infection and subsequent development. Crop density can be
manipulated in various ways, for example, sowing, pruning and fertilisation.
Reducing the sowing density of barley can decrease severity of R. secalis epidemics
(Hoad & Wilson, 2006), probably by decreasing the density of the crop canopy, thereby
altering microclimate and ensuring inadequate leaf wetness for germination of R. secalis
conidia (Davis & Fitt, 1994). A similar effect can also be achieved in barley by reducing
nitrogen applications (Hoad & Wilson, 2006), mediated possibly by altered microclimate,
although there might also be effects of reduced nitrogen on the pathogen (Zhan et al.,
2008).

2.4 Soil amendments and mulching

2.4.1 Mulching

Mulches are used to conserve organic matter and moisture and to reduce soil erosion.
A variety of materials can be used as mulches, including straw, manure, plastics and paper.
Mulching can lead to water retention and nutrient enrichment in the soil and can decrease
soil temperature, all of which can infl uence pathogen infection and disease development
in plants. Although mulching can reduce the spread of splash-dispersed pathogens, by
altering the environment, it could lead to increased severity of some diseases. Further, if
crop residues are used in mulching, disease incidence could increase, since the residues
could be used as a food source by a range of pathogens.
Working on capsicum, Stirling & Eden (2008) found that damage from Pythium root rot
was more severe with plastic mulches than mulches of plant residue, probably because the
organic mulch reduced soil temperatures by roughly 12C. However, mulching increased
losses from cutworms and increased the severity of infection by Xanthomonas campes-
tris pv. vesicatoria and moreover, the organic mulch reduced fruit yield, mainly due to
nutrient leaching from the soil. These workers suggest that mulches have the potential
to reduce losses from soil-borne pathogens in vegetable crops, providing crop nutrition
is managed adequately (Stirling & Eden, 2008). UV-refl ective mulch was found to be
much more effective than black polythene mulch in reducing colonisation of tomato by
thrips and subsequent infections by tomato spotted wilt virus (Momol et al., 2004). In
some years, virus incidence was reduced further by use of the mulch and application of
the plant activator acibenzolar- S -methyl (Momol et al., 2004). In fact, the use of refl ec-
tive mulches to delay the onset of infestations of whitefl y and associated viruses is well
documented (e.g. Summers & Stapleton, 2002; Summers et al., 2004).

2.4.2 Fertilisers

Adequate mineral nutrition is central to crop production. However, it can also exert con-
siderable infl uence on disease development (Datnoff et al., 2007a; Walters & Bingham,
2007). Fertiliser application can increase or decrease development of diseases caused by
different pathogens, and the mechanisms responsible are complex, including effects of
nutrients on plant growth, plant resistance mechanisms and direct effects on the pathogen
(Walters & Bingham, 2007). The effects of mineral nutrition on plant disease and the