Managing crop disease through cultural practices - Disease Control in Crops - page 11

(Naseri et al., 2008). This effect may be due to the mycobiota associated with the buried
stubble and these workers suggest that it might be possible to manipulate the popula-
tion of saprophytic microbiota present on oilseed rape stubble to facilitate the decline of
L. maculans (Naseri et al., 2008).

2.3.2 Sowing practices

Altering sowing practices such as time of sowing, sowing depth and crop density can help
to protect crop plants from pathogens they are susceptible to at particular stages of their
development.

2.3.2.1 Time of sowing

Altering the time of sowing to avoid high levels of pathogen inoculum or conditions
conducive for development of a particular disease can lead to reduced severity of sev-
eral crop diseases. For example, in the United Kingdom, sowing winter oilseed rape in
August rather than September exposes the earlier sown crop to inoculum from stubble of
the previous crop, resulting in more severe Alternaria infection on pods. In contrast, the
risk of infection is reduced in the later sown crop because the stubble is buried by tillage
(Humpherson-Jones, 1992). Late sowing may also be recommended for autumn-sown
barley crops, in order to decrease exposure of newly emerging seedlings to inoculum of
R. secalis produced on previous barley crops in the area (Zhan et al., 2008). In Turkey,
rainfall during spring increases the risk of infection of chickpea by the ascochyta blight
pathogen, A. rabei, while severe drought conditions from late April onwards can lead to
reduced crop yields (Dusunceli et al., 2007). Therefore, determining the best time to sow
chickpeas requires a balance between the resistance rating of the chickpea cultivar and
the weather conditions. Thus, it is recommended that susceptible varieties are sown later,
in late March to early April, since this will avoid the precipitation required for A. rabei
infection and development, while resistant varieties can be sown early (Dusunceli et al.,
2007).

2.3.2.2 Depth of sowing

Sowing depth can infl uence the risk of infection, since the pre-emergence stage of the
seedling, which is usually more susceptible to pathogen infection, is longer when seeds
are sown deeper. In Brassica rapa , for example, rapid emergence of seedlings reduces
pre-emergence damping -off because the period of contact between the emerging seed-
lings and R. solani in the soil is reduced (Nuttall, 1982). Thus, signifi cantly higher seed-
ling emergence was reported for several cultivars of B. rapa sown at a depth of 1.5 cm
compared to 3.0 cm (Nuttall, 1982).

2.3.2.3 Crop density

Crop density can exert considerable infl uence over disease incidence due to the ease with
which pathogen inoculum can be transferred between closely spaced plants and altera-
tions in crop microclimate. In densely planted crops, temperatures are more uniform,