Agriculture Reference
In-Depth Information
a regional basis, the efficacy was 61% in west Europe, 56% in North America and Oceania, and
37% in the rest of the world. Both the potential and actual losses have increased in both actual and
relative terms since the early 1960s, when the yields were lower and cropping less intensive. In 205
German wheat trials, the losses in 1985 and 1990 owing to diseases increased from 11% when the
attainable yield was 4 Mg ha
−1
to 20% when the yield was 11 Mg ha
−1
(Jaggard et al., 2010). This
indicates that as the yield rises in the future, more efficient crop protection measures will be needed
to sustain productivity.
The control of diseases, insects, and weeds is an important factor in improving N use efficiency
in crop production (Fageria and Gheyi, 1999). Crops infested with diseases, insects, and weeds
have a lower photosynthetic efficiency and lower rate of absorption of water and nutrients, and the
competition for light, water, and nutrients consequently reduces yields and results in low N use
efficiency. In cereals or legumes, N is responsible for increasing yield components such as panicle
or head numbers, grain numbers, grain weight, and pod numbers. When these components are
adversely affected, N utilization by crop plants is decreased. In wheat crop, yield components such
as spike weight, kernel weight, kernel number per spike, and number of spikes per plant can be
negatively affected by stripe rust when infestation intensity is high at the heading and milk stages
(Schultz and Line, 1992).
The presence of necrotic lesions on plant leaves is well known to decrease light interception and
consequently leaf photosynthesis (Fageria and Gheyi, 1999). Boote et al. (1983) proposed a clas-
sification of diseases based on their effect on the physiology of the crop. For cercospora leaf spots,
they hypothesized that disease effects on photosynthesis were mediated through loss of leaf area
index (LAI), senescence acceleration, self-shading of healthy leaf area by leaf spots (light stealing),
and a toxic effect of leaf spot disease on the photosynthetic mechanisms of the remaining leaves.
Bourgeois and Boote (1992) reported that the late leaf spot in peanut induced by
Cercosporidium
personatum
decreased the LAI, and that canopy photosynthesis was inversely proportional to total
disease severity, which is an expression of both defoliation and necrotic area.
Planting disease- and insect-resistant crop cultivars is an important strategy in controlling
the adverse effects of these biotic factors (Lynch and Mack, 1995; Sherwood et al., 1995) and
improving yield and consequently N use efficiency. Soybean cultivars differ in sclerotinia stem
rot tolerance and yield losses may not always occur under low levels of disease due to the yield
compensation of nearby soybean plants (Hart, 1998). Yield reduction caused by sclerotinia stem
rot or white mold may range from 147 to 370 kg ha
−1
for every 10% increase in disease sever-
ity, depending on the environment and cultivar (Nelson et al., 2002). Planting soybean cultivars
that are tolerant to selerotinia stem rot is strongly recommended to reduce yield loss (Grau and
Radke, 1984; Boland and Hall, 1987; Kim et al., 1999; Yang et al., 1999) and improves nutrient
use efficiency.
Disease, insects, and weeds can also be controlled by other crop management practices. One
typical example is the control of irrigated rice diseases by an appropriate planting date in the
state of Rio Grande do Sul of Brazil. Rio Grande do Sul is the largest lowland rice-producing
state in Brazil. The average lowland rice yield in this state is about 7 Mg ha
−1
, the highest in the
country. Disease severity (brown spots, grain spots,
Rhizoctonia solani
, and rice blast fungus)
was minimum when the rice crop was planted from October 1 to October 10 and maximum when
it was planted from December 1 to December 12 (Table 8.13). Grohs et al. (2010) reported that
brown spot disease infestation in lowland or irrigated rice was almost nil when planted between
October 1 and October 15 and increased linearly when sowing was delayed from November 15 to
December 1. The timing of crop management practices is increasingly based on the crop develop-
ment stage as a means of improving efficacy (McMaster et al., 2012). For example, management
practices for controlling
Fusarium graminearum
head blight in wheat fields focus on timing the
fungicide application at flowering (Del Ponte et al., 2007). The efficacy of the fungicide applica-
tion decreases as the variation of plant reaching flowering increases within the field (McMaster
et al., 2012).
