Agriculture Reference
In-Depth Information
2.4.2.6
Crop nutrition and plant disease
The sections above on nitrogen, phosphorus, potassium, calcium and silicon show that,
in spite of some inconsistencies, crop nutrition clearly infl uences disease incidence and
severity in a range of pathosystems. Many of the studies suggest that managing crop
nutrition through appropriate fertiliser practice could be a useful aid to control plant dis-
ease. However, because nutrition will also affect crop yield and quality, a balance needs
to be struck between maximising yield and quality and minimising disease. Therefore,
the extent to which fertiliser regimes can be modifi ed to enhance disease control will
depend on the relative effects of crop nutrition on disease development, the response of
crop yield and quality to disease, and the crop's potential yield and quality in the absence
of disease.
2.4.3
Organic amendments
Organic amendments cover a range of inputs, including animal manure, solid wastes and
composts. These amendments are often used to improve soil quality, usually by con-
tributing to general suppressiveness through enhanced microbial biomass and activity
(Janvier
et al.,
2007). Organic amendments are rich in labile carbon fractions which are
an energy source for microorganisms and moreover, they can themselves contain antago-
nistic microbes. A substantial body of data indicates that organic materials can reduce
incidence of diseases caused by a range of plant pathogens (see Bailey & Lazarovits,
2003). Since composts are dealt with in Chapter 5, this section will look at the effects of
high nitrogen amendments and manures on plant disease.
2.4.3.1
Animal manures
The impact of animal manures on disease incidence and severity is much less predict-
able than that of composts. Thus, fresh chicken manure was shown to reduce survival
of
Phytophthora cinnamomi
and disease incidence on seedlings of
Lupinus albus,
while
cow, sheep and horse manure did not consistently suppress populations of
P. cinnamomi
or disease symptoms (Aryantha
et al.,
2000). Interestingly, in this work, only chicken
manure stimulated populations of endospore-forming bacteria, a factor that was strongly
associated with seedling survival. Animal manures have been implicated in increasing
the incidence of common scab on potato (Bailey & Lazarovits, 2003). However, Conn
& Lazarovits (1999) found that a single application of liquid swine manure reduced
the incidence of wilt and common scab in potato and reduced numbers of plant para-
sitic nematodes for three years after the treatment. Microsclerotia of the wilt pathogen
Verticillium dahliae
were killed by exposure to liquid swine manure (Conn & Lazarovits,
2000), apparently due to the presence of volatile fatty acid mixtures in the manure (Tenuta
et al.,
2002). Subsequent work demonstrated that in acidic soils, liquid swine manure
killed microsclerotia of
V. dahliae
by volatile fatty acids and/or nitrous acid toxicity, while
in alkaline soils, microsclerotia were killed by ammonia toxicity (Conn
et al.,
2005). The
authors suggested that for these mechanisms to be operational and effective in practice,
the chemical composition of the manure, rate of application and soil characteristics need
to be determined in each case (Conn
et al.,
2005).
