Agriculture Reference
In-Depth Information
Storage pests are a particularly difficult problem; although storage facilities are often
patchy, pests and pathogens tend to either move with the produce from the crop field to the
storage facility or reside in the storage facility, making isolation difficult for many pest taxa in
both organic and conventional farming operations.
Regulation of established pests and pathogens in organic agriculture
Once a pest or pathogen becomes established in a crop, field or storage facility various proc-
esses act to either enhance its abundance and spread in the system (invasion occurs) or to
suppress its abundance and spread (persistence at low levels). These processes involve either
host or product quality or the presence of suppressive agents in the community that regulate
population growth of the pathogen or pest in the crop environment. In addition, physical
impedance to spread by enhanced distances between hosts or physical barriers contributes to
pest and pathogen regulation. Host plant quality can be optimised for minimal disease severity
or reduced success of herbivorous insects (Scriber 1984). Indeed, crop resistance to pests and
pathogens is a mainstay of organic agriculture. Resistance to pest and pathogen exploitation is
brought about by selecting varieties with genetically based resistance traits, managing the phe-
notype, health and nutrient concentration to reduce its suitability for pests and pathogens, or
managing crop and non-crop vegetation to reduce the concentration of food plants for herbiv-
ores (see
Community
resistance
). Suppressive agents include competitors (neutral herbivores
and microbes avirulent or less damaging to the crop) and natural enemies (predators and par-
asitoids of the pestiferous and pathogenic organisms). Included in this arsenal of crop defences
are tactics that can be used together to protect the crop from yield loss either in the field or in
the storage facility (Table 4.1). Integrative strategies are a cornerstone of successful crop pro-
tection in organic agriculture. In the following sections we explore the tactics available in
organic agriculture, their effectiveness and limitations.
Host plant resistance
Host plant quality is optimised for crop protection when:
1 adequate nutrient status for plant productivity and health is maintained, without excess
nutrients or imbalances that support high levels of herbivores or pathogens; and
2 toxic or repellent properties are sufficient to directly reduce pest or pathogen exploitation
and survival.
The first approach is somewhat f flexible, allowing for a grower to respond to pest and
pathogen dynamics. However, the decision to use a resistant variety is set for the season. Its use
will be determined by the probability of invasion, the severity of the pest or pathogen, any
associated loss of yield quality or quantity, marketability, complementarity with other crop
protection tactics, and the effectiveness of the resistant cultivar against the target and other
possible exploiters. A caveat to these generalisations is that 'one organism's famine is another
organism's feast', that is, very high nitrogen may attract an insect and deter a plant parasitic
nematode (if ammonia is released from the nitrogen source). High mustard oil content deters
insects that find it toxic, and attracts those that specialise on mustard family crops.
Depending on the target pests or pathogens, plant quality-based resistance can be induced
by regulating the type and quantity of nutrients and moisture applied to the crop. For example,
high N levels can enhance the population growth of certain aphids, but when potassium (K) is
in ample supply, the amount of soluble N circulating in the phloem tissue is reduced, thus
retarding aphid fecundity (van Emden 1966). Water shortage, in contrast, accelerates the
breakdown and mobilisation of proteins and enriches the phloem nutrient quality for aphids,
whereas excess moisture may predispose the crop to root-rotting pathogens. Sometimes
