Agriculture Reference
In-Depth Information
nutrient sources used by organic growers can produce mineral balances that reduce the suita-
bility of crop plants for pests and vectors, such as European corn borer (Phelan 1997) or the
bean f fly (Letourneau and Msuku 1992, Letourneau 1994). Thus, management practices can
enhance or reduce host plant resistance by regulating the quality of food source for insects or
pathogens (but see Letourneau 1997). Likewise, certain physiological conditions increase the
incidence and severity of disease, and can be mitigated by management practices. For example,
high N concentrations in soil and plant tissues may predispose a crop to diseases like powdery
mildew, rust and certain root-rotting pathogens (Daamen
et
al
. 1989, Tamis and van den Brink
1998). However, shortages of some elements may also enhance the susceptibility to certain
diseases; for example, K shortages increase the risk of
Verticillium
wilt in cotton, and calcium
(Ca) shortages enhance susceptibility to
Pythium
root rot (Engelhard 1989).
Inherently resistant cultivars have been available for many crops, providing resistance
against diseases caused by fungi, bacteria, viruses and nematodes as well as certain insect
pests. The mechanisms underlying the resistance range from physical features such as tough
leaves, and hairy or waxy tissues to deterrent or toxic secondary plant compounds in the
foliage, fruits or seeds. Some of the resistance features have a broad activity against many pests
and diseases and are based on multiple genes. For example, leaf toughness forms a significant
impediment to insect herbivore feeding and pathogen ingression on many crops (Bergvinson
et
al
. 1994, Agrios 1997). Alkaloids such as nicotine, glucosinolates and cyanogenic glycosides,
found in tobacco, cabbage and cassava respectively, are not only toxic to most herbivores but
also to many plant pathogens (Rosenthal and Janzen 1979, Agrios 1997). The inhibiting agents
can be present continuously (constitutive resistance) or can be induced by stress, insect feeding
or infection by pathogens and symbionts.
If a single gene governs resistance to pest exploitation, a cascade of biochemical reactions is
usually triggered by a particular elicitor of a pathogen or pest, resulting in strong resistance. In
many cases a pathogen or pest population can adapt relatively easily to this kind of resistance
through heavy selection pressure (Riggs 1959), while counter-resistance is not so easily selected
against multiple, mild resistance factors. For this reason, organic growers prefer to use plant
cultivars and animal breeds with broad resistance based on multiple genes (see
Chapters 5
and
6
). Although this means that a limited level of infection or feeding may occur, organic growers
take this for granted, since they value a greater genetic variation and the associated yield stabil-
ity. For the same reason, many organic growers prefer open-pollinated varieties over hybrids.
Moreover, mild resistance based on multiple genes can still be effective, when combined with
other tactics such as biological control of pests, pathogens or vectors, even when it is insufficient
to control a pathogen or pest on its own (Wyss
et
al
. 2001, Vaarst
et al
. 2003).
Plant resistance traits may work indirectly through their effects on natural enemies. For
example, certain maize plants (
Zea
mays
), when fed upon by caterpillars, release a mixture of
volatile compounds that attract parasitic wasps. Varieties known to produce these induced
odour emissions will likely maximise biological control by being particularly attractive to par-
asitoids (e.g. Degen
et
al
. 2004). Varietal selection for maximum effectiveness for biological
control is in its infancy, and is often targeted for the discovery of gene sequences that can be
transferred to conventional cultivars. The production of varieties particularly suited to organic
production systems is progressing in recent years, but the choices are limited compared to
varieties for conventional conditions (Jahn 2003, see
Chapter 5
).
Community resistance - vegetation
A key characteristic of natural plant populations is their genetic and phenotypic heterogeneity.
Individual plants tend to occur in natural habitats displaying a mosaic of resistance levels
due to genetic variability (Whitham and Slobodchikoff 1981) and induced responses. Such
