Agriculture Reference
In-Depth Information
directions that will advance our knowledge and capacity for effective crop protection in
organic agriculture.
Pests and diseases in organic ersus conentional agriculture
Pests and diseases that plague conventional farming operations, causing yield loss or the appli-
cation of costly inputs, are often the same species that challenge organic growers producing the
same crops. One significant difference is that organic growers avoid the use of broad-spectrum
synthetic pesticides, which severely disrupt natural controls in the system and promote the
occurrence of secondary pests (Johnson and Tabashnik 1999). Well-known secondary pests in
pesticide intensive systems include spider mites in temperate orchards treated for codling
moth, rice brown planthopper in pesticide-treated tropical paddy rice,
Rhizoctonia
black scurf
in potato after nematicide applications that reduce fungi-feeding collembola (Hofman 1988),
and apple scab as a result of decimation of earthworm populations by the fungicide benomyl
(slowing down decomposition of infected leaves). A substantial number of major pests in con-
ventional systems, then, are regulated at low levels in organic systems by virtue of the conser-
vation of their natural enemy complex.
Natural pest and pathogen controls are not only conserved (not disrupted) but are also
promoted in organic farming conditions. Most soilborne plant pathogens causing root and
foot rots in older plants are usually less prevalent in organic than in conventional farms (van
Bruggen and Termorshuizen 2003). This kind of disease suppression has frequently been asso-
ciated with higher microbial activity and diversity, with higher microfaunal numbers and
diversity, and/or with lower soil and crop N concentrations in organic than in conventional
soils. Attacks by some airborne diseases (in particular many powdery mildew and rust diseases)
and by sucking insect pests (aphids and whitef lies) can also be less severe in organic than in
conventional crops due to lower nitrogen concentrations in foliar tissues or phloem on organic
than on conventional farms (van Bruggen 1995).
Some arthropods are favoured under conditions of organic farming practices, however,
particularly below-ground pests that are fostered by rich organic matter such as the garden
symphylan, cutworms, wireworms (Jansson and Lecrone 1991, Peachey
et
al
. 2002) and slugs
or hardy pest insects that have few biological controls and are not effectively controlled with
allowable organic inputs such as the strawberry weevil or
Lygus
bug. Similarly, damping-off
causing pathogens such as
Pythium
species can wreak havoc in organic crops, since these can
multiply quickly in fresh organic materials incorporated into soil (van Bruggen 1995). Certain
foliar diseases that can spread quickly and are controlled by frequent fungicide sprays in con-
ventional farms, such as potato late blight and onion downy mildew, can be devastating on
organic crops in humid climates (Piorr and Hindorf 1986, van Bruggen 1995). Stored products
pests should also be a particularly challenging problem for organic agriculture, since synthetic
insecticides are prohibited. However, it seems that stored products pests are a universal problem
posing challenges for conventional and organic farmers alike. Haines (2000) describes many
problems with synthetic chemical approaches in the past decades to long-term storage pest
control, and proposes new innovations in alternative control strategies. These control strate-
gies include those used by organic growers (Table 4.1).
Vertebrate pests, such as deer and other ungulates, fruit-eating and seed-eating birds,
rodents, rabbits and squirrels colonise or intermittently visit both organic and conventional
farms, potentially reducing yields and/or affecting food quality. Some of the practices more
common on organic farms such as cover cropping, farmscaping with non-crop vegetation, and
mixed cropping encourage beneficial fauna and repel some vertebrate pests, but may also
improve the habitat for other vertebrates, such as gophers, voles and noxious birds. Organic
practices overlap with those of conventional farming for control of these pests, including
