Agriculture Reference
In-Depth Information
weeds, supplies N to the next crop, and contributes to disease control, particularly of take-all
caused by
Gaeumannomyces
graminis
(G.A. Hiddink, A.J. Termorshuizen, J.M. Raaijmakers
and A.H.C. van Bruggen, unpublished data).
Thus, organic practices that promote biodiversity both above ground and below ground
tend to enhance the resilience of the arable farming system so that many pests and diseases are
controlled by natural enemies. These practices include an extensive rotation, soil organic
matter management fostering high turnover rates and relatively low residual mineral nutrients
with minimal f luctuations, crop or cultivar mixtures, and varied field margins. Although this
is a desirable scenario for cereal production, it is not always practiced, and the great variation
in organic farming practices understandably results in large variations in pest and disease
intensities.
Case study 2 - pest and pathogen regulation in organic ersus conentional
tomato fields in California
Tomato is a relatively high input crop in Californian agriculture. Of the top 15 vegetable crops,
11 field crops, and 11 fruit or nut crops produced in the USA, Pimentel
et
al
. (1981) listed
tomato as having the highest percentage of acreage treated with insecticides (93%) and fungi-
cides (98%) and the ninth highest for acreage treated with herbicides (67%). A 36% yield
reduction was predicted if pesticides (insecticides, fungicides) were not applied to the tomato
crop (Agricultural Issues Center 1988). To check this premise, pests, diseases and their natural
enemies were monitored in two complementary comparative studies in California. Van
Bruggen participated in the Sustainable Agriculture Farming Systems (SAFS) experiment in
Davis, California, where irrigated conventional, low-input and organic cropping systems with
4-year rotations were compared with a 2-year conventional rotation from 1989 through 2001
(Poudel
et
al
. 2001). All crops in the rotation were represented each year, and there were four
replicated blocks for a total of 56 plots on 11 ha. Tomatoes were the most intensively investi-
gated crop species in this experiment with respect to disease incidence and severity, insect
pests, weed infestations and N dynamics. Letourneau and van Bruggen carried out a two-year
survey of 18 organic (ORG) and conventional (CNV) commercial tomato production systems
to compare:
1 the incidence of pests and pathogens;
2 injury levels and pest damage to the crop;
3 pest abundance and disease severity;
4 biodiversity of the microbial and arthropod communities associated with the crop; and
5 nutritional status of the crop.
The survey was carried out on nine ORG and nine CNV tomato fields in a 600 km
2
area,
encompassing five counties in the Central Valley of California. This sample covered a repre-
sentative spectrum of actual commercial farming practices, and the variability needed to
identify particular practices that affect pest management. All 18 farms used the same tomato
cultivar Blazer® (Drinkwater
et
al
. 1995). Farms in both ORG and CNV management categories
included sites bordered by various combinations of annual crop fields, orchards, oak woodland
and riparian habitats. All fields were maintained reasonably weed-free within the beds during
the growing season, but annual weeds were abundant along roadsides and field edges, especially
where sufficient moisture was available. The cropping history of the fields, however, was not
independent of management category. Most conventional farms were maintained as bare
ground fallows over winter through initial tillage and subsequent herbicide applications,
whereas organically managed fields had a vegetative cover with annual weeds and/or cover
