Crop protection in organic agriculture - Organic Agriculture: A Global Perspective - page 110

crops (see Workneh et al . 1993, Drinkwater et al . 1995, Letourneau et al . 1996, and Letourneau
and Goldstein 2001 for details of sites, design and sampling methods).
Root disease incidence and microbial community structure
In both the experimental study and the field surveys, foliar diseases were not important on
irrigated tomatoes (mostly by furrow irrigation) in the semi-arid climate of California. Only
occasionally, diseases such as bacterial spot ( Xanthomonas campestris ) occurred when it rained
early in the season (Clark et al . 1998). Virus symptoms were also seldom observed. There were
no differences in foliar disease incidence and severity between organic and conventional
farming systems. Root diseases were quite common and sometimes severe in conventional
tomato fields, but were absent or only slight in low-input and organic fields.
In the SAFS experiment, corky root ( Pyrenochaeta lycopersici ) was significantly more severe
in the conventional system with a 2-year rotation and only slightly more severe in the conven-
tional system with a 4-year rotation compared to the low-input and organic systems (Clark et al .
1998, van Bruggen and Termorshuizen 2003). The same was true for root rot caused by Pythium
aphanidermatum . Differences in severity of other root rots were mostly not significant. These
observations were made six, seven, eight and nine years after the start of the experiment. In the
last two years, there were significant differences between both conventional treatments and the
alternative treatments. Both alternative systems had winter cover crops, and much better water
penetration than the conventional systems, which developed a hardpan over time. The organic
plots had lower nitrate concentrations in soil and plant tissues and a higher microbial biomass
and associated food web (particularly bacteria-feeding nematodes) than the conventional plots
(Ferris et al . 1996). There were positive correlations between corky root severity and N concen-
trations in soil and plant tissues (A.H.C. van Bruggen, unpublished data, 1998).
In the field survey of tomato-producing farms in California both the incidence and severity
of corky root were lower in well-established and recently converted organic farms (ORG) than
in conventional farms (CNV). The main variables explaining corky root incidence and severity
levels were N concentrations in both soil and tomato tissue (Workneh et al . 1993). Corky root
was more numerous and severe at higher N concentrations. However, N mineralisation poten-
tial and f fluorescein diacetate (FDA) hydrolysis, both measures of microbial activity, were nega-
tively correlated with disease severity. These relationships were confirmed in greenhouse
experiments (Workneh and van Bruggen 1994a). In other greenhouse and laboratory experi-
ments, corky root suppression in ORG soils was associated with larger populations and higher
diversity of actinomycetes in the rhizosphere (,Workneh and van Bruggen 1994b, Drinkwater et
al . 1995). The community composition of actinomycetes and bacteria were more similar among
samples with the same soil management (CNV or ORG) than between different management
types (CNV v. ORG). Phytophthora root rot ( Phytophthora parasitica ) was also more severe in
CNV than ORG fields, but this difference was primarily associated with soil texture, structure
and moisture content instead of microbial and nutritional factors (Workneh et al . 1993).
Plant-parasitic nematode populations, in particular Pratylenchus spp., were significantly
lower in the organic and low-input systems than in the conventional systems of the SAFS
experiment as early as 1993, four years after initiation of the experiment. This difference was
maintained until the end of the experiment in 2000 (Ferris et al . 1996, Clark et al . 1998, Berkel-
mans et al . 2003). Populations of Meloidogyne spp. were not consistently lower in the alterna-
tive systems than in the conventional systems (Clark et al . 1998, Berkelmans et al . 2003). Root
knot symptoms on tomatoes were rare and differences were not consistent among treatments
(Clark et al . 1998). Bacterivorous nematodes were generally more abundant in the organic and
low-input than in the conventional treatments (Ferris et al . 1996). Accordingly, the enrich-
ment index, a measure of resource availability, was higher, and the channel index, a measure of
domination of the fungi-based over the bacteria-based food web, was lower in the organic and