Winter 1994 (v6n1)
	Variables associated with corky root and Phytophthora root rot of tomatoes in organic and conventional farms. F. Workneh, A.H.C. van Bruggen, L.E. Drinkwater and C Shennan Phytopathology 83:581-589. 1993 Previous studies comparing different organic and conventional farming systems have shown that soil organic matter and polysaccharide content, microbial biomass, soil bulk density, soil erosion, and available water are higher in organic or ecologically-managed systems than in conventional farming systems (Doran et al., 1987; Lockeretz et al., 1981; Reganold et al., 1987). The purpose of this comparative study was to see if the same conclusions could be drawn for a California cropping system. Specifically, this study was conducted to determine the effects of different management practices on soil physical, chemical and biological properties and to relate these effects to disease severity in tomatoes. Agronomic differences were reported in a previous publication (Shennan et al., 1991); this paper focuses on two important tomato diseases in California. corky root and Phytophthora root rot. Methods Summary Twenty sampling locations were randomly selected at each of nine farms in 1989 and 18 farms in 1990. Of the nine farms selected in 1989, two were organic, three were transitional and four were conventional. The 18 farms sampled in 1990 were evenly split, nine organic and nine conventional. Two to three days after irrigation at the green fruit stage, 810 soil cores were taken at each sampling location. These cores were used to determine ten soil variables: water-stable aggregates, percent clay, organic carbon content, nitrate and ammonium, soil pH and electrical conductivity, total nitrogen content, nitrogen mineralization potential, and fluorescein diacetate hydrolytic activity. Two to three weeks after these soil samples were collected, researchers returned to the same sampling sites to collect data on plant biomass and tissue nitrogen content, and the incidence and severity of Phytophthora root rot and corky root. The use of simple statistical techniques such as analysis of variance would have required the selection of paired organic and conventional farms. This selection was not possible. Therefore, multivariate statistics were used to relate the different measurements to each other for each individual sample. Samples could not be combined per field, because roughly ten times as many observations as variables are required for multivariate analyses. Three types of discriminant analyses were performed on the data: discriminant function, canonical discriminant, and stepwise discriminant analyses. These analyses are described in detail in the original paper Disease Incidence The disease organism Phytophthora parasatica was detected by leaf bait assay in soil samples from five of the nine conventional farms; disease symptoms were observed on three of these farms. Phytophthora was not found on any of the organic farms. Corky root. on the other hand, was found on most plants in most locations, but the incidence and severity of the disease were higher on conventional farms than on organic farms (table 1). Influence of Soil Variables Corky root-severity values, estimated populations of P. parasitica, and Phytophthora root rot-severity values were grouped into classes for use in stepwise and canonical discriminant analyses with the ten soil variables and plant tissue nitrogen. These analyses "indicated that corky root severity was positively associated with nitrogen in tomato tissue and nitrate concentrations in soil, which were higher in conventional farms than in organic farms....In addition to the positive association of corky root of tomato with inorganic nitrogen concentrations in soil and plant tissue, there was a negative association between this disease and microbial activity in soil (as expressed by FDA hydrolytic activity and nitrogen-mineralization potential)." Microbial activity (FDA hydrolytic activity and nitrogen mineralization potential) in both years was higher in the organic farms where compost and green manure had been incorporated prior to planting. In contrast to corky root, the severity of Phytophthora root rot and the presence of P. parasitica in the soil were associated with variables other than nitrogen. Clay content, soil water content, organic carbon, and water-stable aggregates were the major factors that distinguished between the presence and absence of the disease organism in the soil. In summary, corky root severity was primarily affected by biological factors and tissue nitrogen, while Phytophthora root rot was affected mainly by factors related to soil texture, structure, and chemistry. This conclusion is supported by consistent results over two years of study. The authors caution that other factors may have affected the development of the two diseases. Preliminary results from greenhouse tests in which the corky root pathogen was added to various soils showed that corky root was suppressed in organically-managed soils. Nonetheless, lower corky root severities on organic farms may have been partially the result of lower levels of disease inoculum. In the case of Phytophthora root rot, the disease organism may not have been introduced in some of the organic fields, or different methods of irrigation on the organic farms could have controlled the disease. "Results obtained in this study cannot be considered conclusive but have led to hypotheses about the mechanisms that suppress corky root and Phytophthora root rot in organic relative to conventional farms. Additional greenhouse experiments will be needed to test these hypotheses." Table l. Incidence (percentage of plants affected) and severity (percentage of root system affected) of corky root and Phytophthora root rot of tomato in soil in organic, transitional, and conventional farms in 1989 and 1990. Corky roota Phytophthoraa Year Management type Number of farms Incidence (%) Severity (%) Incidence (%) Severity (%) 1989 Organic 2 0.1 0.0(0-0.1)b 0.0 0.0(0-0)b Transitional 3 51.6 1.0(0-7) 0.0 0.0(0-0) Conventional 4 91.1 3.9(0-25) 0.0 0.0(0-0) 1990 Organic 9 23.3 0.49(0-8) 0.0 0.0(0-0) Conv. (small) 4 72.5 2.2(0-13) 7.5 0.3(0-8) Conv. (large) 5 88.0 11.0(0-40) 17.0 2.1(0-32) a Incidence and severity values are means of the corresponding farms. b Range References Doran, J.W., D.G. Fraser, M.N. Culik and W.C. Liebhardt. 1987. Influence of alternative and conventional agricultural management on soil microbial processes and nitrogen availability. Amer. J. Altern. Agric. 2:99-106. Lockeretz, W., G. Shearer and D.H. Kohl. 1981. Organic farming in the corn belt. Science 211:540-546. Reganold, J.P., L.F. Elliott and Y.L. Unger. 1987. Long-term effects of organic and conventional farming on soil erosion. Nature 330:370-372. Shennan, C., L.E. Drinkwater, A.H.C. van Bruggen, D.K. Letourneau and F. Workneh. 1991. Comparative study of organic and conventional tomato production systems: An approach to on- farm systems studies. In Sustainable Agriculture Research and Education in the Field. National Academy Press. Washington, DC pp. 109-132. For more information write to: A.H.C. Van Bruggen, Department of Plant Pathology, University of California, Davis, CA 95616. (DEC.518) Contributed by David Chaney [ Back | Search | Feedback ]