Development and Deployment of Systems-Based Approaches for the Management of Soilborne Plant Pathogens.

Biological suppression of soilborne diseases with minimal use of outside interventive actions has been difficult to achieve in high input conventional crop production systems due to the inherent risk of pest resurgence. This review examines previous approaches to the management of soilborne disease as precursors to the evolution of a systems-based approach, in which plant disease suppression through natural biological feedback mechanisms in soil is incorporated into the design and operation of cropping systems. Two case studies are provided as examples in which a systems-based approach is being developed and deployed in the production of high value crops: lettuce/strawberry production in the coastal valleys of central California (United States) and sweet basil and other herb crop production in Israel. Considerations for developing and deploying system-based approaches are discussed and operational frameworks and metrics to guide their development are presented with the goal of offering a credible alternative to conventional approaches to soilborne disease management.

Optimizing Soil Disinfestation Procedures for Fresh Market Tomato and Pepper Production.

Combinations of soil fumigation with a mixture of 1,3-dichloropropene (1,3-D) plus chloropicrin (Pic) and soil solarization for 7 days were evaluated under different plastic films and sequences of application for their effects on soilborne pests and marketable yield of fresh market tomato (Lycopersicon esculentum) and pepper (Capsicum annuum). Shank injection of fumigants under a virtually impermeable film (VIF) using a novel application apparatus dramatically improved their retention in the soil. Survival of Fusarium oxysporum f. sp. lycopersici in soil declined significantly when fumigation or solarization was combined with VIF compared with either soil disinfestation treatment applied under low-density polyethylene. When compared with an untreated control, significant reductions in yellow nutsedge (Cyperus esculentus), purple nutsedge (C. rotundus), and root-knot nematodes (Meloidogyne spp.) were achieved with a reduced dosage of fumigant when applications were made 7 days after planting beds were covered with VIF. A 7-day delay in fumigant application in beds covered by low-density polyethylene significantly increased marketable yield of pepper when compared with an untreated control. The results demonstrate that chemical and nonchemical soil disinfestation methods can be combined with novel application technology and procedures to improve their spectrum of pest control and reduce fumigant application rates.

Effect of Soils from Six Management Systems on Root-knot Nematodes and Plant Growth in Greenhouse Assays.

The effects of soil management systems on root-knot nematode (Meloidogyne incognita) eggs and gall incidence on tomato (Lycopersicon esculentum) and cucumber (Cucumis sativus) following tomato were evaluated. Soil was collected from a replicated field experiment in which six management systems were being assessed for vegetable production. Soil management systems were conventional production, organic production, bahiagrass (Paspalum notatum) pasture, bahiagrass: Stylosanthes (Stylosanthes guianensis) pasture, bare ground fallow, and weed fallow. Soil was collected from field plots and used in greenhouse experiments. Identification of egg-parasitic fungi and the incidence of root-knot nematode galling were assessed both on tomato and cucumber planted in the same pots following the removal of tomato plants. Organic, bare ground fallow and conventional production treatments reduced galling both on tomato and on cucumber following tomato. Although no treatment consistently enhanced egg-parasitic fungi, management system did affect egg viability and the types of fungi isolated from parasitized eggs.

Nonchemical management of soilborne pests in fresh market vegetable production systems.

ABSTRACT Nonchemical methods including host resistance, organic amendments, crop rotation, soil solarization, and cultural practices have been used to control soilborne pests in fresh market vegetable production systems. Their suitability as alternatives to methyl bromide will depend on the approach to pest management used by the grower. Traditionally, methyl bromide is used in production systems that rely on the single application of a broad-spectrum biocide to disinfest soils prior to planting. Non-chemical methods are not suitable for a single tactic approach to pest management because they do not provide the same broad spectrum of activity or consistency as fumigation with methyl bromide. Nonchemical methods are compatible with an integrated pest management (IPM) approach, where multiple tactics are used to maintain damage from pests below an economic threshold while minimizing the impact to beneficial organisms. However, adoption of IPM is hindered by the paucity of economically feasible sampling programs and thresholds for soilborne pests and by a reluctance of growers to commit additional resources to the collection and management of biological information. A novel approach to the management of soilborne pests is to design the crop production system to avoid pest outbreaks. Using this "proactive" approach, a tomato production system was developed using strip-tillage into existing bahia-grass pasture. By minimizing inputs and disruption to the pasture, growers were able to reap the rotational benefits of bahiagrass without cultivating the rotational crop. While minimizing the need for interventive procedures, a proactive approach is difficult to integrate into existing crop production systems and will require several years of testing and validation.

Pythium spp. Associated with Bell Pepper Production in Florida.

Ten species of Pythium and a group of isolates that produced filamentous sporangia but did not form sexual structures (Pythium 'group F') were recovered from the root systems of fresh market bell pepper plants grown on polyethylene-mulched production systems in Florida. Pathogenicity tests using pasteurized field soil inoculated with infested wheat seed demonstrated that P. aphanidermatum, P. myriotylum, P. helicoides, and P. splendens can cause significant root rot and reductions in root growth of pepper. P. aphanidermatum and P. myriotylum caused the most severe root rot, the greatest reductions in plant weight, and 42 and 62% plant mortality, respectively. In pathogenicity tests with tomato plants, these four species produced similar plant weight losses and disease ratings to those observed in pepper, but little or no plant mortality. Low incidences of root tip necrosis in pepper plants were observed with P. arrhenomanes, P. catenulatum, P. graminicola, and P. irregulare, but none of these species caused losses in root weight and only P. irregulare reduced shoot weight. P. periplocum, P. spinosum, and Pythium sp. F colonized root tissue of pepper but caused no significant root rot and did not adversely affect growth. Similar trends were observed with tomato, except that P. arrhenomanes caused limited root tip necrosis without affecting plant growth and P. catenulatum, P. graminicola, P. irregulare, P. spinosum, and Pythium sp. F colonized at least some of the plants but did not cause root disease. A significant interaction between temperature and P. aphanidermatum or P. myriotylum was observed on pepper transplants. The greatest reductions in growth occurred at 28°C, whereas plant mortality only occurred at 34°C.

Thermal Inactivation of Phytophthora nicotianae.

ABSTRACT Phytophthora nicotianae was added to pasteurized soil at the rate of 500 laboratory-produced chlamydospores per gram of soil and exposed to temperatures ranging from 35 to 53 degrees C for 20 days. The time required to reduce soil populations to residual levels (0.2 propagule per gram of soil or less) decreased with increasing temperatures. Addition of cabbage residue to the soil reduced the time required to inactivate chlamydospores. Temperature regimes were established to simulate daily temperature changes observed in the field, with a high temperature of 47 degrees C for 3 h/day, and were good estimators of the efficacy of soil solarization for the control of P. nicotianae in soil. Cabbage amendment reduced the time required to inactivate chlamydospores of P. nicotianae and its effect was more pronounced at lower temperature regimes.

Efficacy of Solarization and Cabbage Amendment for the Control of Phytophthora spp. in North Florida.

The effects of soil solarization with or without cabbage leaf amendments on the survival of Phytophthora spp. were evaluated in several North Florida soils. Soil temperature under solarization treatments reached a maximum of 47°C at a 10-cm depth, but only 41°C at 25 cm. Solarization with a clear, gas-impermeable film was as effective as methyl bromide in reducing populations of P. nicotianae at a 10-cm depth but had no effect on populations at a depth of 25 cm. Populations of P. capsici after solarization with either a clear, low-density polyethylene or a clear, gas-impermeable film were similar to methyl bromide treatment at the 10-cm depth, while at the 25-cm depth, no reduction in populations was observed. Incorporation of cabbage into the soil at a rate of 6.6 to 8.9 kg/m2 did not enhance the effectiveness of solarization in reducing populations of either Phytophthora sp.

Occurrence of Bacterial Stem Rot, Caused by Erwinia chrysanthemi, on Field-Grown Tomato in Florida.

In September 1997, wilted 4-week-old tomato (Lycopersicon esculentum Mill.) plants were observed in a commercial production field in St. Lucie County, FL. Closer inspection of affected plants revealed hollow stems and petioles with dark, water-soaked lesions. Diseased tissue was macerated and streaked onto nutrient agar (NA) and crystal violet pectate (CVP) agar. After incubation for 2 days at 30°C, isolates produced pits on the CVP agar. Isolates were transferred onto NA and the incubation and transfer procedure was performed two additional times to obtain pure cultures. Suspensions of bacterial cells were injected into tomato and tobacco leaves to test for a hypersensitive or pathogenic reaction. Isolates produced collapsed necrotic tissue on tomato while no reaction was observed on tobacco. Tests for differentiating species and subspecies in the 'carotovora' group of Erwinia were conducted following the protocol of Dickey and Kelman (1). With known cultures of E. carotovora subsp. carotovora and E. chrysanthemi as controls, the isolate from tomato was determined to function as a facultative anaerobe, utilize asparagine as a sole source of carbon and nitrogen, and give positive reactions for pectate degradation, phosphatase, and growth at 37°C. Known cultures of E. carotovora subsp. carotovora, E. chrysanthemi, and the tomato isolate were grown on trypticase soy broth agar for 24 h at 28°C and their cellular fatty acids derivatized to fatty acid methyl esters (FAMEs). Statistical analyses of FAME profile data (MIDI Microbial Identification System, Newark, DE, version 3.60) identified the tomato isolate as Erwinia chrysanthemi. Pathogenicity was determined by inoculating 50-day-old tomato plants (cv. SunPride) with a suspension of E. chrysanthemi obtained from nutrient broth plates incubated at 24°C for 60 h. Three plants each were inoculated with the E. chrysanthemi identified from tomato, sterile water, and known cultures of E. chrysanthemi and E. carotovora subsp. carotovora by placing a drop at the junction of the petiole and stem and passing a sterile needle through the drop into the stem. Plants were maintained in a greenhouse. Dark, water-soaked cankers were observed on the stems of plants inoculated with E. chrysanthemi, including the tomato isolate and E. carotovora subsp. carotovora, after 7 days. No symptoms were observed on plants inoculated with sterile water. Reisolation of the pathogen and identification was performed with tissue from one of the symptomatic inoculated plants. Analyses of FAMEs confirmed E. chrysanthemi as the causal agent. This is the first report of E. chrysanthemi causing a vascular disease of field-grown tomato in Florida. Reference: (1) R. S. Dickey and A. Kelman. 1988. Pages 44-59 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria. N. W. Schaad, ed. American Phytopathological Society, St. Paul, MN.

Adaptation of soil solarization to the integrated management of soilborne pests of tomato under humid conditions.

ABSTRACT Soil solarization was shown to be cost effective, compatible with other pest management tactics, readily integrated into standard production systems, and a valid alternative to preplant fumigation with methyl bromide under the tested conditions. Solarization using clear, photoselective, or gas-impermeable plastic was evaluated in combination with metham sodium, 1,3-dichloropropene + chloropicrin, methyl bromide + chloropicrin, pebulate, or cabbage residue. Strip solarization, applied to 20-cm-high, 0.9-m-wide beds, was conducted to achieve compatibility with standard production practices and resulted in soil temperatures 2 to 4 degrees C above those temperatures resulting when using conventional flatbed solarization. Soil temperatures were 1 to 2 degrees C higher at the edges of the raised beds, eliminating any border effects associated with solarization. Following a 40- to 55-day solarization period, the plastic was painted white and used as a production mulch for a subsequent tomato crop. The incidence of Southern blight and the density of Paratrichodorus minor and Criconemella spp. were lower (P < 0.05) in solarized plots. No differences (P < 0.05) in the incidence of Fusarium wilt and the density of nutsedge and Helicotylenchus spp. were observed between plots receiving solarization and plots fumigated with a mixture of methyl bromide + chloropicrin. The severity of root galling was lower (P < 0.05) when soil solarization was combined with 1,3-dichloropropene + chloropicrin (16.2 + 3.4 g/m(2)) and a gas-impermeable film. The incidence of bacterial wilt was not affected by soil treatments. Marketable yields in plots using various combinations of soil solarization and other tactics were similar (P < 0.05) to yields obtained in plots fumigated with methyl bromide + chloropicrin. The results were validated in several large scale field experiments conducted by commercial growers.

Reduction of phytoparasitic nematodes on tomato by soil solarization and genotype.

The effects of soil solarization and tomato (Lycopersicon esculentum) genotype on populations of plant-parasitic nematodes and bacterial wilt were examined in North Florida. Maximum soil temperatures achieved under solarization treatments using a photoselective polyethylene mulch were 49.5, 46, and 40.5 C at depths of 5, 15, and 25 cm, respectively. Soil solarization reduced (P < 0.05) populations of Paratrichodorus minor, Rotylenchulus reniformis, and Criconemella spp. 85 days after transplanting on the cultivar Solar Set. Soil solarization reduced (P < 0.10) populations of P. minor, R. reniformis, and Criconemella spp. on the breeding line Fla. 7421. Reductions of P. minor and Criconemella spp. on Solar Set and Fla. 7421 were similar to those achieved by fumigation with a 67:33 mixture of methyl bromide and chloropicrin (448 kg/ha). Fla. 7421 reduced (P < 0.10) populations of R. reniformis compared with Solar Set. Neither soil solarization nor fumigation reduced the incidence of bacterial wilt on the susceptible cultivar Solar Set. This study demonstrates the ability of soil solarization to provide season-long control of plant-parasitic nematodes of tomato under a climatic regime characterized by periods of abundant rainfall and extended cloud cover.