Nancy Kokalis-Burelle

Lack of Induced Systemic Resistance in Peanut to Late Leaf Spot Disease by Plant Growth-Promoting Rhizobacteria and Chemical Elicitors

A disease assay was optimized for late leaf spot disease of peanut using Cercosporidium per-sonatum in the greenhouse, and this assay was used in attempts to elicit induced systemic resistance using strains of plant growth-promoting rhizobacteria (PGPR) and chemical elicitors. Nineteen strains of spore-forming bacilli PGPR, including strains of Paenibacillus macerans, Brevibacillus brevis, Bacillus laterosporus, B. subtilis, B. pumilus, B. amyloliquefaciens, B. sphaericus, B. cereus, and B. pasteurii, which previously elicited systemic disease control activity on other crops, were evaluated in greenhouse assays. Seven PGPR strains elicited significant disease reduction in a single experiment; however, none repeated significant protection achieved in the greenhouse assay, while significant protection consistently occurred with the fungicide chlorothalonil (Bravo). In other greenhouse trials, neither stem injections of C. personatum nor foliar sprays of chemicals, including salicylic acid, sodium salicylate, isonicotinic acid, or benzo[1,2,3]thiadiazole-7-carbothioc acid S-methyl ester (Actigard), which elicit systemic acquired resistance on other crops, elicited significant disease protection. In contrast, foliar sprays with DL-β-amino-n-butyric acid (BABA), which is an elicitor of localized acquired resistance, resulted in significantly less late leaf spot disease in one of two tests. Combination treatments of four PGPR strains with BABA in the greenhouse did not significantly protect peanut from late leaf spot. Field trials conducted over two growing seasons indicated that none of the 19 PGPR strains, applied as seed treatments at two concentrations, significantly reduced late leaf spot disease. The same chemical elicitors tested in the greenhouse, including BABA, did not elicit significant disease protection. Some combinations of four PGPR and BABA significantly reduced the disease at one but not at two sample times. Collectively, these results suggest that late leaf spot resistance in peanut is not systemically inducible in the same manner as is resistance to diseases in other crops by PGPR and chemical inducers.

Exploring warm-season cover crops as carbon sources for anaerobic soil disinfestation (ASD)

Background and aimsAnaerobic soil disinfestation (ASD) has been shown to be an effective strategy for controlling soilborne plant pathogens and plant-parasitic nematodes in vegetable and other specialty crop production systems. Anaerobic soil disinfestation is based upon supplying labile carbon (C) to stimulate microbially-driven anaerobic soil conditions in moist soils covered with polyethylene mulch. To test the effectiveness of warm-season cover crops as C sources for ASD, a greenhouse study was conducted using a sandy field soil in which several warm-season legumes and grasses were grown and incorporated and compared to molasses-amended and no C source controls.MethodsGreenhouse pots were irrigated to fill soil porosity and covered with a transparent polyethylene mulch to initiate a 3-week ASD treatment prior to planting tomatoes. Soilborne plant pathogen inoculum packets, yellow nutsedge (Cyperus esculentus L.) tubers, and Southern root-knot nematode (Meloidogyne incognita (Kofoid & White) Chitwood; M.i.) eggs and juveniles were introduced at cover crop incorporation.ResultsIn nearly all cases, ASD treatment utilizing cover crops as a C source resulted in soil anaerobicity values that were equal to the molasses-amended fallow control and greater than the no C source fallow control. In trial 1, Fusarium oxysporum Schlechtend.:Fr. (F.o.) survival was reduced by more than 97% in all C source treatments compared to the no C source control but there was no effect of C source in Trial 2. Carbon source treatments were inconsistent in their effects on survival of Sclerotium rolfsii Sacc. (S.r). In general, the number of M.i. extracted from tomato root tissue and root gall ratings were low in all treatments with cover crop C source, molasses C source, or composted poultry litter. Germination of yellow nutsedge tubers was highest in the no C source control (76%), lowest in the molasses control (31%), and intermediate from cover crop treatments (49% to 61%).ConclusionsWarm-season cover crops have potential to serve as a C source for ASD in vegetable and other crop production systems, but more work is needed to improve consistency and further elucidate mechanisms of control of soilborne plant pathogens and weeds during ASD treatment utilizing cover crops.

Cover crops and organic mulches for nematode, weed and plant health management

Summary – Traditional cover cropping systems for nematode management seldom consider weed and soil nutrient management concurrently. Integrating cover crops suppressive to plant-parasitic nematodes with a cover crop mulching system could improve traditional approaches. Two field experiments were conducted in 2003 and 2004 to evaluate ‘Tropic Sun’ sunn hemp (Crotalaria juncea) and ‘Iron Clay’ cowpea (Vigna unguiculata) as summer cover crops and as organic mulches. Both experiments were in a 3 × 3 split-plot design in which the main plots were summer planting of sunn hemp, cowpea or fallow, and the subplots were organic mulch of sunn hemp, cowpea or no mulch. The summer cover crop was followed by turnip (Brassica rapa) and lima bean (Phaseolus lunatus )i n the autumn. Using sunn hemp as organic mulch suppressed root-knot nematodes more effectively than using it as a cover crop, but only on a less susceptible host such as turnip, and not on a very susceptible host such as lima bean. While sunn hemp as a cover crop failed to enhance beneficial free-living nematodes, sunn hemp as an organic mulch enhanced bacterial-feeding nematode population densities. Sunn hemp mulch also suppressed broadleaf weeds but not grasses or nutsedges. Although sunn hemp and cowpea cover crops did not increase lima bean N and K content, their mulches increased N and K content. Similar results were observed for turnip and lima bean yields. Population density of root-knot nematodes was positively related to abundance of omnivorous nematode in 2003. The abundance of plant-parasitic nematodes was negatively related to the infestation levels of Pasteuria penetrans, and the abundance of predatory nematodes in 2004. Factors that might have affected the performance of sunn hemp on nematode communities are discussed.

Anaerobic Soil Disinfestation and Soilborne Pest Management

Anaerobic soil disinfestation is a biologically based, preplant soil treatment that provides an alternative to chemical fumigation for soilborne pest and disease management. The method involves the incorporation of organic amendments that contain a labile carbon source, covering soil with clear or gas-impermeable polyethylene tarp, and irrigating until saturation. Populations of facultative anaerobes utilize the carbon source and deplete the soil of oxygen for a short period of time. The approach has been tested for use in multiple crops, soil types, and regions and inputs can be modified to target specific pathogens. It is a particularly valuable tool for use in organic production, for use in fumigant buffer zones, and for use in regions where soil solarization is not feasible due to environmental constraints. In addition to oxygen depletion, the mechanisms involved in disease suppression include biological control and the production of organic acids. Significant shifts in the composition of the soil microbial community occur during the treatment, and the effects of ASD can be seen beyond the first cropping system and have the potential to contribute to the development of a disease-suppressive soil.

ALLELOCHEMICALS AS BIOPESTICIDES FOR MANAGEMENT OF PLANT-PARASITIC NEMATODES

Many allelopathic compounds in their native or processed forms have potential for development as viable components of plant-parasitic nematode management strategies. Allelochemicals have been identified that possess differing levels of activity against a wide range of plant-parasitic nematodes. In general, these compounds are less toxic to nontarget species, and less persistent in soil than chemical nematicides. Operative mechanisms for plant-parasitic nematode control with allelopathic compounds include nematicidal activity, nematostatic activity, and nematode behavior modification. Allelochemicals are sometimes produced in large quantities in plant material or as agricultural waste, making the use of rotation crops, cover crops, and organic amendments effective means for production and/or distribution of the active compounds. A greater understanding of the effects of soil microbes and environmental conditions on allelopathic compounds is necessary to improve their efficacy for control of parasitic nematodes. Use of allelochemicals for nematode control will require that growers know specifically what types and population levels of nematodes are present in their production fields. Development of improved production and incorporation methods for rotation and green manure crops, and appropriate application methods for processed allelochemical compounds, will also enhance the efficacy and consistency of these compounds for nematode control.

Nonfumigant alternatives to methyl bromide for management of nematodes, soil-borne disease, and weeds in production of snapdragon (Antirrhinum majus)

Two field experiments were conducted in north Florida to examine effects of solarization and reduced-risk pesticides on weeds, nematodes, soil-borne diseases, and yield of snapdragon (Antirrhinum majus). Five treatments were replicated five times in a randomized complete block design: fumigation with 80:20 methyl bromide:chloropicrin, solarization, solarization + Kodiak® (product with Bacillus subtilis), solarization + Biophos™ (product with dipotassium phosphonate and dipotassium phosphate), and non-treated control. Methyl bromide was generally superior to treatments involving solarization, which in turn were superior to the control, for improving flower yield and managing weeds, nematodes, and mortality due to plant disease. In the second year, solarization was more effective than methyl bromide in reducing plant mortality; however, surviving plants were larger and more productive following methyl bromide. In the second year, Biophos™ + solarization was as effective as methyl bromide in improving plant height, but not yield. However, integrating solarization with Biophos™ did not provide benefits over solarization alone in terms of flower yield. In some cases, solarization alone provided results similar to methyl bromide for managing weeds, nematodes, and plant disease mortality.

Anaerobic soil disinfestation is an alternative to soil fumigation for control of some soilborne pathogens in strawberry production

Alternatives to soil fumigation are needed for soil-borne disease control. Our goal was to test anaerobic soil disinfestation (ASD) as an alternative to soil fumigation for control of critical soil-borne pathogens in California strawberry production. Controlled environment experiments were conducted at 25°C and 15°C testing different materials as carbon sources for ASD using soil inoculated with Verticillium dahliae. Field trials were conducted in three locations comparing ASD with 20Mg ha−1 rice bran (RB) against fumigated and untreated controls, steam, mustard seed meal and fish emulsion. In ASD treated soils, temperature and anaerobicity were critical for control of V. dahliae, but multiple carbon inputs reduced inoculum by 80-100%. In field trials, ASD with RB provided control of a number of pathogens, and in three of four trials produced marketable fruit yields equivalent to fumigation. Little weed control benefit from ASD was found. ASD with RB also induced changes in the soil microbiome that persisted through the growing season. When equivalent yields were obtained, net returns above harvest and treatment costs with ASD RB were 92–96% of those with bed fumigation based on average prices over the previous 5 years. ASD can be a viable alternative for control of some soil-borne pathogens. Growers are adopting ASD in CA strawberry production, but research to determine optimal soil temperatures, anaerobicity thresholds and carbon sources for effective control of specific pathogens is needed. This article is protected by copyright. All rights reserved.

The Effect of Transitional Organic Production Practices on Soilborne Pests of Tomato in a Simulated Microplot Study

The perceived risk of pest resurgence upon transition from conventional to organic-based farming systems remains a critical obstacle to expanding organic vegetable production, particularly where chemical fumigants have provided soilborne pest and disease control. Microplots were used to study the effects of soil amendments and cropping sequences applied over a 2-year transitional period from conventional to organic tomato (Solanum lycopersicum) cultivation on the incidence of bacterial wilt caused by Ralstonia solanacearum, purple nutsedge (Cyperus rotundus) reproduction, root galling by Meloidogyne incognita, and soil nematode populations. A continuation of tomato monoculture during the transitional period resulted in a disease incidence of 33%, as compared with 9% in microplots that were rotated with sunn hemp (Crotalaria juncea) and Japanese millet (Echinochloa crusgalli var. frumentacea). The benefits of disease control from a crop rotation extended into to a second season of organic tomato cultivation season, where bacterial wilt declined from 40% in microplots with a tomato monoculture to 17% in plots with a crop rotation sequence. Combining applications of urban plant debris with a continued tomato monoculture increased the incidence of bacterial wilt to 60%. During the transition period, tomato plants following a cover crop regime also had significantly lower levels of root galling from root-knot nematode infection compared with plants in the continuous tomato monoculture. Nutsedge tuber production was significantly increased in plots amended with broiler litter but not urban plant debris. Compared with a continuous monoculture, the results illustrate the importance of a systems-based approach to implementing transitional organic practices that is cognizant of their interactive effects on resident soilborne disease, weed, and pest complexes.

First Report of Colletotrichum lupini on Lupinus hartwegii and L. mutabilis

During the 2013 winter cut flower production season, a severe anthracnose epidemic was observed on Lupinus mutabilis (syn L. cruckshanksii) on a commercial flower farm in Martin County, FL. Approximately 50% of the crop was lost to the disease. Symptoms included dark brown, irregularly shaped leaf spots, but more typically, there was a single severe twist in the stem, forming a distinctive necrotic crook. Margins of necrotic lesions were excised and surface sterilized by immersion in 1% sodium hypochlorite for 90 s, rinsed in sterile deionized water, and plated onto potato dextrose agar (PDA). Plates were incubated at approximately 27°C with cycles of 12 h light/12 h darkness. Infected tissue consistently produced colonies that were typical of the genus Colletotrichum. Conidia were primarily oval, with one rounded end and one pointed end, and were highly variable in size, ranging from 10 to 15 μm in length and 3.5 to 5.5 μm in width. Cultures were gray with orange spots, and no setae were observed. These morphological characteristics are consistent with those of Colletotrichum lupini (2). Identification of this species was confirmed by performing a BLASTn search with ITS sequence data (primers ITS4 and ITS5), which shared 99% identity with GenBank submission AJ301968, C. lupini var. setosum strain BBA 71310, isolated from L. luteus in Poland. Inoculum was produced by flooding PDA cultures with sterile deionized water, scraping with a rubber policeman, and passing the suspension through four layers of sterile cheesecloth. This preparation was used to inoculate 10 L. mutabilis and 10 L. hartwegii plants by injecting 10 μl of a suspension of 105 conidia/ml into the stem using a hypodermic needle (1). Ten additional plants were injected with sterile deionized water and maintained with the inoculated plants in the greenhouse for 4 weeks. All of the inoculated plants developed the previously-observed necrotic crook in the stem, whereas control plants developed no symptoms. The same organism was isolated from all inoculated plants. The ITS region was again sequenced, and the Polish strain was the closest match. The Floridian isolate sequence was deposited in GenBank (KF207599). Epidemics of anthracnose on ornamental lupins are common in most areas in which they are grown. In 1939, research plots of L. angustifolius were found with symptoms of anthracnose caused by Glomerella cingulata (3). Although it is not possible to determine if this isolate would be redefined as C. lupini, it does not seem likely since pathogenicity was confirmed on L. angustifolius and L. albus, but it did not cause infection on L. luteus (3) as has been reported for C. lupini (2). The finding of a lupin anthracnose in southeastern Florida is important to both the cut flower producers as well as vegetable producers who might consider some species of Lupinus as potential green manure crops. To the best of our knowledge, this is the first report of C. lupini or any Colletotrichum species on L. hartwegii and L. mutabilis in Florida. References: (1) W. H. Elmer et al. Plant Dis 85:216, 2001. (2) H. I. Nirenger et al. Mycologia 94:307, 2002. (3) J. L. Weimer. Phytopathology 43:249, 1943.

USDA-ARS Research on Practices Compatible with Organic Agriculture for Management of Plant-Parasitic Nematodes on Vegetable Crops

Abstract The market for organically grown fruits and vegetables has been increasing in recent years, and research is vital for obtaining optimal quality and yields in organic production systems. Scientists at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) are investigating methods for managing plant-parasitic nematodes on these crops, and studies that involve practices appropriate for organic vegetable production are reviewed in this paper. The projects summarized here focus primarily on suppression of root knot nematode species, including Meloidogyne arenaria (Neal) Chitwood, M. hapla Chitwood, M. incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood. Projects from Florida include investigations of plant growth-promoting rhizobacteria (PGPR) and chitin amendments for management of nematodes on pepper (Capsicum annuum L.), muskmelon (Cucumis melo L.), and tomato (Solanum lycopersicum L.). In South Carolina, research programs focus on the identification, characterization, and development of host plant resistance to root-knot nematodes in bell and hot peppers (Capsicum L.), southernpea [cowpea; Vigna unguiculata (L.) Walp.], and watermelon [Citrullus lanatus (Thunb.) Matsumura and Nakai]. Collaborative research in Georgia and South Carolina concentrates on the utilization of root-knot nematode-resistant bell pepper for managing root-knot nematodes in double-cropped squash (Cucurbita pepo L. cv. Cougar) and cucumber (Cucumis sativus L.). Research conducted in Maryland involves the use of rye (Secale cereale L.) and velvetbean (Mucuna Adans.) cover crops as nematotoxin-producing soil amendments, and application of beneficial microbes and their metabolites for suppression of root-knot nematodes on bell pepper, cucumber, tomato, and muskmelon. This research contributes to development or improvement of nematode management strategies that do not rely on the use of synthetic nematicides.