A. Gamliel

Involvement of soluble organic matter in increased plant growth in solarized soils

Abstract Although soil solarization is used to control soil-borne pests, it also results in increased growth response (IGR) of plants, beyond the effect of pest control. IGR is attributed to various abiotic factors (e.g. increased mineral nutrient concentrations) and biotic factors. In this work, we studied the role played by dissolved organic matter (DOM) in soil extracts in the IGR. DOM concentrations were about twice as high in solarized soil than in untreated soil. In two out of three soils, solarization appeared to increase amino acid synthesis, indicating that it had a favorable effect on microbial activity. Elemental composition, carbohydrate levels, E4 : E6 ratios and FTIR spectra did not differentiate between DOM extracted from solarized soils and DOM extracted from untreated soils. Growth of corn plants increased with increasing concentrations of DOM. Addition to the soil of DOM extracted from leonardite increased populations of fluorescent pseudomonads, known as beneficial bacteria, and reduced fungal populations. We conclude that the increase in DOM concentration following soil solarization is a potentially positive plant-growth-enhancement factor.

Combined soil treatments and sequence of application in improving the control of soilborne pathogens.

ABSTRACT The effects of reduced doses of methyl bromide (MB) or metham sodium, heating, short solarization, and soil microbial activity, alone or in combination, on survival of soilborne fungal pathogens were tested in a controlled-environment system and field plots. Sublethal doses of heating or MB delayed germination of Sclerotium rolfsii sclerotia. Combining MB and heating treatments was more effective than either treatment alone in controlling S. rolfsii and Fusarium oxysporum f. sp. basilici. The application heating followed by fumigation with MB, was significantly more effective in delaying and reducing germination of S. rolfsii sclerotia and in controlling F. oxysporum f. sp. basilici than the opposite sequence. Further, incubation in soil and exposure to microbial activity of previously heated or MB-treated sclerotia increased the mortality rate, indicating a weakening effect. Similarly, incubation of chlamydospores of F. oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici in soil in the field after fumigation further reduced their survival, confirming the laboratory results. In field tests, combining MB or metham sodium at reduced doses with short solarization was more effective in controlling fungal pathogens than either treatment alone. Treatment sequence significantly affected pathogen control in the field, similar to its effect under controlled conditions. This study demonstrates a frequent synergistic effect of combining soil treatments and its potential for improving pathogen control and reducing pesticide dose, especially when an appropriate sequence was followed.

Reduced Dosage of Methyl Bromide for Controlling Verticillium Wilt of Potato in Experimental and Commercial Plots

The use of gas-impermeable films to reduce the dosage of methyl bromide (MB) required to control Verticillium wilt in potatoes was examined in field experiments, conducted in soils naturally infested with Verticillium dahliae. The incidence and severity of Verticillium wilt were significantly reduced (by 74 to 94%) by fumigation with MB at 50 g/m2 under standard low density polyethylene (LDPE) or at 25 g/m2 under gas-impermeable films. Fumigation at 25 g/m2 under LDPE was less effective. Disease severity was inversely correlated (r2 = 0.89 to 0.91) with chlorophyll content in the leaves. Fumigation also reduced (by 89 to 100%) stem colonization by the pathogen. Potato yield in the fumigated plots was significantly higher (26 to 69%), than in their nonfumigated counterparts, and was inversely correlated with disease index (r2 = 0.69 to 0.9). The percentage of high-value tubers (above 45 g) was 52 to 56% of total yield in the fumigated plots as compared with 32 to 40% in the nonfumigated controls. Thus, fumigation also improved the commercial quality of tuber yield. Effective control of V. dahliae and yield increases following MB fumigation at the recommended dosage or at a reduced dosage with gas-impermeable films was also observed in a consecutive crop. These results were verified in a large-scale field experiment using commercial applications, further demonstrating the feasibility of reducing MB dosages under farm conditions, without reducing its effectiveness in terms of disease control and yield improvement.

Glutathione and glutathione-S-transferase in fungi: Effect of pentachloronitrobenzene and 1-chloro-2,4-dinitrobenzene; Purification and characterization of the transferase from Fusarium

Abstract Shortly (3–6 hr) after exposure to either the fungicide pentachloronitrobenzene (PCNB) or 1-chloro-2,4-dinitrobenzene (CDNB) the pool of reduced glutathione (GSH) in Fusarium oxysporum f.sp. melonis and Rhizoctonia solani is drastically depleted, followed by a gradual increase in GSH levels. A similar pattern regarding the effect of the above compounds on glutathione-S-transferase (GST) specific activity was observed in both fungi. There is no significant differences in either GSH levels or GST specific activities in F. oxysporum and R. solani, tolerant and sensitive to PCNB, respectively. The GST was purified from the cytosolic fraction of F. oxysporum using glutathione-agarose affinity chromatography as a major purification step. The enzyme was purified about 753-fold to apparent homogeneity, and its Michaelis-Menten parameters were calculated. Sodium dodecyl sulfate-polyacrylamide electrophoresis revealed one polypeptide of about 25,000 molecular weight.

Permeation of soil fumigants through agricultural plastic films

Plastic films are used in soil fumigation to control fumigant emission into the atmosphere. In previous studies it was shown that the plastic films are permeable to fumigant vapors. Virtually impermeable films (VIF) have been developed to reduce such emission and to increase the efficacy of pest control. A rapid, accurate, sensitive and simple method to measure the permeability of plastic films to soil fumigants that was developed in the present study is described in this paper. The method uses a static, closed system in which the tested film is fixed between two cells. The fumigant is sampled by a solid-phase microextraction method and measured quantitatively by gas chromatography. The method was used to assess the permeability of two plastic films — a low-density polyethylene film (LDPE) and a VIF — to commercial soil fumigants formulated individually or in mixtures. All the tested fumigants permeated through the commonly used LDPE film, in the following descending order of permeability: methyl isothiocyanate (MITC), methyl bromide (MBr), 1,3-dichloropropene (1,3-D; Telone), chloropicrin (CP). The VIF was impermeable to all the tested fumigants except MITC, the permeation of which was reduced by 40%. The permeation of some fumigants through LDPE films was influenced by the formulation used. The permeation of CP was increased when it was combined with MBr in Bromopic. With Telopic, a mixture of 1,3-D and CP, the permeation of 1,3-D through LDPE film was 62% greater than that of Telone, whereas that of CP was not affected. The permeation rates of both MBr and CP were 25–30% greater when they were formulated as a mixture in Bromopic than when they were formulated individually. The formulation of fumigants as mixtures of two components did not affect their permeability through VIF. This study showed that differences in the suitability of plastic films for soil fumigation can be measured easily in a laboratory. It also showed that the VIP was more effective than LDPE in reducing losses of fumigant to the atmosphere, thus allowing more efficient use of fumigants to manage soilborne pests. The presented method helps us to choose the most adequate film for optimizing fumigation efficacy, and reducing costs and environmental risks.

The border effect of soil solarization

Abstract A gradient of reduced effectiveness of solarization toward the edge of the plastic mulch is known as a ‘border effect’, corresponding with decreasing soil temperatures from the middle toward the edge of the mulched area. The cyst nematode Heterodera avenae , of wheat was completely controlled by solarization at 90 cm or more inward from the edge of the mulched plots. A gradient of decreasing effectiveness of nematode control in the mulched plot, toward the edge of the mulch, was observed. There was a similar gradient with height of wheat planted after solarization, and in the incidence of pod wart disease of peanuts caused by Streptomyces spp., planted as a second crop after soil disinfestation. Disease reduction was correlated with a decrease in population of streptomycetes. A similar, but less pronounced, border effect was observed when solarization was combined with methyl bromide or formalin at reduced dosages.

Toxicity of chloronitrobenzenes toFusarium oxysporum and rhizoctonia solani as related to their structure

The structure-activity relationship of several chlorinated nitrobenzenes was studied using two soilborne fungi,Fusarium oxysporum f. sp.melonis Snyder and Hansen andRhizoctonia solarii Kühn. Fungitoxicity increased with the increase in number of chlorine substituents and was also affected by the position of the halogens on the phenyl ring. A linear relationship was obtained when the fungitoxicity values (EC50) of the compounds were plotted against their lipophilicity values calculated from octanol-water partition coefficient π.R.solarii was much more sensitive thanF. oxysporum to chloronitrobenzenes, particularly with respect to the pentachloro derivative.

Improved technologies to reduce emission of methyl bromide from fumigated soil

Methyl bromide (MB) is the chemical most widely used for soil fumigation in intensive agriculture, and for commodity and postharvest quarantine treatments. MB was listed by the Montreal Protocol in 1992 as a controlled ozone-depleting substance, and a phase-out process has been initiated. Several technologies to reduce the fumigation dosage and subsequent emission of MB from the fumigated soil were tested and applied in field trials and commercial application. These include dosage reduction by using impermeable films, improving uniformity of distribution, and preventing possible escape sources such as the edges of the fumigated plot. Combining MB with other pesticides, solarization, or biocontrol agents is another approach to reducing MB emission and dosage. Adapting these technologies may result in a 60–90% reduction of MB emitted from fumigated soil.

Plant Health Management: Soil Solarization

Soil solarization is a method of soil-disinfestation based on its solar heating by mulching a soil with a transparent polyethylene during the hot season, thereby controlling soilborne pests. Pathogen and disease control are attributed to microbial, chemical, and physical processes in addition to the thermal killing. These occur in the soil during the solarization treatment and even after its termination. Frequently, a beneficial microbial shift is created in the solarized soil, resulting in soil suppressiveness. Soil solarization can be combined with other control measures for an integrated approach, thus improving its performance. The uses of soil solarization have expanded beyond soil disinfestation including structure disinfestation, sanitation, controlling human pathogens, and more.