Thermal Mechanism

Solarization process increases soil temperatures up to levels lethal to many plant pathogens and pests and, therefore, direct thermal inactivation is the most important and normally expected mechanism. Some studies on the biochemical bases of sensitivity of organisms to high temperatures hypothesized that heat sensitivity is related to small differences in cell macromolecules, leading to a lethal increase of intra-molecular hydrogen, ionic, and disulfide bonds (Brock 1978). Sundarum (1986) suggested a reduced cell membrane function beyond an upper limit fluidity exceeded by high temperatures, concluding that mesophylic organisms, including soilborne pathogens and pests, are more sensitive to high temperatures due to the presence of low-melting-point unsaturated lipids in cell membranes, whereas ther-motolerant and haemophilic soilborne organisms survive soil solarization due to macromolecules stability at temperatures up to 60°C. Heat inactivation of respiratory enzymes was found by the same authors as a further cause for thermal decline of soilborne microorganisms and weed seeds (Brock 1978; Sundarum 1986).

Pullman et al. (1981) described the thermal effect of solarization on microorganisms and weeds as a function of a logarithmic inverse relationship, changing for the different target organisms, between soil temperature and exposure time. Damage thermal thresholds were found to begin around 39-40°C for most mesophilic soil organisms, whereas temperatures normally achieved during solarization treatment were survived by thermophilic and thermotolerant organisms (Stapleton and DeVay 1995). Thermal inactivation thresholds have been experimentally calculated for a number of important plant pathogenic fungi, nematodes, and weeds. Under laboratory conditions, Porter and Merriman (1983) found that a variety of fungal pathogens, viz. Fusarium oxysporum Schltdl., Pythium irregulare Buisman, Plasmodiophora brassicae Woron, Sclerotium cepivorum Berk, S. rolfsii Sacc, Sclerotinia minor Jagger, Verticillium dahliae Kleb., were killed by temperatures within the range 38-55°C, with the lowest sensitivity for F oxysporum, P. irregulare, and P. bras-sicae, whereas V. dahliae, S. cepivorum, and S. minor were the most sensitive species. The ED90 of V. dahliae was about 14 h at 37°C and reduced to about 9 min at 50°C (Pullman et al. 1981a), whereas Rosellinia necatrix Berl. ex Prill. (syn. Dematophora necatrix R. Hartig) was found highly heat-sensitive, as 50-100% mortality was recorded after 4 h at 38°C (Sztejnberg et al. 1987). Phytophtora cin-namomi Rands, P. cactorum (Lebert and Cohn) Schrot, and P. megasperma Drechsler were killed within 20, 30, and 30 min, respectively, in soil exposed at 45°C (Juarez-Palacios et al. 1991), whereas thermal death of fungal mycelium of P. cinnamomi was reported after 1-2 h at 38-40°C (Gallo et al. 2007). In other experiments, chlamydospores of P. cinnamomi were killed after only 30 min when directly suspended in water at 38°C (Barbercheck and von Broembsen 1986), or after a 25 min immersion in water at 41°C of infested wheat (Triticum aestivum L.) grains (Theron et al. 1982), suggesting that water was probably a more effective heat conveyor than either soil or agar (Benson 1978; Coelho et al. 2001). Other soilborne pathogenic fungi, such as Macrophomina phaseolina (Tassie) Goid and Pythium aphanidermatum (Edson) Fitzp., showed a lower heat sensitivity, as in soils with a high moisture content microsclerotia of M. phaseolina strongly declined within 24 h at 40°C, but were completely inactivated only at 50°C or higher (Mihail and Alcorn 1984; Sheikh and Ghaffar 1987).

Pullman et al. (1981a) reported a gradual accumulation of heat damage by the application of progressive time and temperature levels, up to complete fungal inac-tivation, suggesting that detrimental effects may be caused to target organisms also by sublethal thermal doses. Heat shock proteins involved in the acquisition of ther-motolerance or thermostability were found to be produced by fungal cells during sublethal heating (Plesofsky-vig and Brambl 1985; Lindquist 1986; Freeman et al. 1989). Sublethal temperatures may damage fungal pathogens, reducing germinability and aggressiveness of their propagules and increasing vulnerability to other biotic or abiotic agents (Freeman and Katan 1988; DeVay and Katan 1991). Sclerotia of S. rolfsii weakened by sublethal heating were found intensely colonized by Trichoderma rolfsii Rifai and other microorganisms (Lifshitz et al. 1983; Greenberger et al. 1984). Under laboratory conditions, vulnerability of propagules of R. necatrix to colonization by Trichoderma spp. was highly increased after an exposure to sublethal temperatures (Sztejnberg et al. 1987), whereas field tests showed that sublethal heating caused by a short solarization effectively controlled S. rolfsii, F. oxysporum f. sp. basilici (Dzidzariya) Armstr. & Armstr., F. oxysporum f. sp. melonis W.C. Snyd. and Hans., and F. oxysporum f. sp. radicis-lycopersici Jarv. and Shoem. when combined with reduced doses of methyl bromide or metham sodium (Eshel et al. 2000). Detrimental effect of sublethal temperatures on soil-borne pathogens was also suggested to explain the higher suppressiveness often observed in solarized soils (Greenberger et al. 1987).

Prolonged permanence at temperatures above 35°C were also found to be lethal to phytoparasitic nematodes or to reduce their infectivity and to increase their biotic and abiotic stresses vulnerability (Heald and Robinson 1987), though thermal effect of solarization on nematodes was stated as strictly species-related (Greco et al. 1998; D'Addabbo et al. 2005). Giblin-Davis and Verkade (1988) reported the death of Belonolaimus longicaudatus Rau and Hoplolaimus galeatus (Cobb) Thorne after a 2 h exposure of infested soil to 48°C ± 2°C. Juveniles within cysts of Heterodera glycines Ichinohe were completely killed within 1 s, 8 min, and 8 h at 63°C, 52°C, and 44°C, respectively (Endo 1962), whereas a 30 min exposure at 60°C was reported to be lethal for the potato (Solanum tuberosum L.) cyst nematode Globodera rostochiensis Woll. (Evans 1991). Walker (1962) reported a 6 min permanence at 48°C as the thermal death threshold for the eggs and the egg masses of root-knot nematodes, Meloidogyne spp., in accordance with the studies of Gokte and Mathur (1995), in which the eradication of root-knot nematodes from grapevine rootstocks was achieved after a 10-20 min treatment at 48-53°C. In recent studies, application of constant temperature-time dosages to soil infested by M. incognita Kofoid et White Chitw. resulted in LD95 values of 813, 281 and 32.4 min at 39°C, 42°C, and 46°C, respectively (Ruiz et a95l. 2003).

Time and temperature requirements for thermal death of weeds were also reported to change considerably among the target species. Egley (1990) indicated a 12 h exposure at temperatures from 50°C to 66°C as the LD50 of eight common weed seeds, confirming findings of Horowitz et al. (1983) that found an effective weed control by soil temperatures above 45°C. Similarly, in recent laboratory studies Dahlquist et al. (2007) observed that seeds of all tested weed species were killed at 50°C and above, though Portulaca oleracea L., Amaranthus albus L., Echinochloa crus-galli L., and Solanum nigrum L. were not affected by heating up to 46°C, 42°C, and 39°C. Purple nutsedge (Cyperus rotundus L.) was found to be less heat-sensitive, as its tubers viability decreased in an inverse linear pattern after a 30 min exposure at a 30-90°C temperature range (Rubin and Benjamin 1984). Simulation models were also developed to describe weed mortality as a function of heat treatment duration or daily fluctuating soil temperatures and, therefore, to predict time x temperature combinations needed for an effective weed control by solarization under field conditions (Dahlquist et al. 2007; Miles et al. 2002).

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