Effect on Bacteria and Viruses

Survival of plant pathogenic bacteria in solarized soil was investigated in a limited number of studies. Agrobacterium species were found highly sensitive to solariza-tion, as bacterial population was reduced up to 72% in the studies of Stapleton and DeVay (1984), and decreased by 99% and 92% after solarization treatment in two italian nurseries (Raio et al. 1997). In this study, two strains of A. tumefaciens Smith and Townsend were eliminated within 4 weeks or markedly reduced after 2 months in sandy loam and silty clay soil, respectively. Khlaif (2003) reported consistent, though related to temperature and soil type, effects of soil solarization on Agrobacterium spp. population and on number of crown galls on peach, Prunus persica (L.) Batsch, and bitter almond roostock seedlings. Causal agent of tomato canker, Clavibacter michiganensis subsp. michiganensis (Smith) Davis et al., was also stated highly heat-sensitive, as Shlevin et al. (2004) estimated 7 days ED80 values and Antoniou et al. (1995) reported that population of bacteria strains of C. michiganensis subsp. michiganensis sharply declined 4-6 weeks after soil tarping in plastic tunnels. Tent solarization of apple and pear (Pyrus communis L.) trees infected by Erwinia amylovora (Burrill) Winslow et al. stopped canker expansion and eradicated bacterial population from many treated trees, though the results were dependent on tree species and ambient temperatures (Thomson 1996). Soil solarization failed to reduce the incidence of tomato bacterial diseases caused by Ralstonia solanacearum (Smith) Yabuuchi et al. (syn. Pseudomonas solanacearum Smith) either in the field (Chellemi et al. 1994, 1997, 1999) or in plastic greenhouse (Horiuchi 1991), though bacterial population was reduced by combining solariza-tion with a chemical treatment (Chellemi et al. 1999).

Fate of plant viruses after a soil solarization treatment was almost completely ignored by researchers, as only one study documented a stabilizing effect of heat treatment on tobacco mosaic virus degradation in a sandy loam soil (Triolo and Materazzi 1992).

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