Effectiveness of soil solarization on phytoparasitic nematodes was generally found less consistent than on phytopathogenic fungi and weeds (Lamberti and Greco 1991; McGovern and McSorley 1997). Greater soil depths inhabited by phytopathogenic nematodes and their rapid migration to upper soil layers after solarization treatment, as resulting in a quicker recolonization of solarized soil compared to fungal pathogens and weeds, were hypothesized to account for this lower effectiveness (Porter and Merriman 1983; Heald and Robinson 1987; Abdel-Rahim et al. 1988; Cartia et al. 1989; Stapleton and Heald 1991). Nematode soil recolonization was found to be delayed by repeating solarization treatment for 2 or 3 years (Candido et al. 2008), though Sharma and Nene (1990) reported a similar suppression of nematode population for single- and two-season field solarization. Moreover, solarization effect on soil nematode densities was also observed to be sometimes not immediately evident or, inversely, to last for several months, due to biotic and abiotic changes occurring in the solarized soil. Stapleton and DeVay (1983) observed that suppressive effects of solarization on Helicotylenchus digonicus Perry became evident only 3 months after the treatment, whereas Walker and Wachtel (1988) reported the infection of juveniles of Meloidogyne javanica Treub by Pasteuria penetrans Sayre and Star as increased for 10 months after soil solarization.
Since the first demonstration of soil solarization effectiveness against phytone-matodes (Katan et al. 1976), nematicidal effects of solar heating were largely investigated on many genera of plant-parasitic nematodes.
Porter and Merriman (1983) documented a high sensitivity to solarization treatment for Mesocriconema xenoplax (Raski 1952) Luc & Raski 1981, M. javanica, Pratylenchus penetrans Cobb and Tylenchulus semipenetrans Cobb within a 38-55°C temperature range. Densities of reniform nematode, Rotylenchulus reni-formis Linford & Oliveira, were strongly reduced by solarization in field experiments on cowpea (Heald and Thomas 1983; Heald and Robinson 1987), though a quick nematode recolonization was observed in other studies (McSorley and Parrado 1986; Sharma and Nene 1990). Thickness, but not color, of mulching film and season of treatment were found to affect suppressivity of heat treatment on R. reniformis (Coates-Beckford et al. 1997; Coates-Beckford et al. 1998). Soil solarization showed a great effectiveness against the bulb nematode Ditylenchus dipsaci Filipjev, as only 10%, 6%, and 2% of D. dipsaci population was found still viable after 4, 6, and 8 week solarization, respectively, in southern Italy, (Greco et al. 1985), and garlic bulbs were found throughout the growing season in solarized fields heavily infested by D. dipsaci in Israel (Siti et al. 1982). Cyst-forming nema-todes have been also reported to be effectively controlled by soil solarization, as hatching of the golden cyst nematode G. rostochiensis from solarized soil was reduced by 100%, 68%, and 59%, at 5, 10, and 15 cm depth, respectively, compared to nontreated soil (LaMondia and Brodie 1984). Soil solarization in southern Italy strongly suppressed viability and reproduction of G. rostochiensis either in the soil or on potato roots (Greco et al. 2000) and reduced to only 24-38% egg survival of the carrot cyst nematode, Heterodera carotae Jones (Greco et al. 1985). A suppres-sive effect of soil solarizing treatment was reported also on the population of Heterodera ciceri Vovlas et al. in soil and in chickpea roots (Di Vito et al. 1991) and on Heterodera cajani Koshi (Chauhan et al. 1988; Sharma and Nene 1990).
Some field studies documented the failure of soil solarization for the control of root-knot nematodes, Meloidogyne spp. (Greco et al. 1985; Barbercheck and Von Broembsen 1986). Failure was generally attributed to a protective role against stress factors exerted by the gelatinous matrix aggregating Meloidogyne egg masses (Daulton and Nusbaum 1961; Orion 1995), though Nico et al. (2005) adversely found no effect of this gelatinous matrix on survival of M. incognita eggs in solarized soil. However, a successful application of solar heating against root-knot nematodes was also reported in other field researches, as soil solarization significantly decreased root galling of M. incognita on lettuce and cantaloupe in a 3-year experiment in Italy (Lamberti et al. 2000), almost completely suppressed root-knot nematode down to 20 cm depth in a 4-year investigation in Croatia (Ostrec and Grubisic 2003), and reduced root-knot severity and population densities of M. incognita in Florida (McGovern et al. 2002). An excellent control of root-knot nematodes by soil solarization was generally found under greenhouse conditions (Cenis 1984; Cartia et al. 1989), though Ioannou (2000) achieved only a 50% reduction of M. incognita infestation on tomato after 8-week soil solarization in Cyprus. In more recent greenhouse experiments, percentage plant infestation, root galling, and soil population densities of Meloidogyne spp. were strongly reduced or almost completely suppressed following a solarization treatment (Ostrec and Grubisic 2003; Candido et al. 2008) (Fig. 9.4).
An effective application of solarization was demonstrated against many other nematode species, such as Pratylenchus thornei Sher and Allen (Katan et al. 1976; Grinstein et al. 1979b; Greco et al. 1990a), Radopholus similis (Cobb) Thorne (Bhattacharya and Rao 1984), Hirschmanniella mucronata (Das) Luc and Goodey (Sivakumar and Marimuthu 1987), Helicotylenchus spp. (Sharma and Nene 1990;
Coates-Beckford et al. 1997; Coates-Beckford et al. 1998), or rice and wheat nematodes Aphelenchus spp., Helicotylenchus spp., Hirschmaniella spp., Pratylenchus spp., and Tylenchorhynchus spp. (Stapleton and Heald 1991; Pokharel 1995; Ganguly et al. 1996). Kluepfel et al. (2002) found that solarization in a peach orchard shifted soil microflora toward microbial species suppressive to M. xenoplax. Moreover, field trials in Florida reported also the suppression of B. longicaudatus, Criconemella spp., and Dolichodorus heterocephalus Cobb throughout the growing season following solarization, whereas uncertain results were found for Paratrichodorus minor (Colbran) Siddiqi (Chellemi et al. 1993; McSorley and McGovern 2000; McGovern et al. 2002). Bello et al. (2004) suggested that the ineffective management of the virus-vector nematode Xiphinema index Thorne et Allen by soil solarization in vineyard replant could be due to nematode survival on grapevine roots still viable up to 1 year after soil heating. Potential of soil solarization was tested also for the control of phytoparasitic nematodes in confined volumes of soils (Giblin-Davis and Verkade 1988). Heat treatment of soil in black polyethylene sleeves reduced by 89-100% the populations of T. semipenetrans, Pratylenchus vulnus Allen et Jensen, or M. xenoplax (Stapleton et al. 1999), and solarization of soil piles reduced by 95% egg hatch of M. incognita (Nico et al. 2003). Population density and infectivity of both M. incognita and R. reniformis were reduced by solarization in nursery beds, though R. reniformis showed a higher heat susceptibility (Gaur and Dhingra 1991).
A number of studies investigated the impact of solarization treatment on total soil nematophauna, agreeing that solarization dramatically also decreased the abundance of free-living nematodes (Stapleton and DeVay 1983; Stapleton and Heald 1991). However, free-living nematodes were found more likely to survive solariza-tion or rapidly colonize soil after solarization compared to the plant-parasitic nema-todes (Stapleton 2000; Ostrec and Rubisic 2003) and Nasr Esfahani (2007) recently reported that recover of soil nematophauna can be accelerated by combining the heat treatment with organic amendments. Overman (1985) found that soil solariza-tion was more effective at reducing total soil nematophauna than cover crops or herbicide fallow, and Culman et al. (2006) documented a significant decrease of nematophauna in solarized rice fields. Analysis of the impact of soil solarization on different soil nematode trophic groups showed that solarization disturbance on nematode communities disappeared at the end of the experiment and that omnivorous nematodes were more heat-sensitive than bacterivores and fungivores, whereas lowest sensitivity was exhibited by herbivores (Wang et al. 2006).
Long-term effectiveness of solarization on phytoparasitic nematodes was investigated with contrasting results, as under greenhouse conditions visual symptoms of root-knot nematode infection on cucumber were eliminated up to 1 year after soil solarization in Quatar (Atta-Aly 2007), whereas slight or no effects on M. incognita or M. javanica infestation on tomato were found after the same time interval in other trials in Cyprus and southern Italy (Ioannou 2000; Candido et al. 2008). Under field conditions, several reports stated that M. incognita infestation on cabbage and sweetpotato was reduced significantly for 2 consecutive years following solarization (Stevens et al. 2003). The rapid nematode soil recolonization after the thermal treatment is the main reason for the shorter residual effect of solarization on nematodes, though repeated solarization treatments may delay recolonization and progressively reduce population densities under economic threshold, thus enhancing the nematicidal effect (Candido et al. 2008).
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