Effect on Fungal Pathogens

Suppressivity of soil solarization on soilborne fungal pathogens is generally related to numerous variables in climate, soil conditions, population, and distribution of fungal propagules in soil (Pullman et al. 1979). However, a satisfactory fungicidal effect of solar heating treatment was documented on the most economically important plant pathogenic species.

Suppression of Fusarium spp. wilt diseases by soil solarization was largely investigated on many crops and in different experimental conditions. Gamliel and Katan (1993) hypothesized that, in addition or in the absence of direct thermal effect on pathogen survival, fluorescent pseudomonads and other microorganisms may be involved in this suppressive effect. Heat sensitivity of F oxysporum was found to change among the different special forms (Shlevin et al. 2004), but field solarization effectively reduced incidence of F. oxysporum wilt in cumin, Cuminum cyminum L. (Lodha 1995; Israel et al. 2005), corn, Zea mays L. (Ahmad et al. 1996), cotton, Gossypium hirsutum L. (Katan et al. 1983), watermelon, Citrullus lanatus (Thunb.) Matsum and Nakai (Martyn and Hartz 1986), and cabbage, Brassica oleracea var. capitata L. (Ramirez-Villapudua and Munnecke 1987, 1988). Tomato Fusarium wilt disease was successfully controlled by soil solarization in field trials in Florida (Chellemi et al. 1997), Israel (Gamliel and Katan 1993), and Turkey (Yücel and Qinar 1989), but reduction of fungal density was limited to the upper 5 cm of solarized soil in another study (Chellemi et al. 1994). Field studies in Australia reported that solarization treatment reduced inoculum levels of Fusarium oxysporum sp. dianthi Snyd. and Hans., but not disease symptoms in carnation (Dianthus caryophyllus L.) and watermelon (Porter and Merriman 1985). Field tests in potato and watermelon crops documented also a great reduction or a complete eradication of F. solani between 0 and 30 cm depth after a 30-60-day soil solarization (Mansoori and Jaliani 1996; Triki et al. 2001). Under greenhouse conditions, solarization was effective against F. oxysporum f. sp. dianthi in carnation (Elena and Tjamos 1992), successfully controlled Fusarium wilt disease in tomato (Ioannou 2000) and reduced by 69-95% population of Fusarium oxysporum f. sp. radicis-lycopersici in Israel (Shlevin et al. 2003). Moreover, greenhouse solarization provided an almost complete control of Fusarium oxysporum f. sp. niveum in watermelon (Gonzales-Torres et al. 1993) and F. oxysporum f. sp. melonis in melon (Tamietti and Valentino 2006).

A satisfactory control of soil densities of Phytophthora spp. and related diseases was documented in several solarization studies (Pinkas et al. 1984; Lopez-Herrera et al. 1997; McGovern et al. 2000; Benlioglu et al. 2005). Pinkerton et al. (2002)

observed a complete eradication of Phytophthora cinnamomi Rands from solarized soil, confirming similar findings of Barbercheck and Von Broembsen (1986) in South Africa and absence of activity of P. cinnamomi at 30 and 45 cm depth 2 or 4 weeks after the solar treatment, respectively, previously found by Pinkerton et al. (2000). Soil population of P. cactorum was strongly reduced or completely suppressed by soil heating in field trials in California and Spain (Hartz et al. 1993; Porras et al. 2007b), as well as a significantly lower infection of Phytophthora fragariae Hickman and Phytophthora citricola Sawada was also observed in strawberry (Fragraria x ananassa Duch.) roots from solarized soil (Hartz et al. 1993; Pinkerton et al. 2002). Field solarization trials in watermelon showed that propagules of Phytophthora drechsleri Tucker were greatly reduced or completely eliminated at 0-25 cm depth in solarized soil (Mansoori and Jaliani 1996), and soil densities of Phytophthora nicotianae Breda de Haan, Phytophthora solanacearum (Mont.) de Bary and Phytophthora capsici Leonian significantly decreased down to a 10-15 cm depth (Chellemi et al. 1994; Coelho et al. 1999, 2000). Finally, cherry (Prunus cerasus L.) crown rot caused by Phytophthora cambivora (Petri) Buisman was controlled for more than 12 months after solarization in Australia (Wicks 1988).

Application of solarization was successfully investigated also for the control of Verticillium wilt caused by V. dahliae in olive (Olea europaea L.) orchards (Tjamos et al. 1991; Lopez Escudero and Blanco Lopez 2001). Moreover, population of V. dahliae was eliminated down to 120 cm depth in a solarized grove of pistachio, Pistacia vera L. (Ashworth and Gaona 1982), and symptoms of Verticillium wilt were reduced by 86-100% also in apricot (Prunus armeniaca L.) and almond (P amygdalus Batsch.) (Stapleton et al. 1993). Trials on field vegetable crops indicated an effective control of artichoke (Cynara scolymus L.) verticillium wilt throughout three successive cropping seasons following solarization (Tjamos and Paplomatas 1988) and a consistent reduction also in potato (Davis and Sorensen 1986; Lazarovits et al. 1991), eggplant (Pinkerton et al. 2000; Ioannou 2001), and cotton (Pullman et al. 1981b; Melero-Vara et al. 1995). Morgan et al. (1991) demonstrated an effective control of Verticillium wilt in established tomato plants also by the application of solarizing mulches to planted soil. Positive results against Verticillium wilt were provided also by greenhouse solarization studies on tomato in Crete (Bourbos and Skoudridakis 1996), and Cyprus (Ioannou 2000), and on eggplant in Northern Italy (Tamietti and Valentino 2001).

Variable, though always significant, effects of solarization on incidence of diseases caused by Sclerotium rolfsii were described in various studies (Reynolds 1970; Grinstein et al. 1979a; Mihail and Alcorn 1984; Stevens et al. 1990b; Ristaino et al. 1991; Chellemi et al. 1997; Shlevin et al. 2003). Sclerotial viability of S. rolfsii was quickly reduced by more than 95% at 2.5 cm depth in solarized fruit orchards soil, though lower effects were found in deeper soil layers (Stapleton et al. 1989). Negligible or undetectable levels of inoculum of S. cepivorum, the agent of white rot in garlic (Allium sativum L.), were observed after solarization treatments in Spain (Basallote-Ureba and Melero-Vara 1993; Melero-Vara et al. 2000) and in New Zealand experimental sites (McLean et al. 2001). Lettuce drop caused by

Sclerotinia minor was also largely documented for a positive control by solarization (Hawthorne 1975; Vannacci et al. 1988; Sinigaglia et al. 2001; Patricio et al. 2006, 2007). Mortality of sclerotia of S. sclerotiorum (Lib.) de Bary exceeded 90% after soil solarization in Israel (Ben-Yephet 1988) and ranged from 53% to 100%, according to solarization period and experimental site, in New Zealand (Swaminathan et al. 1999). Phillips (1990) hypothesized this reduction of sclerotial viability as mainly due to microbial colonization and degradation of sclerotia weakened by sublethal temperatures raised by soil solar heating.

Control of M. phaseolina (Tassie) Goid by soil solarization was similarly reported to require optimal temperature and moisture conditions (Mihail and Alcorn 1984; Sheikh and Ghaffar 1987), due to the lower heat sensitivity of this species (Bollen 1985). Following soil solarization in field, no reduction of M. phaseolina inoculum was found at any soil depth by Hartz et al. (1987), whereas significant suppressions of pathogen population and reduced incidence of dry root rot symptoms were observed in clusterbean, Cyamopsis tetragonoloba (L.) Taub. and corn (Lodha 1995; Hameed and Aslam 1996; Lodha et al. 1997).

A number of reports documented also a consistent effect of soil solarizing on Rhizoctonia solani Kühn in various crops (Pullman et al. 1981; Chet et al. 1982; Kaewruang et al., 1989a,b; Keinath 1995; Katan 1996). In particular, final incidence and progress of crown rot and blight in impatiens, Impatiens walleriana Hook., and of strawberry and lettuce bottom rot caused by R. solani were efficiently reduced following a solarization treatment (McGovern et al. 2002; Pinkerton et al. 2002; Patricio et al. 2006; Patricio et al. 2007).

Under greenhouse conditions, tomato corky root rot disease caused by Pyrenochaeta lycopersici was effectively controlled by soil solarization in Canary Islands (Díaz Hernández et al. 2005), in Italy (Garibaldi and Tamietti 1984; Cartia et al. 1989; Cascone G D'Emilio A 2000), and in Portugal (Moura and Palminha 1994). Positive results were reported also in field experiments on furrow-irrigated soils in Egypt (Abdel-Rahim et al. 1988), whereas only a partial control was observed in four trials in Cyprus (Ioannou 2001). Pink root disease induced by P. terrestris (Hansen) Gorenz et al. in chive (Allium schoenoprasum L.) roots and in onion (Allium cepa L.) seedlings and bulbs was also generally found as dramatically reduced by soil solarization (Hartz et al. 1989; Gamliel et al. 2004; Chan-Jung et al. 2007).

Field soil solarization was effective in dramatically reducing or completely eliminating the infection of Pythium spp. in carrot (Daucus carota L.) and strawberry roots (Becker and Wrona 1995; Pinkerton et al. 2002), as well as reduced inoculum levels of P. aphanidermatum in watermelon and potato down to 25-30 cm soil depth (Mansoori and Jaliani 1996; Triki et al. 2001). Summer soil solarization in greenhouse reduced Pythium root rot even in the temperate climate of Denmark (Christensen and Thinggaard 1999).

Other fungal pathogens were also investigated for a potential control by soil solar heating treatment. Field and glasshouse experiments documented a high effectiveness of solarization against Plasmodiophora brassicae, the agent of crucifers clubroot disease (White and Buczacki 1979; Horiuchi and Hori 1983; Myers et al. 1983), though heat sensitivity of this pathogen was related to suitable soil moisture conditions (Porter et al. 1991). An effective management of Rosellinia necatrix (syn. Dematophora necatrix) was achieved by soil solarization in apple (Malus domestica Borkh.) and avocado (Persea americana Mill.) orchards (Sztejnberg et al. 1987; Freeman et al. 1990; Lopez-Herrera et al. 1998) and in apple nurseries (Sharma et al. 2002). Survival of the melon necrotic spot virus-vector Olpidium bornovanus (Satiyanci) Karling (syn. O. radicale) was strongly reduced in greenhouse trials, either in soil (Horita and Manabe 2005) or soilless cultivation (Guirado et al. 2005). Soil solarization in greenhouse demonstrated also to be an effective tool for the control of stem canker by Didymella lycopersici Kleb. in the hot summer climate of Morocco (Besri 1982).

Suppressive effect of soil solarization on fungal pathogens was investigated also in nursery or containerized soil, where solar heating demonstrated to be as effective as steaming or fumigation in reducing soil infectivity of Pythium spp., Fusarium spp., and R. solani in forest nurseries (Annesi and Motta 1994; Le Bihan et al. 1997; Salerno et al. 2000). Summer soil solarization in forest plant nursery resulted in a significant reduction of preemergence damping-off disease of seeds of Pinus radiata D. Don and Eucalyptus obliqua L'Herit (Kassaby 1985). In the same study, solarization reduced also post-emergence mortality of P. radiata seedlings and eradicated P. cinnamomi, F. oxysporum and Pythium spp. from artificially inoculated pine roots. Kaewruang et al. (1989) reported an effective control of root rot of gerbera (Gerbera jamesonii Bolus ex Hook.) by polyethylene bag solarization of potting mixes infested with P. cryptogea Pethybr. and Laff., F. oxysporum and R. solani. Inocula of P. myriotylum Drechsler, P nicotianae, and S. rolfsii were killed within 3-10 days, according to species sensitivity and inoculum depth, in solarized mounds of potting medium (Duff and Barnaart 1992). Solarization of shallow layers of growth medium in containers provided a high reduction of melon collapse caused by Monosporascus cannonballus Pollack and Uecker and a fast decline of ascospore viability (Cohen et al. 2000; Pivonia et al. 2002). Finally, soil solarization was successfully evaluated also for recycling piles of spent potting media (Zinati et al. 2002).

Beneficial effects of soil solarization on fungal pathogens control were commonly reported to last for about two growing seasons (Usmani and Ghaffar 1982; Stapleton and DeVay 1986; Greenberger et al. 1987; Davis 1991), and other studies also documented a suppression of soilborne disease for 1-3 years following solarization (Stapleton and DeVay 1982; Gamliel and Katan 1993; Ioannou 2000; Stevens et al. 2003). Solarization was found effective for at least two or three seasons for the control of Fusarium wilt in cotton, Verticillium wilt and shellspots in peanut, pink root of onion and Pythium tomato root rot (Katan 1981; Katan et al. 1983; Rabinowitch et al. 1985). Long-term effects of solarization against Verticillium wilt were documented in successive cropping seasons of safflower, Carthamus tinctorius (L.) Mohler et al., cotton and artichoke (Pullman et al. 1981; Tjamos and Paplomatas 1988), but also in the control of corky root in tomato and S. cepivorum white rot disease in onion (Abdel-Rahim et al. 1988; Satour et al. 1989). Diseases caused by S. minor and R. solani on lettuce were severely reduced in the second crop following solarization in a 2-year experiment (Patricio et al. 2006). Long-term effects were also reported on soilborne pathogens of tree crops, as no R. necatrix white root rot disease developed over 3 years and no death of replanted apple trees occurred up to 2 years in solarized apple orchards (Freeman et al. 1990). Effects of heat treatment on suppression of V. dahliae soil population and on tree recovery was observed for at least 3 years in olive orchards (Tjamos et al. 1991; López Escudero and Blanco López 2001). Effect of preplant soil solarization against P. cinnamomi extended over more than 5 years in avocado orchards, but Gallo et al. (2007) found that dead plant percentage was significantly lower than in nonsolar-ized soil also 11 years after the heat treatment. Long-term effect of solarization on soil pathogens cannot be explained solely by thermal inactivation, as it also involves a rapid soil recolonization by aggressive populations of heat-tolerant bacteria, actinomycetes and fungi, antagonistic to plant pathogens and contributing to soil suppressiveness (Tjamos and Paplomatas 1988; Kaewruang et al. 1989; DeVay 1991; Gamliel and Katan 1993; Pinkerton et al. 2000).

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