Weed management by soil solarization was widely investigated with variable responses either in field or greenhouse studies (Elmore 1991b; Yaduraju and Mishra 2004), though the best results were always reported in hot climate countries (Al-Masoom et al. 1993; Saghir 1997) (Fig. 9.5).
Solarization effects on weed population was hypothesized to be due to different mechanisms, such as changes in cell metabolism and ultrastructure (Singla et al. 1997), microbial parasitism on seeds weakened by sublethal temperatures, seed dormancy interruption by raising temperatures, and foliar scorching of weeds under the plastic mulch (Egley 1990; Katan and DeVay 1991). Moreover, imbalance of O2 and CO2 or release of acetaldehyde, ethylene, and other volatile toxic compounds were also reported as accounting for weed death (Rubin and Benjamin 1984; Gamliel et al. 2000).
Weed sensitivity to solarization treatment may be largely variable according to the different species (Economou et al. 1997; Elmore 1998). Based on heat sensitivity, Restuccia et al. (1994) classified weed species as sensitive, resistant, or not well defined (Table 9.1). The first group included about 80 annual species, further sub-
Table 9.1 Classification of weed species according to sensitivity or resistence to soil solarization as reported by literature
Sensitive species Annuals autumn-winter cycle Anagallis cemlea (Gouan) Schreb., Aram italicum Mill., Avena fatua L., A. sterilis L., Borago officinalis L., Brassica nigra Koch, Capsella bursa-pastoris (L.) Medicus, C. ribella Reuter, Centaurea ibérica Trevir., Coronopus didymus (L.) Smith, Chrisantemum coronarium L..Daucus aureus Desf., Emex spinosa L., Erodium spp., Heliotropium suaveolens Caruel, Hordeum leporinum LK., Lactuca scariola L., Lamium amplexicaule L., Medicagopolymoipha L., Mercurialis annua L., Montiapetfoliata (Donn. ex Willd) Howell, Notobasis syriaca (L.) Cass., Papaver dibium L., P. roheas L.a, Phalaris brachystachys LK., Ph. paradoxa L., Poa annua L., Polygonum equisetifonne S., Raphanus raphanistrum L., Senecio vemalis Raf., S. vulgaris L., Sinapis atyensis L., Sisymbrium spp., Solanum nigrum L., Sonchus oleraceus L., Stellaria media (L.) Villars, S. neglecta Weihe, Urtica membranacea Poir, U. wens h..Veronica spp.a Annuals spring-summer cycle Abutilón theophrasti Medic., Amaranthus albus L., A. blitoides S. Watson, A. retroflexus L.,
Cartamus syriacum Willd, Chenopodium album L., Ch. pumilio R.BR., Ch. murale L., Commelina communis L., Conyza bonariensis (L.) Cronq., Crysanthemum segetum L., Datura stramonium L., Digitaria sanguinalis (L.) Scop., Echinochloa crus-galli (L.) Beauv., E. colomum (L.) L.K., Elusine indica Gaertn, Eragrostis megastachya L.K., Fumaria judaica Boiss, F. mura-lis Koch, F. officinalis L., Geranium ¡nolle L., Ipomea lacunosa L., Malva pannflora L., M sylvestris L., Orobanche aegyptiaca L., O. crenata Forsk., O. ramosa L., Setaria glauca P.B., S. viridis (L.) P.B.a, Polygonum persicaria L., Sida spinosa L., Solanum nigmm L., Tribulus terrestris L., Vicia sativa L.a, Xantium pensilvanicum Wallr., Xantium spinosum L. Perennials Chloris gayaría Kunth, Cirsium atyense (L.) Scop. (rizoma-propagated)a Convolvulus althaeoides
L., C. atyensis L. (seed and rizoma-propagated), Cynodon dactylon (L.) Pers. (seed-propagated), Equisetum atyense L., E. ramosissimum Desf., Oxalis corniculata L., Plant ago spp., Sorghum halepense (L.) Pers. (seed-propagated) Resistent species Annuals Conyza canadensis L., Coronilla scorpioides (L.) Koch, Anchusa aggregata Lechm., Astragalus boeticus L., Melilotus sulcatus Desf., Scotpiorus muricatus L., Lavatera crética L., Malva nicaeensis All., Lathyrus ochrus (L.) DC Perennials Cyperus esculentus L., C. rotundus L.
Not defined species Annuals Xantium strumarium L., Portulaca oleráceo L., Solanum luteum L.
Perennials Cynodon dactylon (L.) Pers.(rizoma-propagated), Sorghum halepense (L.) Pers. (rizoma-propagated)
a Candido et al. 2006, 2008
divided into autumn-winter, spring-summer, and indifferent time germinating weeds. Compared to summer annual species, winter annual species, as they germinate in shorter day and lower temperature conditions, are more temperature-sensitive and require smaller temperature increases to be effectively controlled. A 1-week solarization was enough to control many susceptible winter annuals such as Poa, annua, Montia perfoiata (Donn ex Willd.) Howell and Senecio vulgaris L. (Katan and DeVay 1991), whereas summer annual species required higher solariza-tion temperatures and/or a longer duration (Egley 1990). Solarization-resistant group included either annual species, like leguminous Astragalus boeticus L., Scorpiurus muricatus (L.) Lam., Coronilla scorpioides (L.) Koch, and Melilotus sulcatus Desf., asteraceous Conyza canadiensis ( L.) Cronquist, and malvaceous Lavatera cretica L. and Malva nicaeensis All., or perennials, like Cyperus spp. Annual weeds P. oleracea, X. strumarium, and S. luteum Miller, and perennials C. dactylon and Sorghum halepense L. Pers. were classified as undefined behaviour species. Moreover, recent solarization studies in lettuce crop included leguminous weed Lathyrus ochrus (L.) D.C. among solarization-resistant species in field, while perennial Cirsium arvense (L.) Scop. was stated as sensitive to solarization treatment by Candido et al. (2006).
An effective control of annual weeds by solarization was generally documented, as in a prolonged experimental program Stapleton et al. (2005) found that solar heating reduced by nearly 100% a wide range of annual weed species, including yellow sweetclover (Melilotus officinalis L. Lam.), chickweed (Stellaria spp.), annual blue-grass (P. annua L.), shepherdspurse (Capsella bursa-pastoris L. Medikus), crabgrass (Digitaria spp.), and spotted spurge (Euphorbia maculata L. Small). Emergence of other annual summer or winter species, such as Amaranthus spp, Chenopodium spp, Coronopus didymus (L.) Sm., Digitaria sanguinalis (L.) Scop., E. crus-galli, Eleusine indica (L.) Gaertner., Galinsoga parviflora Cav., Medicago arabica (L.) Huds., S. nigrum, and Sonchus oleraceus L., was almost completely suppressed by soil solarization in other field and greenhouse trials (Elmore,1993; Moya and Furukawa 2000; Patricio et al. 2006; Candido et al. 2008). Egyptian broomrape (Orobanche aegyptiaca L. Pers.) and bean broomrape (O. crenata Forsk) were effectively controlled in carrot and eggplant and tomato crop, respectively, following a solarization treatment (Jacobsohn et al. 1980; Abdel-Rahim et al. 1988), though the best control of the above Orobanche species by solarization was found in hot seasons (Sauerborn et al. 1989). Hemp (O. ramosa L.) and nodding broomrape (O. cernua Loefl) were completely absent from solarized soil in field and greenhouse experiments on tomato (Abu-Irmaileh 1991a; Mauromicale et al. 2005). A solarization-tolerant behavior was often reported for P. oleracea (Elmore 1991b), and its seed germination was found to decrease only after a 2 or 1 h exposure to 60°C or 65°C, respectively (Verdu and Mas 2004). However, Dahlquist et al. (2007) reported a 39°C temperature sublethal to seeds of P. oleracea, and Patricio et al. (2006) found infestation of P. oleracea drastically reduced by field soil solarization. Cuscuta spp. was found tolerant to soil solarization by Abu-Irmaileh and Thahabi (1997), but Haidar and Iskandarani (1999) observed a strong reduction of soil seed bank Cuscuta spp. after a solarization treatment.
Perennial weeds were more difficult to control than annual species, maybe due to the occurrence of propagules at soil depths not exposed to lethal temperature (Rubin and Benjamin 1984). Thermal death of seeds of S. halepense and Convolvulus arvensis L. was observed only in the upper 3-4 cm of solarized soil (Standifer et al. 1984; Katan and DeVay 1991). Rubin and Benjamin (1984) reported that the heat sensitivity of rhizomes of C. dactylon (L.) Pers. and S. halepense, whereas sprouting of tubers of C. rotundus was increased by solarization in several field trials ((Egley 1983; Kumar et al. 1993; Elmore et al. 1997; Miles et al. 2002; Roe et al. 2004). Stimulation of Cyperus spp. emergence was found related to pronounced diurnal temperature fluctuations during solarization and was also influenced by polyethylene mulch properties (Miles et al. 1996; Webster 2005). Most studies documented failure of solarization for the control of perennial weeds and in particular of Cyperus spp. (Duranti and Cuocolo 1988; Rosskopf et al. 1999; Stapleton et al. 2005; Candido et al. 2008). Other authors observed only a partial elimination (Herrera and Ramirez 1996; Kamra and Gaur 1998), or a fast recover of C. rotundus following the removal of plastic film (Lira-Saldivar et al. 2004). A number of reports documented also an effective control of Cyperus spp. and other perennial weeds by solarization, either alone or combined with low rates of herbicides (Mushobozy et al. 1998; Marenco and Lustosa 2000; Ozores-Hampton et al. 2001; Gilreath et al. 2005). Extending solarization period up to 8-10 weeks was found to improve the control of C. rotundus and other perennial weeds (Rubin and Benjamin 1983; Chase et al. 1998), and in other studies 90-day solarization provided a significant reduction of the weed population which included C. rotundus (Stevens et al. 1990a; Ricci et al. 2000). Effect of extended soil solarization on Cyperus spp. was found relatively improved by the application of heat-retentive mulches, which provided the death of greater proportions of emerged weeds by foliar scorching (Chase et al. 1998, 1999a) and, moreover, a better residual weed control and a reduced effect of seed depth (Chase et al. 1999b). In other field and greenhouse reports, Patterson (1998) observed that the translucent solarizing mulches were strongly reduced or had totally suppressed emergence and growth of C. rotundus, compared to a conventional, opaque, white/ black polyethylene film.
Species-dependent heat sensitivity of weeds does not allow a complete control of weed flora by solarization. In a large field survey in food legume crops, solariza-tion suppressed 80% of weeds, without affecting or stimulating species with bulbs, heat-tolerant seeds, deep root systems, or perennial organs (Linke 1994). Approximately 40% of present weed species were found not sensitive to solariza-tion by Marenco and Lustosa (2000), and several weed species were reported as differently tolerant to solar heating by Satour et al. (1991). Tamietti and Valentino (2000) found monocotyledon species less controlled by solarization than dicotyledons, whereas Abdel-Rahim et al. (1988) documented the heat treatment as very effective on a wide spectrum of weeds, except annuals species like Avena sterilis L., A. retroflexus L., P. oleracea, Xantium strumarium L., Malva spp. and perennials Cirsium arvense (L.) Scop., Cynodon dactylon (L.) Pers., Cyperus spp., and Sorghum halepense (L.) Pers.
Length of mulching period, maximal soil temperatures, seed's vigor, and germination depth were also identified as main factors for solarization effectiveness on weeds (Abu-Irmaileh 1991b; Elmore 1991a). Horowitz et al. (1983) found that weed control was related to the number of days with temperatures above a 45°C threshold, though heat-sensitive species were killed after shorter solarization periods than heat-tolerant weeds (Standifer et al. 1984). Arora and Yaduraju (1998) reported that weed control was inversely related to seed depth in soil, as seeds in deeper soil layers were often found to escape the solarization effect (Egley 1983; Horowitz et al. 1983; Rubin and Benjamin. 1984). Standifer et al. (1984) showed that seeds of C. rotundus and E. crus-galli were killed only in the upper 3-4 cm and those of E. indica and Commelina communis L. within the upper 5 and 11 cm, respectively. Similarly, survival of Poa annua seeds was reduced in the upper 5 cm solarized soil and enhanced at deeper profiles (Peachey et al. 2001). As a consequence, soil disturbance after soil solarization reduces treatment effectiveness, due to the recontamination by viable seeds in deeper soil layers (Egley 1983; Abu-Irmaileh 1991a). Soil and seeds moisture are also involved in weed thermal death, as dry seeds were found still viable up to 120°C while exposure to 50°C was lethal to hydrated seeds (Rubin and Benjamin 1984). Moreover, seeds of various weeds survived a 70°C exposure for up to 3 and 7 days at 19% and 2% soil moisture regimes, respectively (Egley 1990). Wet soil solarization generally provided a more effective weed control than dry soil treatment (Horowitz et al. 1983; Arora and Yaduraju 1998), though Sales Beuno et al. (2003) found weed emergence negatively related to substrate permanence under moist conditions prior to solar-ization, and other authors reported a single preliminary irrigation as effective as repeated watering (Grinstein et al. 1979c; Horowitz et al. 1983). No effect of irrigation was found on germination or seed viability of Striga asiatica (L.) Kuntze in solarized soil (Osman et al. 1991). Color of solarizing mulches may also influence the herbicidal effect of solarization, as both black and clear films reduced weed populations, but a lower control and a shorter residual activity were provided by black compared to clear polyethylene film (Horowitz 1980; Horowitz et al. 1983; Standifer et al. 1984; Campiglia et al. 2000). Finally, Abu-Irmaileh (1991b) found that a further black polyethylene soil mulch after solarization may improve the treatment effect.
Solarization demonstrated to be an effective and inexpensive herbicidal treatment also in nursery beds (Patel et al. 1995; Eleftherohorinos and Giannopolitis 1999; Kumar and Sharma 2005), containerized soil and potting mixes (Stapleton et al. 2002), and even in newly established fruit orchards (Abu-Irmaileh 1994).
Residual effect of solarization treatment was found much more pronounced on weeds than on nematodes and most fungal pathogens, as Candido et al. (2008) reported a consistent reduction or a total suppression of annual and some perennial species present in a solarized greenhouse throughout 2 years following the treatment, and also later for C. dactylon. In previous experiments, soil of an olive orchard was weed-free for at least 3 years after solarization (Lopez-Escudero and Blanco-Lopez 2001), and residual effects of soil solarization on C. rotundus and C. esculentus were observed during four cropping seasons in a tomato-cucumber rotation (Gilreath et al. 2005). Moreover, persistence of weed control may be prolonged by the absence of soil disturbances after the treatment (Bell and Elmore 1983).
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