Weeds in a study of Postic et

Weeds are among the major pests in the Philippine rice fields. If left uncontrolled, weeds can reduce yields tremendously. Some farmers, however, do not recognize the yield reducing effect of weeds as these do not cause visible damage and symptoms like those of insect pests and diseases(Philippine Rice Research Institute, 2001). Weeds affect crops by competing with them for different environmental resources like water and sunlight. When these effects occur concomitantly, the harm caused by weeds becomes even greater (Kadioglu, I. & Yanar, Y., 2004). Losses in crop yield and production caused by weeds are well documented in many studies (Akobundu, 1987; Swanton et al., 1993).
More importantly, weeds may serve as alternative hosts of some insect pests and plant pathogens that causes reduced yields (Philippine Rice Research Institute, 2001) but when colonized, may not exhibit disease symptoms (Postic et al., 2012).
Many plant pathogens colonize weeds or other non-cultivated plant species as alternative hosts (Caesar 1996; Leslie et al. 2004; Wisler and Norris 2005; Vrandecic et al. 2006; Cosic et al. 2008; Inderjit and Callaway 2008). Examples of bacteria and fungi having alternative host weeds occurred with the bacteriumRalstonia solanacearum and with fungiFusarium spp. and Bipolarissorokiniana.
Ralstonia solanacearum is known to infect a wide range of economically important crops and weeds. Since 1976 (Olsson, 1976) outbreaks of brown rot or bacterial wilt of potato caused by the bacterium R. (Pseudomonas) solanacearum race 3, biovar 2 have been reported in several Western European countries (Stead et al., 1996).
The bacterium infects natural solanaceous weed hosts like Solanum dulcamara (Olsson, 1976), S. nigrum (Hayward, 1991), S. cinereum (Graham and Lloyd, 1978). Tusiime et al. (1998) also reported a number of latently infected non-solanaceous weeds such as Amaranthus spp., Bidenspilosa, Galinsogaperviflora, Oxalis latifolia, Spergula arvensis, Rumexabyssinicum, Tagetesminuta, and Stellariasennii in highland Uganda.
Also, in a study of Postic et al. (2012) on 2008 and 2009, samples of weeds and plant debris were collected from 12 locations in eastern Croatia, and 300 Fusarium isolates colonizing them were identi?ed. They concluded that Fusarium spp. may be isolated from numerous alternative hosts during the off season and that weeds and plant debris can serve as a reservoir of genetically diverse inoculum.
Weeds and wild plants external to crop ?elds and within them can serve as alternative hosts, usually when the economically important host plant is not present (Kaloostian et al. 1976; Bandyopadhyay et al. 2006). Plant debris also can serve as a reservoir for plant pathogens, under similar conditions.
Communities of plant pathogenic fungi are usually more diverse in organic debris than in the soil (McMullen and Stack 1983, Jeschke et al. 1990; Inch and Gilbert 2003a). Carry over of pathogens in plant debris can be especially problematic in zero or minimal tillage cropsystems, where plant residue is left on the soil surface (Sumner et al. 1981; Krupinsky et al. 2002). For example, Gibberellazeae can survive and produce inoculums (aeciospores) on wheat residue for at least two years after harvest (Pereyra et al. 2004).
In another study conducted in Eastern India concerning spot blotch caused by B.sorokiniana(syn. Helminthosporium sativum, teleomorph Cochliobolus sativus), a serious disease of wheat in the Mega Environment 5 (ME 5) characterized by a warm humid climate (Duveiller et al., 1998a; Joshi et al., 2002), the researchers concluded that only three weeds among the twenty-two collected samples of weeds and grasses, namelySetariaglauca, Echinochloacolona and Pennisetumtyphoids, were found to naturally harbor B. sorokiniana. It was also reported that the average yield loss caused by the said leaf blight (spot blotch, tan spot, and Alternaria blight) in south Asia is around 20% (Saari, 1998), but yield losses between 20% and 80% have also been reported by Duveiller and Gilchrist (1994). Under severe conditions, the yield losses may be as high as 100% (Srivastava et al., 1971; Mehta, 1994).
Some weed species play an important role as secondary hosts for the virus and the vector (Tamada and Baba, 1973; Schlosser, 1989; Hugo et al., 1996; Subikova&Bojnansky, 1998, Mouhanna, 2008).
Echinochloa colona (L.) Link. is an annual weed belonging to the family Poaceae and is known to have become one of the world’s most serious grass weeds (Holm et al., 1991; Chauhan ; Johnson, 2009). It is a major weed in many crops, including rice, corn, sorghum, sugarcane, cotton, peanut, and cassava (Holm et al., 1991). The losses in crop yield production due to E. colona have been reported in several studies (Chander et al., 2008). This weed is also an alternate host of diseases, insects, and nematodes (Holm, et al., 1991). E. colona is widespread throughout different habitat types. It is the dominant species of weed communities of summer crops and orchards in Egypt (Shaltout et al., 1992; Hegazy et al., 2004). It is characterized by a high relative growth rate (RGR) together with a high dry matter investment into leaves, during seedling and juvenile stages. This promotes the competitive ability of the species and may ensure a resource turnover from vegetative to reproductive structures later in the plant life cycle (Hegazy et al., 2005). E. colona begins to produce flower buds early in its lifespan, a behavior that ensures some seed production even in years with a short growing season (Hegazy et al., 2005).
Xanthomonas oryza epv. oryzae is a major pathogen of rice and is a threat to rice production in both temperate and tropical rice-growing regions, due to its high epidemic potential (Mew, 1987; Sakthivel, 2001). Yield losses of 10–20% are common and losses of 50–70% have been recorded in severely infected fields (Raina et al., 1982; Mew, 1987; Mew et al. 1993; Sakthivel, 2001). The spread of bacterial leaf blight (BLB) occurs through plant debris (Goto et al., 1953; Guo et al. 1980; Sakthivel, 2001), wild rice (Aldrick et al., 1973; Sakthivel, 2001), weeds (Goto et al., 1953; Valluvaparadesasan and Mariappan, 1989; Sakthivel, 2001) and water (Singh, 1971; Srivastava, 1972; Sakthivel, 2001).
Knowing that E. colona may be an alternative host to the plant pathogenXoo, farmers may have the option to directly weed them out or spray fungicides. With this knowledge, farmers not only downgrade the risk of their rice crops having BLB symptomsbut at the same time, save time and money from using fungicides.


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