Chenopodium album (fat hen)

C. album is responsible for important economic losses in agriculture around the world. Except in the extreme desert climate, C. album is found in all inhabited areas of the world where it thrives on all soil types and over a wide range of pH values (Holm et al., 1977). A survey conducted in Canada in 1991 showed that weeds caused estimated average annual losses of $984 million (Swanton et al., 1993). In the USA, a similar survey conducted on 46 crops showed average annual losses of $4.1 billion with current control strategies and $19.6 billion if herbicides were not available (Bridges and Anderson, 1992). Among the weeds implicated with those losses, C. album has been classified as one of the world's worst (Holm et al., 1977; Mitich, 1988). Its worldwide distribution, its ability to colonise new habitats and produce large quantities of seeds with viability extended over several years, its allelopathic potential, as well as the evolution of herbicide resistant biotypes have made C. album a major weed problem in agriculture (Holm et al., 1977; Mitich, 1988; Holt and Lebaron, 1990).

C. album reduces crop yield by direct competition for light and nutrients. In field and greenhouse experiments, important losses due to C. album have been reported on many crops including maize, soyabeans, tomato, oat, barley, lucerne, and sugarbeet. At the density of 172 to 300 plants/m², C. album was reported to cause between 6 and 58% yield loss in maize in field experiments in Canada (Sibuga and Bandeen, 1980; Ngouajio et al., 1999). In Spain, Torner et al. (1995) reported 22.3% maize yield loss in irrigated field experiments when maize was allowed to compete with C. album at equivalent densities. In the USA, 59% maize yield losses were attributed to uncontrolled populations of C. album in field experiments (Dyck and Liebman, 1995). At a density of 1.6 C. album plants/m of soyabean row, Shurtleff and Coble (1985) observed 15% loss of soyabean seed yield in North Carolina, USA. In Iowa, USA, Staniforth and Lovely (1964) observed about 35% soyabean yield losses due to a natural weed population composed mainly of C. album. In tomato, field experiments conducted in the USA using 64 C. album plants/m of row showed 36% losses of marketable fruits (Bhowmik and Reddy, 1988). In oats, C. album interference in field experiments conducted in Canada caused about 60% losses of grain yield when the weed was allowed to compete with the crop for the entire growing season (Lapointe et al., 1985). Losses of 23-36% of barley yield were attributed to C. album competition in the USA (Conn and Thomas, 1987). Under greenhouse conditions in Canada, about 23% reduction of lucerne biomass yield was recorded by Lapointe et al. (1985) as a direct result of C. album competition.In field studies conducted in Colorado, USA, sugarbeet root yield was reduced by 48% when competing with ca one C. album plant/m of row (Schweizer, 1983). When sugarbeet was grown with 13 C. album plants/m of row that emerged 10 days after the crop, root yield was reduced by 72% in Japan (Watanabe and Hirokawa, 1975). In contrast, when 22 C. album plants/m² were allowed to emerge simultaneously with sugarbeet in Wageningen, The Netherlands, yield losses as high as 93% were obtained (Kropff and Spitters, 1991; Kropff et al., 1992). When grown with a high density of 170 C. album plants/m², sugarbeet root yield was reduced by 86% (Holm et al., 1977). While yield losses due to C. album vary according to crop, weed density and location, in all cases reported, the crop losses were of significant economic impact.Crop seed contamination by weed seeds not only contributes to weed propagation, but also causes important losses in crop seed quality and value. C. album seeds are very small and frequently contaminate crop seeds harvested from weed infested fields (Holm et al., 1977). For example, C. album seeds are frequently found as impurities in many cereal seeds. Williams (1963) reported a carrot contamination rate of one-third at an official seed testing station in the UK. In the USA, contamination of legume seed by C. album has also been reported (Isely, 1960).While allelopathic effects of crop plants or crop residues on weeds are beneficial to farmers, the reverse may cause important economic losses. C. album has been reported to exhibit allelopathic effects on crop plants including maize, soyabeans, carrots, cucumbers, onions, tomatoes, sunflowers, lettuce, squash (Cucurbita maxima) and oats (Bhowmik, 1982; Reinhardt et al. 1994). In the USA, C. album residues were reported to cause 15-30% reduction of maize and soyabean growth under field conditions (Bhowmik and Doll, 1980) and 16-20% soyabean yield losses under glasshouse conditions (Bhowmik, 1982). In laboratory and greenhouse studies conducted in South Africa, Reinhardt et al. (1994) reported 68, 85, 47 and 51% growth inhibition by C. album residues on cucumbers, onions, tomatoes and sunflowers, respectively.The loss of herbicide activity as a result of evolution of resistance among weed populations has become a major concern in agricultural communities over the last three decades (Holt and Lebaron, 1990; Holt, 1992). C. album has been selected for resistance to several herbicides including triazines, substituted ureas, bromoxynil and pyrazon (Solymosi et al., 1986; Vencill and Foy, 1988; De Prado et al., 1989; Hagood, 1989; Holt and Lebaron, 1990; Myers and Harvey, 1993; Glenn et al., 1997). Herbicide-resistant biotypes of C. album have been reported in many countries including Belgium, Bulgaria, Canada, Chile, Czech Republic, Italy, France, Germany, New Zealand, Norway, Poland, Slovenia, Spain, Switzerland, The Netherlands, the UK and the USA (Heap, 2000). C. album resistance to herbicides has an important economic impact on agricultural production. Resistant biotypes cause direct losses from competition, especially in no-till production systems. Their control requires the use of alternative herbicides or integrated management systems that include herbicide combinations as well as non-chemical methods (Hagood, 1989; Holt and Lebaron, 1990; Holt, 1992; Myers and Harvey, 1993; Glenn et al., 1997). The additional cost of controlling resistant weed biotypes may increase total farm inputs.

As an alternate host of several economically important pests and diseases, C. album is responsible for important indirect losses in agriculture. C. album was reported to be the host of a new plant disease caused by the fungus Stagonospora atriplicis in New Zealand (McKenzie and Dingley, 1996). In Japan, C. album was reported to be a host for Polymyxa betae (Abe and Ui, 1986). This fungus is a vector of rhizomania of sugarbeet caused by beet necrotic yellow vein virus (BNYVV) (Abe and Tamada, 1986). P. betae isolated from C. album was also reported to thrive on other plant species including spinach (Spinacia oleracea) (Abe and Ui, 1986). C. album is also the alternate host of several crop viruses. In the UK, Stevens et al. (1994) showed that C. album was susceptible to beet yellows virus (BYV). This disease, transmitted primarily by the aphid Myzus persicae, is responsible for up to 47% sugarbeet losses (Smith and Hallsworth, 1990). In the USA, C. album was reported to be a successful host of peanut stunt cucumovirus (Gillaspie and Ghabrial, 1998). In India, Sharma et al. (1998) showed that prunus necrotic ring spot virus (PNRSV) was transmitted to C. album, which was also shown to be susceptible to potato viruses M and S (Ksiazek, 1976).In Quebec, Canada, Bélair and Benoit (1996) reported C. album as an alternate host for the northern root-knot nematode Meloidogyne halpa. This nematode is a major constraint to carrot production in southwestern Quebec. The potato root-knot nematode, Ditylenchus destructor, was shown to infest C. album in South Africa, and thereby survive between crop seasons (De Waele et al., 1990). In South Africa, this nematode is also an important pest of groundnut. In Utah, USA, the insect Pemphigus betae (Homoptera: Aphididae) was shown to have a life cycle that alternates between cottonwood trees (Populus angustifolia) and the roots of C. album (Moran and Whitham, 1988). The beet leafhopper and the common stalkborer are insects that live on C. album, but spread to sugarbeet, tomatoes, corn and certain flowers (Wright, 1972; Mitich, 1988).

C. album is toxic to humans and animals. It produces pollen that causes hay fever (Wodehouse, 1971). C. album produces high concentrations of nitrate and oxalic acid, which are poisonous to many animals including swine and sheep when eaten in large quantities (Kingsbury, 1964; Schmutz et al., 1968; Everist, 1979). When eaten by dairy cows, C. album causes taint in milk (Mitich, 1988). Between 1951 and 1960 the estimated losses of beef cattle due to poisonous plants in 11 western states of the USA were over $17 million (Schmutz et al., 1968). According to the same source, losses of sheep and wool were estimated at nearly $6 million. In 1988, estimated losses of cattle and sheep in 17 western states of the USA were $145,330,080 and $23,779,350 respectively (Nielsen et al., 1988; Frandsen and Boe, 1991); C. album is one of the major species associated with those losses (Lorenz and Dewey, 1988).