Changes in functional structure of soil bacterial communities due to fungicide and insecticide applications in canola (original) (raw)

2009, Agriculture, Ecosystems & Environment

Cabbage seedpod weevil (Ceutorhynchus obstricus (Marsham)) has become a major pest of canola (Brassica spp.) in western Canada (Dosdall et al., 2002), often warranting chemical control (Cá rcamo et al., 2005). Two insecticides that are registered for use to control this pest in Canada are l-cyhalothrin (Matador 1) and deltamethrin (Decis 5EC 1). The same insecticidal compounds can be applied to control other damaging insect pest infestations in canola, including lygus bugs (Lygus spp.), bertha armyworm (Mamestra configurata (Walker), and diamondback moth (Plutella xylostella (L.)) (Anonymous, 2008). Scelerotinia stem rot (caused by Sclerotinia sclerotiorum (Lib) de Bary) is a major canola disease in western Canada (Turkington and Morrall, 1993; Bailey et al., 2003). Because cultural control measures such as crop rotation and sanitization are relatively ineffective (Williams and Stelfox, 1980), and management by canola host resistance has been difficult (Morrall and Dueck, 1982), effective stem rot control has been achieved by application of fungicides such as vinclozolin (Ronilan 1) and iprodione (Rovral Flo 1) (Anonymous, 2008). These fungicides also control alternaria blackspot, which is also an important disease affecting yield, pod shattering and green seed counts (Bailey et al., 2003) and is caused by Alternaria brassicae (Berk.) Sacc., A. brassicicola (Schwein.) Wiltshire, and A. raphani (Groves and Skolko). Information regarding non-target effects of these insecticides and fungicides on soil microorganisms in western Canada is required. Most field studies on fungicide or insecticide effects indicate that when they are applied at recommended rates, they usually have no significant effects or have transitory effects on soil microbial characteristics (Ahtiainen et al., 2003; Griffiths et al., 2006; Vig et al., 2008). In a study of the effects of 19 years of cumulative annual field applications of five pesticides, either singly or in combination at recommended or slightly above recommended rates, Hart and Brookes (1996) reported no measurable harmful effects on soil microbial biomass or its activity (C or N