Fungicide Resistance in Cucurbit Powdery Mildew Fungi

Abstract

Powdery mildew is the major cause of losses in production of cucurbits worldwide (Cohen et al., 2004; Křistkova et al., 2009; Lebeda et al., 2007b; McCreight, 2006) (Fig. 1a,b and 2a,b). This disease is caused by two obligate biotrophic ectoparasites: Golovinomyces cichoracearum s.l. (Gc) (syn. Erysiphe cichoracearum s.l.) and Podosphaera xanthii (Px) (syn. Sphaerotheca fuliginea) (Křistkova et al., 2009; Lebeda, 1983) (Fig. 3a,b and 4a,b). Their distribution and relative occurrence varies throughout the world (Bardin et al., 2009; Křistkova et al., 2009; Perez-Garcia et al., 2009). For example, both Px and Gc are important in Central Europe whereas Px occurs almost exclusively in the USA. Both cucurbit powdery mildew (CPM) species (Gc and Px) have high evolutionary potential and according to the terminology of McDonald & Linde (McDonald & Linde, 2002) could be considered as “risky” pathogens (Lebeda et al., 2007a). Pathogen populations with a high evolutionary potential are more likely to overcome plant genetic resistance and/or develop fungicide resistance than pathogen populations with a low evolutionary potential (Kuck & Rusell, 2006). CPM species are highly variable in their pathogenicity and virulence which is illustrated by the existence of a large number of different pathotypes and races (Jahn et al., 2002; Lebeda & Sedlakova, 2006; McCreight, 2006). Breeding of cucurbit crops for powdery mildew resistance has a long and successful history, with many resources of race-specific resistance now known in muskmelon (Cucumis melo) (McCreight, 2006). There is also excellent resistance in cucumbers (Cucumis sativus) (Jahn et al., 2002). Resistance has been bred in some cultivars of squash and pumpkin (Cucurbita pepo) and in gourds (Cucurbita spp.) (Jahn et al., 2002; Lebeda & Křistkova, 1994). Degree of suppression achieved with resistant cultivars is variable, partly due to pathogen adaptation. Additionally, there is tremendous variation within the different cucurbit crops, and incorporating resistance into all horticultural types is an enormous task. Thus utilising plant disease resistance is not an option for managing CPM for all farmers. Furthermore, resistant cultivars do not always provide adequate suppression to be utilized as the sole management practice.