Dodders (Cuscuta spp.)

Dodders (Cuscuta spp.) are holo-parasites, usually annuals, or perennials under certain conditions. They are members of the morning glory family (Convolvulaceae) in old references that include Cuscuta, Convolvulus and Ipomoea and of dodder family (Cuscutaceae) in the more recent publications. Cuscuta is a yellow or orange plant, twinged with purple or red color sometimes but generally white. The stems can be very thin and thread like or relatively stout. However, all Cuscuta species are herbaceous vines with twining, slender, pale stems, with reduced, scale-like leaves, and have no roots. Plants have no or merely low amounts of chlorophyll, and usually do not have a photosynthetic activity (Hibberd et al., 1998; Garcia et al., 2014) or may be placed as intermediate between hemi- and holoparastic conditions (Nickrent and Muselman, 2004) hence some species show residual photosynthesis (Dawson et al., 1994; Hibberd at al., 1998) and have thus been designated as cryptically photosynthetic (Funk et al., 2007; McNeal et al., 2007a, b). All Cuscuta species are fully dependent on host plants to complete their life cycle, and therefore regarded as obligate holoparasites. Cuscuta spp. are the most problematic parasitic weeds in agriculture in the semi-humid and semiarid areas of Africa and Asia (Sauberborn, 1994). They are nearly cosmopolitan, occurring in a wide range of habitats and hosts with a high number of species in the Americas (Yuncker, 1932; Hunziker, 1950; Mabberley, 1997; Stefanović et al., 2007). Their wide geographical distribution and host range make them among the most damaging parasitic weeds worldover (Ashton and Santana, 1976; Holm et al., 1997). Damage can ultimately lead to total destruction and death of the host plants. The genus Cuscuta comprises some 200 species (Costea, 2007), usually grouped in three subgenera; Monogyna, Cuscuta and Grammica. Molecular studies on subgenera Grammica and Cuscuta, delimit major clades within these groups. Differences in seed morphology between these subgenera were reported. Seed exostructure features supported the subgeneric classification based on exosomorpholigical (vague term) characters (Kim et al., 2000). However, the sequences used were unalienable among subgenera, preventing the phylogenetic comparison across the genus (García et al., 2014). A new phylogenetic classification proposed by Costea et al. (2015) placed 194 accepted species of Cuscuta into four subgenera and 18 sections. While only 10–15 of these species are considered economically important pests (Parker and Riches, 1993; Costea, 2007), their broad geographical distribution and host ranges contribute to impacts on a wide range of agricultural cropping systems (Dawson et al. 1994; Costea and Tardif, 2006). Cuscuta spp. are not host-specific, although they show a wide variation in their host ranges. Severe infestations can stunt, smother and kill host plants, cause severe yield and stand reductions. The worldwide plant species that are commonly parasitized by certain parasitic genera, including Cuscuta spp. were thoroughly reviewed and documented (Qasem, 2006). Cuscuta spp. parasitize many wild and cultivated plant species. They are destructive to certain high-value crops and particularly troublesome where alfalfa, clover, and onion are grown for seeds (Ristau, 2001; Swift, 2001). Some species can parasitize a wide range of host of herbaceous and woody plants, while others may be more host restricted (Qasem, 2006). Cuscuta’s successful parasitism depends critically on efficient host-location mechanisms, as the free-living seedlings of these vine-growing species (Lyshede, 1985; Koch et al.,2004; Furuhashi et al., 2011) and, being obligate parasites, must rapidly make an attachment to a susceptible host before exhausting their limited stored food (Lanini and Kogan, 2005; Costea and Tardif, 2006).Lyshede (1985) indicated that host plants less than about three weeks old seem not to be attacked by Cuscuta seedlings.