Abstract
Aster yellows (AY) group phytoplasmas affect plants by causing a general reduction in quantity and quality of yield. The most severely affected hosts are carrot, lettuce, onion, spinach and several ornamental crops, including aster, gladiolus, hydrangea, chrysanthemum and purple coneflower. Disease incidence may vary from year to year depending on the population trend of the vectors in the field. Infection rates varying from 20 to 30% were observed in lettuce and ranunculus fields in southern Italy (Marcone et al., 1995; Parrella et al., 2008). In Oklahoma, USA, according to Errampalli et al. (1991), AY group phytoplasma infections occurred in 80% of lettuce plants and 28% of carrots. In Ohio, disease incidence of 100% has been recorded in lettuce fields (Zhang et al., 2004). A major outbreak of AY disease occurred in 2000 in Texas that affected several vegetable crops. Among them, carrot was most severely damaged with infection rates that ranged from to 50 to near 100% (Lee et al., 2003). A severe AY disease of chrysanthemum which induced losses of 70 to 80% of the crop has been reported from China (Min et al., 2008) whereas losses of 90% were recorded in AY-affected aubergines in Bangladesh (Kelly et al., 2009). Infection rates of 60 and 99% were recorded in Hungary for AY-affected sugarbeet plants and India for AY-affected Jatropha curcas plants, respectively (Mumford et al., 2000; Kumar et al., 2010b). However, there are also several reports on sporadic occurrence and generally low incidence of AY group phytoplasmas in vegetable and ornamental crops (Smith et al., 1988; Vibio et al., 1995; Bertaccini et al., 1998). Cassava witches' broom, a disease affecting cassava in South East Asia (Vietnam, Thailand, Cambodia, Laos, China and the Philippines) is caused by 16SrI phytoplasma. The disease has resulted in significant reductions in cassava root starch content and up to 80% yield loss in parts of Vietnam (Anon., 2014).