Susceptibility to phytoplasma infection of three pear varieties grafted on different rootstocks

Pear decline is a serious disease that occurs in many of the countries where pear is cultivated. Phytoplasmas are consistently detected in the diseased trees: they are usually present in very low concentrations and located in the phloem of the affected plants. To detect the presence and clarify the role of phytoplasmas in epidemics of pear decline in southern Italy, an experimental field planted in 1992 was monitored for the disease development. Among 406 pear trees 105 were tested by molecular tools to verify phytoplasma presence; these trees showed in several cases severe symptoms of stunting, while others showed leaf rolling or new sprouting in the winter or general stunting and various degrees (from 10 to 100%) of violet leaves. Some plants were asymptomatic, and others died. The phytoplasma presence was determined by molecular assays on samples of cv William grafted on seven rootstocks, of Decana del Comizio grafted on six rootstocks and of Conference grafted on two rootstocks. Two sets of universal primer pairs amplifying sequences in the phytoplasma ribosomal DNA were employed in direct and nested-PCR. Few positive amplifications were obtained with direct-PCR, but with nested-PCR 60 out of the 105 plants tested were positive: RFLP analyses identified that the phytoplasmas detected belonged to subgroup 16SrX-C. Pear cultivars grafted on Quince A rootstock were more susceptible to phytoplasma infection (58-85%). However, cvs William and Decana del Comizio appear to be less susceptible, especially when grafted on "Franco comune" rootstock (29-43% of infection). It is interesting to observe that plants of cv William grafted on Farold 40 and Farold 87 showed similar degree of phytoplasma infection (57%), but all the phytoplasma-positive trees were asymptomatic. It seems there is no direct correlation between phytoplasma presence and symptom expression.