|Lee, Ing Ming|
|LA CARA, FRANCESCO|
Submitted to: Journal Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/1997
Publication Date: N/A
Interpretive Summary: Phytoplasmas, formerly called mycoplasmalike organisms, are associated with many economically important fruit tree diseases in U.S. and Europe. Pear decline is a serious disease that occurs in many of the countries where pear is cultivated. Growing pear that is grafted on tolerant or resistant rootstocks is one of the effective ways to control this disease. However, because phytoplasma concentrations in affected pear trees are usually very low, symptomatology alone is not a sufficient indication of susceptibility of pear cultivars to phytoplasma infection. In this study, a molecular assay, nested polymerase chain reaction (PCR), was used for detection of phytoplasmas in the pear trees. It appeared that there was no direct correlation between the presence of phytoplasma and symptom expression. This implied the necessity of using molecular assays for the evaluation of susceptibility of pear cultivars to phytoplasma infection. In our experimental conditions, cultivars/Quince rootstock combinations were more susceptible to phytoplasma infection compared to cultivars "Franco comune" combinations. The information and pathogen-identification technology will be of benefit to diagnosticians and to APHIS, for implementation of new quarantine regulations, and to the breeders of fruit trees.
Technical Abstract: 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 the presence of phytoplasma; 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 presence of phytoplasma 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 indirect 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). Apparently, there is no direct correlation between the presence of phytoplasma and symptom expression.