Three years of molecular monitoring of phytoplasma spreading in a plumgrowing area in Italy

In the plum growing area of Vignola (Northern Italy) a molecular monitoring was carried out in order to verify possibility to reduce the epidemic phytoplasma infection. Nucleic acids were extracted from plum leaf midribs for molecular tests to verify phytoplasma presence. Direct PCR using universal primer pairs P1/P7 followed by nested PCR with R16F2/R2 and specific primers R16(X)F1/R1 and RFLP analyses mainly with RsaI and SspI were employed to detect/identify phytoplasmas. The presence of phytoplasma specific DNA bands was observed after nested-PCR using either R16F2/R2 and R16(X)F1/R1 primers. From symptomatic and asymptomatic plum 16SrX-B phytoplasmas were identified. Four farms were monitored during 2000 testing 11 Japanese plum varieties: in June among 40 samples, all from symptomatic plants about 23% were positive in the first nested PCR reaction and 92% in the second one performed with group specific primers 16Sr(X)F1/R1. When the same plants were retested in October, 39 out of the 40 samples were positive in direct PCR. During July 2001 in two of the above farms a total of 57 asymptomatic plums were tested: in the first farm 21% of the plants tested were positive and in the second 17%. In July 2002 both symptomatic and asymptomatic plum were monitored in 3 different fields. Ten out of 20 samples from symptomatic plum resulted to be infected by 16SrX-B phytoplasmas, 5 were negative and in the other samples phytoplasmas belonging to groups 16SrXII-A, 16SrI-B and 16rIII-B were identified. Asymptomatic plant that were re-grafted in 2001 on rootstock where symptomatic scions were previous observed showed that 67% of the plant tested were infected by 16SrX-B phytoplasmas showing a clear evidence that the rootstock was colonized by the pathogens that are able to infect the new grafted scions just during one winter season