Reaction of some commercial citrus species and Iranian lime biotypes to witches' broom disease of lime

Abstract

Witches' broom disease of lime (WBDL), associated with Candidatus Phytoplasma aurantifolia, has devastated the majority of Mexican lime orchards in southern Iran. Recognition of resistant cultivars and rootstocks can play a major role in WBDL management. In this study, graft-inoculation was used to address the reaction of 12 citrus commercial species and 39 collected lime biotypes from five main lime production areas of Hormozgan province, Iran, against WBDL phytoplasma. The experimental plants were allowed to grow from seed, then five seedlings of each species and biotypes were grafted with Ca. Phytoplasma aurantifolia infected shoots. Inoculated plants were inspected for symptom expression for two years. Direct and nested-PCR using P1/P7 and R16F2n/R16R2 primer pairs, respectively, were used for confirmation of phytoplasmal infection in the suspected plants. All commercial citrus species and 39 of the lime biotypes showed the symptoms of little leaf, internode shortening and witches' broom after two to twenty months from graft-inoculation. Incubation period in plants varied from two months in Astaraei lemon and R11 biotype of Rudan region to twenty months in S4 and S6 biotypes of Senderk region. With these results, we can say all commercial citrus species were sensitive to WBDL. Among the commercial citrus species, Lisbon lemon, grapefruit and Cleopatra mandarin cultivars showed symptoms of WBDL after 460, 425 and 375 days, respectively. Disease symptoms developed rapidly on Astaraei lemon and Bakraei within 85–105 days, respectively. Among the collected biotypes, one, five and one biotypes from Minab, Rudan and Hashtbandi, respectively, were resistant to WBDL phytoplasma. These biotypes were symptomless; however, the PCR assay for detection of the phytoplasma was positive in all of them. These results are promising regarding the potential of using the suitable biotypes to reduce the development of WBDL.