Role of different phytoplasmas in inducing Poinsettia branching

The phytoplasma associated with poinsettia branching belonging to group 16SrIII-H was detected several times in mixed infection with phytoplasmas of the 16SrI group (B and C subgroup) or 16SrXII-A subgroup. Experiments were carried out in order to evaluate the role of the different phytoplasmas in providing branching to commercial poinsettias. Plants belonging to cultivars Peter Star and Angelica, and tested to be virus and phytoplasma-free, were graft inoculated with patches of infected poinsettias. The plants, maintained during three growing seasons under insect-proof greenhouse were periodically tested to identify phytoplasmas by nested PCR with general ribosomal primers and primers specific for 16SrIII, 16SrI and 16SrXII groups. Results indicate that phytoplasmas show diverse ability to infect poinsettias: plants grafted with 16SrI+16SrIII phytoplasmas show 62% of infection, while in plants grafted with 16SrIII phytoplasmas the percentage of transmission was about 24%. Phytoplasma-infected plants showed several degrees of branching in agreement with phytoplasma identified: 16SrIII-H was associated with normal branching in 78% of plants, while 16SrI phytoplasmas were present only in plants with restricted or no branching. Only one plant with mixed infection was detected and showed restricted branching. Phytoplasmas of 16SrIII-H group were detected also in 3 plants without branching; preliminary RFLP analyses on phytoplasma ribosomal protein rpL22, tuf and spacer region show these isolates molecularly indistinguishable from those detected in restricted branching poinsettias deriving from the same mother plants. Similar results were obtained on poinsettia plants belonging to cultivars Cortez and Freedom and showing severe free branching together with stunting where 16SrIII-H phytoplasmas were detected, sometimes in mixed infection with 16SrI phytoplasmas.