Association of phytoplasmas and viruses with malformed clovers.

Plants of Trifolium spp. exhibiting two different kinds of symptoms--phyllody associated with yellowing/reddening, and dwarf growth habit without floral abnormalities--were observed in several areas of the Czechia. Nested polymerase chain reaction (PCR) with phytoplasma specific primers, and restriction fragment length polymorphism (RFLP) analyses of 16SrDNA revealed that phyllody of T. repens was associated with phytoplasmas belonging to the 16SrI-C subgroup. Similar symptoms in T. hybridum and T. pratense plants revealed the presence of phytoplasmas belonging to two subgroups: 16SrI-C and 16SrIII-B. Dwarf disease of cultivated T. pratense plants was associated with more than one agent: 11 of 20 plants examined by PCR/RFLP analysis revealed the presence of phytoplasmas belonging to four distinct subgroups: 16SrI-B, 16SrI-C, 16SrIII-B and 16SrX-A. Moreover, two kinds of bacilliform virions were observed in ultrathin sections of 15 T. pratense plants. Particles occurred mostly in the parenchymatous cells of vascular bundles and were located in the cytoplasm as aggregates within an extended network of membranous cisternae. Phytoplasmas and rhabdoviruses occurred singly, and both together or in co-presence with filamentous virus-like particles.