In January 2012, disease symptoms including chlorosis, leaf crinkle, leaf curving and stunting of whole plants, virescence, and curving and necrosis of flower stalks were observed in Freesia hybrida cvs. Evone, Honey Moon, Golden Gem, and Pallas in Icheon and Suwon (Gyeonggi Province in Korea). To determine a possible phytoplasma infection, the symptomatic freesia plants were examined for the presence of phytoplasma 16S rDNA fragment by PCR with the primer pair P1/P6 (2) and R16F1/R16R1 (in nested PCR), which amplifies phytoplasma 16S rDNA regions (4). An expected PCR product of ~1,096 bp was obtained from the symptomatic freesia plants, and they were designated as FreLN, Fre-phy-Ev4, Fre-phy-Ev6, Fre-phy-GG, Fre-phy-HM, and Fre-phy-Pal. The PCR products were sequenced and registered as GenkBank accessions AB695174 and AB709951-55. The sequence corresponding to symptomatic freesia had 98.8 to 99.4% identity with Stolbur phytoplasma strains in the 16S rDNA region, and it had only 95.7 to 96.3% identity with AY phytoplasma strains. In the ultra-thin sections of the leaf midribs, globous phytoplasmal bodies 54 to 214 nm in size were observed in sieve tube elements of phloem tissue. Fre-Phy-Ev6 and Fre-Phy-HM were doube-infected with Stolbur phytoplasma and Freesia mosaic virus (FreMV). Fre-Phy-Ev6 and Fre-Phy-HM revealed necrosis of flower stalks and flower color breaking besides curving of flower stalks. Therefore, flower color breaking and flower stalk necrosis were assumed to be caused by FreMV (1). Symptoms of chlorosis and stunting of whole plants shown in FreLN and virescence of Fre-phy-GG were typical symptoms of phytoplasmal diseases, while leaf crinkle, leaf curving, and curving of flower stalks appeared to be unique symptoms in F. hybrida. Stolbur phytoplasma was abundant in commercial freesia cultivation fields. Some of the cultivars, such as cv. Pallas, showed only curving of leaf and flower stalks without any typical symptom of phytoplasmal diseases. A phytoplasmal disease was reported in Poland in 2001 from F. hybrida exhibiting leaf chlorotic and necrotic spots, and classified as AY I-B based on RFLP analysis of PCR products (3). To our knowledge, this is the first report of Stolbur phytoplasma in F. hybrida. This result is significant because F. hybrida could be the infection source of Stolbur phytoplasma disease in floricultural crops. Interestingly, we found a prevalence of Stolbur phytoplasma in Petunia hybrida cultivars (GenBank Accession Nos. AB713757 to AB713758). High nucleotide sequence identity of 99.8% in the 16S rDNA region of Stolbur phytoplasma isolates from petunia and freesia support the inference that those Stolbur phytoplasma isolates could infect both floricultural crops.
References: (1) A. A. Brunt. Freesia. Page 274 in: Virus and virus-like Diseases of Bulb and Flower Crops, John Wiley & Sons, Chichester, 1995. (2) S. Deng and C. Hiruki. J. Microbiol. Methods. 14:53, 1991. (3) M. Kamińska and H. Sliwa. Plant Dis. 85:336, 2001. (4) I. M. Lee et al. Phytopathology 84:559, 1994.