- Molecular detection and identification of phytoplasmas in sugarcane in Hawaii, Thailand, Cuba and Near East (2012)
- The Yellow leaf syndrome (YLS) had been first detected and described in Hawaiian sugarcane plantations. The polerovirus Sugarcane yellow leaf virus was identified as a causal agent of the syndrome; however there was no strict correlation between the degree of symptom expression and the virus titre. Therefore several surveys on breeding station sugarcane plants in Hawaiian Islands were done for Sugarcane yellow leaf phytoplasma (SCYLP), a bacterium which had been hypothesized to be also a causal agent of YLS. Two types of phytoplasmas were found in Hawaiian sugarcane cultivars mainly sugarcane white leaf phytoplasma (SCWL) which is a member in rice yellow dwarf group, in addition to aster yellows group. This was also true for sugarcane plants from Hawaiian plantations, which routinely use hot water-treatment for the seed cane cuttings. Sugarcane samples were obtained also from other countries including Cuba, Egypt, Syria and Thailand where sugarcane plants are also showing symptoms of yellowing or whiting. Aster yellows and X-disease phytoplasmas were found in Cuban cultivars whereas one sugarcane cultivar from Egypt contains grassy shoot phytoplasma that is a member in rice yellow dwarf group, but the other two Egyptian ones were phytoplasma-free. Syrian sugarcane was infected by phytoplasma that identified preliminary in rice yellow dwarf group. To our knowledge, this is the first report for the detection and identification of phytoplasma in sugarcane plants from Hawaii, Egypt and Syria. Our investigation on Thai sugarcane plants was in agreement with previous literature where sugarcane white leaf (SCWL) phytoplasma is associated with white leaf disease (Nakashima et al., 1994; Wongkaew et al., 1997). Q-PCR (real-time PCR) offers the opportunity to detect the phytoplasma in a sensitive, specific and quick manner, but that is not true for sugarcane plants with a very low titer of phytoplasma. Therefore, nested-PCR is better than qPCR for low titer phytoplasma detection and that is true for sugarcane yellow leaf phytoplasma disease. A BLAST search for the 16S rRNA gene sequences reported in this study showed that they shared 99 to 100% sequence identity with those of other phytoplasmas in the Aster yellows, X-disease and Rice yellow dwarf groups. However, no one of these identified strains can be described as a new “Candidatus Phytoplasma species”. On the other hand, Hawaiian sugarcane cultivar H78-7750 as a representative of Hawaiian breeding station sugarcane contains phytoplasma clustered to strain sugarcane white leaf (SCWL) phytoplasma, closely together with sugarcane white leaf phytoplasma from Taiwan (AY139874). It is possible to explain the occurrence of (SCWL) phytoplasma in Hawaiian Islands, by insect vectors or by infected stem cuttings which were obtained from other countries. Thai sugarcane contains phytoplasma isolate closely together with sugarcane white leaf phytoplasma from Myanmar. The transmission electron microscopic (TEM) studies revealed the presence of sugarcane white leaf phytoplasma only in phloem sieve tubes of diseased sugarcane leaves but not in adjacent cells to the sieve elements including companion cells and phloem parenchyma as well. According to ultrastructural observations under TEM, parenchymatic cells of bundle sheath and mesophyll tissue of affected leaves showed some alterations including accumulations of starch granules, increase plastoglobuli number and size in disorganized chloroplasts. Insect vectors of phytoplasmas are phloem feeders. Thus far, none of aphid species has been found to be a vector of phytoplasmas. Our tests showed also that black sugarcane aphids (Melanaphis Sacchari) were unable to transmit the phytoplasmas from infected sugarcane into the phytoplasma-free one. Hot water treatment has been proposed to cure plant material from phytoplasmas. Our tests showed that the appropriate hot water treatment, which recommended for phytoplasma elimination, is immersion of the sugarcane stem cuttings at 50°C for 60 min.