First report of sugarcane yellow leaf disease in Mexico and detection of `Candidatus Phytoplasma asteris'-related strains in affected plants

Authors: PEREZ-LOPEZ, EDEL - University Of Saskatchewan; Wei, Wei; Davis, Robert; WANG, JIAWEI - Shandong Institute; Zhao, Yan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2018
Publication Date: 3/12/2019
Citation: Perez-Lopez, E., Wei, W., Davis, R.E., Wang, J., Zhao, Y. 2019. First report of sugarcane yellow leaf disease in Mexico and detection of `Candidatus Phytoplasma asteris'-related strains in affected plants. Plant Disease. https://doi.org/10.1094/PDIS-09-18-1591-PDN.
DOI: https://doi.org/10.1094/PDIS-09-18-1591-PDN

Interpretive Summary: Sugarcane, being a major source for sugar and biofuel production, is an important cash crop widely cultivated in tropical and subtropical regions. However, the productivity of sugarcane can be seriously hindered by diseases caused by phloem-colonizing bacteria known as phytoplasmas. Sugarcane diseases around the world have been attributed to diverse phytoplasma strains belonging to six mutually distinct species. During the 2015-2016 growing season, sugarcane plants exhibiting leaf discolorations (white edge and yellow stripes) indicative of sugarcane yellow leaf disease were observed in a sugarcane field in Cosamaloapan, Veracruz, Mexico. Using DNA fingerprinting technology, ARS scientists analyzed samples from affected sugarcane plants and identified the disease agent. Base on genetic profiles, the scientists concluded that the sugarcane disease was caused by a phytoplasma closely related to that responsible for yellows diseases in aster and many other herbaceous plants. This is the first report of phytoplasmal sugarcane yellow leaf disease in Mexico. Associations of aster yellows phytoplasma with sugarcane diseases were previously reported in Brazil and Cuba. Our finding of sugarcane-infecting aster yellows phytoplasma in Mexico underscores the need for disease surveillance of sugarcane in neighboring countries, since insect vectors capable of spreading aster yellows phytoplasma strains are known to be present over wide areas, including the Caribbean countries and the United States. The results communicated in this report are important to farmers and extension personnel who are concerned with phytoplasmal disease diagnosis and management. The information is also critical to regulatory agencies for preventing exotic pathogens and diseased plant materials from being introduced into the U.S. and other sugarcane production countries.

Technical Abstract: Sugarcane is a common name for any of several tall perennial grass species of the genus Saccharum. As a major source for sugar production and an efficient feedstock for biofuel generation, sugarcane is widely cultivated in tropical and subtropical regions. Mexico alone produces over six million metric tons of cane sugar annually valued at 1.3 billion U.S. dollars. While economically important, sugarcane is susceptible to devastating diseases caused by phloem-colonizing, cell wall-less bacteria known as phytoplasmas. Different sugarcane diseases around the world have been attributed to diverse phytoplasma strains belonging to six mutually distinct 'Candidatus Phytoplasma' species. During the 2015 -2016 growing season, sugarcane plants exhibiting leaf discolorations (white edge and yellow stripes) indicative of sugarcane yellow leaf (ScYL) disease were observed in a sugarcane field in Cosamaloapan, Veracruz, Mexico, with less than 1% of the plants in the filed being affected. Leaf samples were collected from three symptomatic plants. Total DNA was extracted from leaf midribs using a modified CTAB method. A preliminary diagnostic assay was carried out using polymerase chain reactions (PCR) with phytoplasma-specific primer pair R16F2n/R16R2. All PCR assays with DNA templates from symptomatic plants produced a phytoplasma-characteristic amplicon of 1.25 kb. No amplicon was detected from samples of healthy plant controls. The DNA samples from symptomatic plants were subjected to further PCR analysis with primer pair P1A/16S-SR as previously described. All three samples were PCR positive and each yielded a 1,539 bp amplicon. DNA sequence analysis confirmed that the amplicon represented a near-full-length 16S rRNA gene and a partial 16S-23S RNA gene intergenic spacer. The obtained DNA sequences were deposited into the GenBank. Analysis of the sequences through the iPhyClassifier revealed that the phytoplasmas detected in the ScYL diseased plants were closely related to the reference strain of 'Candidatus Phytoplasma asteris' (aster yellows phytoplasma). While the three 16S rRNA gene sequences differed from each other by two to five bases, all possessed the entire set of signature sequences of 'Ca. Phytoplasma asteris', indicating the three ScYL phytoplasma strains are “sequevars” affiliated with a single phytoplasma species. To our knowledge, this is the first report of phytoplasmal sugarcane yellow leaf disease in Mexico. Associations of aster yellows phytoplasma with sugarcane diseases were previously reported in Cuba and Brazil. Recently, infection of aster yellows phytoplasma in weedy grass growing near sugarcane fields in Mexico was noted. Findings of aster yellows phytoplasma infecting sugarcane in the countries of Brazil, Cuba, and now Mexico underscore the need for disease surveillance of sugarcane in neighboring countries, since insect vectors capable of spreading aster yellows phytoplasma strains are known to be present over wide areas, including the Caribbean countries and the United States.