Key message: Status of the current outbreak of cassava mosaic disease (CMD) in Southeast Asia was reviewed. Healthy cassava seed production and dissemination systems have been established in Vietnam and Cambodia, along with integrated disease and pest management systems, to combat the outbreak. Cassava (Manihot esculenta Crantz) is one of the most important edible crops in tropical and subtropical regions. Recently, invasive insect pests and diseases have resulted in serious losses to cassava in Southeast Asia. In this review we discuss the current outbreak of cassava mosaic disease (CMD) caused by the Sri Lankan cassava mosaic virus (SLCMV) in Southeast Asia, and summarize similarities between SLCMV and other cassava mosaic begomoviruses. A SATREPS (Science and Technology Research Partnership for Sustainable Development) project “Development and dissemination of sustainable production systems based on invasive pest management of cassava in Vietnam, Cambodia and Thailand”, was launched in 2016, which has been funded by The Japan International Cooperation Agency (JICA) and The Japan Science and Technology Agency (JST), Japan. The objectives of SATREPS were to establish healthy seed production and dissemination systems for cassava in south Vietnam and Cambodia, and to develop management systems for plant diseases and insect pests of cassava. To achieve these goals, model systems of healthy seed production in Vietnam and Cambodia have been developed incorporating CMD-resistant planting materials through international networks with The International Center for Tropical Agriculture (CIAT) and The International Institute of Tropical Agriculture (IITA).

Cassava, Manihot esculenta Crantz, is the third-largest source of food carbohydrates, after rice and maize, in the tropics. On the Indian subcontinent, production of cassava can be severely affected by the cassava mosaic disease (CMD) caused by two Begomovirus species (Geminiviridae), Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV). In Southeast Asia, however, CMD was not known until May 2015, when the first CMD outbreak was observed in Cambodia, and the causal agent was identified as SLCMV (Wang et al. 2016). Here, we report the first outbreak of CMD in Vietnam. In May 2017, cassava plants showing typical CMD symptoms, including chlorotic mosaic, leaf distortion, and stunted growth, were observed in the field in Tay Ninh, Vietnam. Two symptomatic and two non-symptomatic cassava leaves as well as two whiteflies on symptomatic leaves were collected from each of four independent farmer's fields. Three whiteflies were also collected from an adjacent field with no symptomatic pl...

An 11.4-kbp region of genomic DNA containing the complete S10-spc operon was constructed by an integrative mapping technique with eight plasmid vectors carrying ribosomal protein sequences from onion yellows phytoplasma. Southern hybridization analysis indicated that phytoplasmal S10-spc is a single-copy operon. This is the first complete S10-spc operon of a phytoplasma to be reported, although only a part of six serial genes of the S10 operon is reported previously. The operon has a context of 5′-rps10, rpl3, rpl4, rpl23, rpl2, rps19, rpl22, rps3, rpl16, rpl29, rps17, rpl14, rpl24, rpl5, rps14, rps8, rpl6, rpl18, rps5, rpl30, rpl15, SecY-3′, and is composed of 21 ribosomal protein subunit genes and a SecY protein translocase subunit gene. Resembling Bacillus, this operon contains an rpl30 gene that other mollicutes (Mycoplasma genitalium, M. pneumoniae, and M. pulmonis) lack. A phylogenetic tree based on the rps3 sequence showed that phytoplasmas are phylogenetically closer to acholeplasmas and bacillus than to mycoplasmas. In the S10-spc operon, translation may start from either a GTG codon or an ATG codon, and stop at a TGA codon, as has been reported for acholeplasmas and bacillus. However, in mycoplasmas, GTG was found as a start codon, and TGA was found not as a stop codon, but instead as a tryptophan codon. These data derived from the gene organization, and the genetic code deviation support the hypothesis that phytoplasmal genes resemble those of acholeplasmas and Bacillus more than those of other mollicutes.

Cytosine methylation is an important defense against invasive DNAs. Here, cytosine methylation profiles of a plant pararetrovirus, Cauliflower mosaic virus (CaMV), were investigated. Nuclear CaMV DNA is highly methylated throughout the genome including at transcription regulatory regions, but the virion DNA is unmethylated. In vitro CG methylation of the viral 35S promoter reduces transcription from the downstream gene. Although nuclear CaMV DNA is highly methylated, its transcripts are accumulated in the nucleus. The data suggest that a small population of unmethylated viral genomes produced through reverse transcription are constantly delivered back to the nucleus. Small RNA profiles suggest that methylation of the CaMV DNA may be due to de novo methylation through 21‐, 22‐, and 24‐nt small RNAs with adenines at their 5' terminus.