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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. Abstract 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).

Key message A molecular analysis using informative SNP markers in 1570 clones of cassava from Vietnam reveals varietal composition from farmers’ field and genebank collections Abstract Cassava is the most important smallholder cash crops in Southeast Asia and is especially used in industrial products. Yet, systematic genetic studies on molecular markers from Vietnamese germplasm have not been considered for breeding and conservation programs. We conducted a molecular analysis of 1570 clones of cassava germplasm from farms across six agro-ecological zones using informative SNP markers. We unraveled the genetic diversity and population structure and provided insights into the value of breeding and conservation programs. Duplicated genotypes comprised 98% of the total sample of the Central Highlands region. Ninety-six SNPs were amplified Central Highlands and South East provinces had the highest allelic richness, covering up to 83% of alleles. The average observed heterozygosity (Ho = 0.43) was slightly higher than expected (He = 0.40) across SNP markers, suggesting an excess of heterozygotes plants. Diversity indexes indicated that cassava populations from North West and Eastern Vietnam are genetically diverse (mean He = 0.40). Genetic parentage tests identified 85 unique genetic groups within the varieties KM94, KM419, BRA1305, KM101, KM140, PER262, KM60, KM57 and two unidentified varieties, which accounted for 82% of the frequency distribution. KM94 is the most dominant variety in Vietnamese farms surveyed (38%), reflecting its superior quality and productivity. Discriminant analysis of principal components (DAPC) revealed four main subgroups, which were partially corroborated by neighbor joining (NJ) analyses. After removing duplicates, 31 unique genotypes were distributed across five of the agro-ecological zones. These were well distributed in the subgroups revealed via DAPC and NJ analyses. The genetic groups identified herein could be used to select unique accessions that should ideally conform with ex situ germplasm collections and identify areas where on-farm conservation programs should be targeted. Newly identified genotypes may also contribute as genetic breeding resources that could be used to adapt cassava to future changes and farmers’ needs.

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Supplementary Figure 1. Work flow of plant propagation using the aeroponic system. (1) Six-week-old tissue culture plants are transferred to the soil. (2) The wrapping film surrounding the plantlet is loosened about 2 weeks and the plantlet is cultivated to grow up for a period of up to 2-4 weeks. (3) Optional step: generation of a new shoot from a properly lignified stem cutting. (4) Obtain explants from a top and middle portion of stems of the plant source: i) plantlets derived from tissue culture, ii) renewal plants and iii) shoot parts obtained from properly lignified stem cuttings. (5) The explants are placed in the aeroponic system and covered with a wrapping film. (6) The film is gradually opened after three days. (7) Transfer the rooted plants to soil in pots. (8) Use the propagated plants for the next cycle of propagation in the aeroponic system after the rooted plants are grown for 2-4 weeks. Blue arrow: Preparation of the source material. Green arrows: Propagation cycle.

Cassava (Manihot esculenta Crantz) is a crucial staple crop, and provides carbohydrate energy to more than half a billion people in the tropics. Cassava mosaic disease (CMD) is the most important disease of cassava in Africa. Since Sri Lanka Cassava Mosaic Virus (SLCMV) was first reported in South East Asia in 2015, establishing sustainable solutions to CMD has become a top priority for the cassava program at the International Center for Tropical Agriculture (CIAT) and its partners. In the present study, we screened two populations for CMD resistance: VNM142, 142 clones collected from farms throughout Vietnam, and CIAT102, 102 clones resistant to CMD or mites, which were introduced from CIAT. High broad-sense heritability was observed in all the trials (>0.80). From the population VNM142, eight clones showed high CMD resistance with CMD severity scores less than 2.0. Two resistant clones had the same DNA fingerprinting with the accessions CR63 (PER262 or TAI9) and KM57 (VNM8) in the genebank, respectively. To our knowledge, this is the first report of CMD resistance in the genebank at CIAT. We also used the two populations to validate the CMD markers S12_7926132 and S14_4626854. Both markers explained 51% of the population variance in the segregating population CIAT102, but only 11% in the diverse population VNM142. Thus, we concluded that the two CMD markers could not be used to select for CMD resistance in diverse populations, but could predict the CMD resistance in segregating populations when the susceptible parents do not have resistant marker alleles and the resistance of the CMD2 donors is confirmed.

It has been commonly recognized by the international research and practice community that the presence of both outer and inner stirrups may significantly enhance the axial load capacity of reinforced concrete (RC) columns. However, there is limited testing evidence to support this conclusion that has been published nationally. This paper reports an experimental programme to study the effectiveness of stirrup detailing on the structural performance of columns having small sectional dimensions that are common in low-rise building structures. Nine column specimens with the same geometrical dimensions of 220 mm x 220 mm x 880 mm in three batches were detailed with different stirrup categories, have been gradually axially loaded to failure. The test data have revealed that although the presence of stirrups can generally enhance the axial load capacity of the column specimens, the enhancing levels are much dependent to the shapes of the stirrups. Selected interesting aspects of the test results have also been discussed, which set a concrete base for recommendations for design and detailing of such vertical structural elements. Keywords: experimental investigation; low-rise building columns; axial load capacity; stirrups.