▪ Abstract Nineteen single dominant genes (R genes) for resistance to viruses, nematodes, and fungi have been positioned on the molecular map of potato using DNA markers. Fourteen of those genes are located in five “hotspots” for resistance in the potato genome. Quantitative trait loci (QTL) for resistance to late blight caused by the oomycete Phytophthora infestans, to tuber rot caused by the bacterium Erwinia carotovora ssp. atroseptica, and to root cyst nematodes have been identified on all 12 potato chromosomes. Some QTL for resistance to different pathogens are linked to each other and/or to resistance hotspots. Based on the genetic clustering with R genes, we propose that some QTL for resistance have a molecular basis similar to single R genes. Mapping potato genes with sequence similarity to cloned R genes of other plants and other defense-related genes reveals linkage between candidate genes, R genes, and resistance QTL. To explain the molecular basis of polygenic resistance in potato we propose (a) genes having structural similarity with cloned R genes and (b) genes involved in the defense response. The “candidate gene approach” enables the identification of markers highly useful for marker-assisted selection in potato breeding.

Public genomic databases have provided new directions for molecular marker development and initiated a shift in the types of PCR-based techniques commonly used in plant science. Alongside commonly used arbitrarily amplified DNA markers, other methods have been developed. Targeted fingerprinting marker techniques are based on the well-established practices of arbitrarily amplified DNA methods, but employ novel methodological innovations such as the incorporation of gene or promoter elements in the primers. These markers provide good reproducibility and increased resolution by the concurrent incidence of dominant and co-dominant bands. Despite their promising features, these semi-random markers suffer from possible problems of collision and non-homology analogous to those found with randomly generated fingerprints. Transposable elements, present in abundance in plant genomes, may also be used to generate fingerprints. These markers provide increased genomic coverage by utilizing specific targeted sites and produce bands that mostly seem to be homologous. The biggest drawback with most of these techniques is that prior genomic information about retrotransposons is needed for primer design, prohibiting universal applications. Another class of recently developed methods exploits length polymorphism present in arrays of multi-copy gene families such as cytochrome P450 and β-tubulin genes to provide cross-species amplification and transferability. A specific class of marker makes use of common features of plant resistance genes to generate bands linked to a given phenotype, or to reveal genetic diversity. Conserved DNA-based strategies have limited genome coverage and may fail to reveal genetic diversity, while resistance genes may be under specific evolutionary selection. Markers may also be generated from functional and/or transcribed regions of the genome using different gene-targeting approaches coupled with the use of RNA information. Such techniques have the potential to generate phenotypically linked functional markers, especially when fingerprints are generated from the transcribed or expressed region of the genome. It is to be expected that these recently developed techniques will generate larger datasets, but their shortcomings should also be acknowledged and carefully investigated.

Sweetpotato is a member of the morning glory family that is thought to have originated in Central or South America but also has a secondary center of diversity in the southwest Pacific islands. It is grown in all tropical and subtropical areas of the world and consistently ranks among the 10 most important food crops worldwide on the basis of dry weight produced, yielding about 130 million metric tons per year on about 9 million hectares. Sweetpotato is an important crop for food security. It has been relied on as a source of calories in many circumstances. Vines and/or storage roots can be used for direct human consumption or animal feed. Growing awareness of health benefits attributed to sweetpotato has stimulated renewed interest in the crop. Orange-fleshed cultivars, a source of vitamin A, were introduced to developing countries with hope that they would replace the white-flesh varieties and help alleviate vitamin A deficiencies. In East Africa, sweetpotato virus disease, which is caused by the synergistic interaction of the whitefly-transmitted crinivirus and the aphid-transmitted potyvirus, can cause losses of 80 to 90% in many high-yielding genotypes. During the past 15 years, as molecular methods have been adopted, much has been learned about the composition of the sweetpotato virus complexes, the effects of virus diseases on production systems, the biology of the virus–plant interaction, and management approaches to sweetpotato virus diseases. This article is intended to summarize what has been learned since earlier reviews, integrate knowledge gleaned from experiences in tropical and temperate production systems, and suggest courses of action to develop sustainable management programs for these diseases