Francisco J. Morales

The emergence and dissemination of whitefly-transmitted geminiviruses in Latin America.

Summary. The proliferation and rapid dissemination of whitefly-transmitted viruses of important food and industrial crops in Latin America, have been the consequence of drastic changes in traditional cropping systems. Some of the expanding non-traditional cash and export crops, such as soybean and several vegetables, have served as suitable reproductive hosts for the whitefly Bemisia tabaci. This insect pest has been shown to transmit at least 20 different geminiviruses that affect different commercial and basic food crops in Latin America. This review summarizes the existing knowledge on this important group of viruses and their vector in this region.

Comparative value of four arcelin variants in the development of dry bean lines resistant to the Mexican bean weevil

Wild Phaseolus vulgaris L. accessions containing arcelin codominant alleles 1 through 5 were reconfirmed and characterized for resistance to the Mexican bean weevil, Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae). Accession G 02771 (arcelin 5) had the highest level of antibiosis resistance, followed by G 12952 (arcelin 4), G 12882 (arcelin 1) and G 12866 (arcelin 2). Arcelin 3 accessions conferred the lowest levels of resistance. As the presence of arcelin is inherited as a single dominant gene, a backcross breeding program has been used to transfer resistance to the Mexican bean weevil from wild beans to bean cultivars using serological techniques to detect the presence of arcelin and replicated insect feeding tests to measure resistance levels. Progeny containing arcelin 1 showed resistance equal or superior to that of the resistant check. Arcelin 2‐derived lines had intermediate levels of resistance while no resistant progenies were obtained from crosses with arcelin 3 and 4 sources. Results are discussed in relation to the deployment of arcelin alleles in bean cultivars.

Conventional breeding for resistance to Bemisia tabaci-transmitted geminiviruses☆

Abstract The whitefly Bemisia tabaci Genn. is a pervasive pest and vector of plant viruses. The existence of B. tabaci biotypes and numerous whitefly-transmitted geminiviruses (begomoviruses) affecting food and industrial crops has become a major constraint to agricultural development in tropical and subtropical regions of the world. The predominant whitefly and begomovirus method of control has been the application of insecticides. The excessive use of agrochemicals over the past decades, has contributed to an exponential increase in B. tabaci populations and incidence of begomoviruses transmitted by this whitefly vector. Under these conditions, biological and integrated whitefly/geminivirus control practices have not met expectations. Incorporating begomovirus resistance in a relatively small number of crops improved by conventional plant breeding methods has been a sustainable and efficient disease control strategy. This review discusses some of the conventional intra- and interspecific hybridization strategies followed to incorporate genetic resistance to begomoviruses in three major crops: cassava, common bean and tomato.

History and current distribution of begomoviruses in Latin America.

Publisher Summary This chapter presents an overview of the history and current distribution of begomoviruses in Latin America. The main crops affected by begomoviruses in Latin America have been common bean, tomato, and sweet and hot peppers. The area planted to tomato and peppers in Latin America is relatively small, but the economic losses caused by different begomoviruses in these crops largely surpass the economic losses reported for common bean in the entire region. These crops and the main begomoviruses that affect them in Latin America have been discussed. These are not the only plant species affected by begomoviruses in the region. Cotton, tobacco, soybean, various native and introduced cucurbits, and some fruit crops have been affected by begomoviruses in past and recent years, and have been mentioned in this chapter. The importance, distribution and incidence of begomoviruses in Latin America is directly associated to the distribution and population dynamics of their insect vector, the whitefly species B. tabaci . Its geographic distribution depends largely on climatic conditions that favor the reproduction of the species: usually warm temperatures, moderate relative humidity and relatively low-to-moderate rainfall. Despite sporadic efforts to educate farmers on the most appropriate methods to manage whitefly pests and whitefly-transmitted viruses these pests continue to cause significant yield losses and to affect more agricultural regions in Latin America. The main reason for the continuous expansion of these biotic problems is the lack of technical assistance in rural areas. The main biological threat remains the continuous invasion of new agricultural areas by biotype B of B. tabaci .

Emergence and partial characterization of rice stripe necrosis virus and its fungus vector in South America

A new disease of rice, known as ‘entorchamiento’ (crinkling), was first noticed in the Department of Meta, Colombia, in 1991. Symptoms include seedling death, foliar striping and severe plant malformation. Tissue extracts and purified preparations from diseased rice plants, contained virus-like particles ca. 20 nm in diameter, with a bimodal length of 260 and 360 nm. Particle aggregates were also observed in the cytoplasm of infected rice leaf cells. Electrophoretic analyses of purified preparations and dsRNA extracts, revealed a single protein species of M 22,500, and four dsRNA bands ca. 6300, 4600, 2700 and 1800 bp in size. Cystosori, characteristic of plasmodiophorid fungal vectors of plant viruses, were consistently observed in the roots of diseased rice plants. PCR and sequence analyses of amplified fungal DNA products from infected rice roots, revealed that the putative fungus vector was Polymyxa graminis. A Western blot of tissue extracts obtained from ‘crinkled’ rice plants from Colombia, using antiserum against a West African isolate of rice stripe necrosis furovirus (RSNV), resulted in the detection of a protein band of approximately M 22,000. The RSNV antiserum recognized the Colombian virus isolate in serologically specific electron microscopy tests. These results confirm the presence of RSNV in the Americas.

Genetics of resistance to bean golden mosaic virus in Phaseolus vulgaris L.

SummaryThe genetics of resistance to bean golden mosaic virus (BGMV) was studied in an 8×8 complete diallel cross of common bean, Phaseolus vulgaris L. The 28F1 hybrids, their reciprocals, and eight parents were artificially inoculated with BGMV under glasshouse conditions. Data were recorded and analyzed for foliar yellowing, plant dwarfing, flower abortion, and pod formation, using a randomized complete block design with two replications.General combining ability (GCA) mean squares were highly significant (P<0.01) and larger than values for specific combining ability for all traits. Reciprocal and maternal effects were nonsignificant. Cultivars Royal Red and Alubia Cerrillos possessed significant negative and positive GCA for all traits, respectively. Porrillo Sintetico also had negative GCA for all traits except plant dwarfing. None of the GCA effects were significant for Great Northern 31 and PVA 1111. Positive associations existed among all traits studied.

Linkage of dominant hypersensitive resistance to bean common mosaic virus to seed color in Phaseolus vulgaris L.

SummaryEvaluation of Phaseolus vulgaris germplasm bank materials and progenies from a large number of crosses using red- or yellow-colored, BCMV-susceptible bean lines, crossed to purple- or grey/brown-colored, hypersensitive-resistant lines, suggested strong trait association between seed color and BCMV resistance. The cross of red-mottled I+I+ (susceptible) BAT 1255R to isogenic purple-mottled II (resistant) BAT 1255M was made to study the segregation of the two characters and to recover red-mottled resistant progenies. No recombinant genotypes were observed among 353 F3 families inoculated with BCMV-NL3, suggesting that linkage of purple-mottled seed color and dominant BCMV resistance is very close.

Potato yellow mosaic virus: a synonym of tomato yellow mosaic virus.

Summary. Tomato yellow mosaic was first described in 1963, as a disease caused by a geminivirus transmitted by the whitefly Bemisia tabaci in Venezuela. In 1981 and 1985, Tomato yellow mosaic virus (ToYMV) was reported to occasionally infect potato plants growing in the proximity of tomato plantings affected by this virus. Despite these previous reports, a virus isolated from yellow mosaic-affected potato plants in Venezuela, was described in 1986 as a “new geminivirus” called potato yellow mosaic virus (PYMV). In recent years, different geminiviruses related to PYMV have been described from tomato fields in Venezuela and other countries in the Caribbean Basin, including Panama. Comparative nucleotide and amino acid sequence analyses of a 1698 bp fragment amplified from the common region and part of the AV1 and AC1 ORFs of ToYMV from Venezuela, yielded 95.7% sequence identity with the corresponding regions of PYMV. Nucleotide and amino acid sequence identities between ToYMV and PYMV, were 96.3% and 95.1% for AC1, and 95.7% and 100% for AV1, respectively. The identity of the nucleotide sequence for the common region of ToYMV and PYMV was 96.5%. Comparative sequence analyses conducted with ToYMV and other tomato begomoviruses present in the Caribbean region, showed only distant relationships. It is concluded here that PYMV is a synonym of ToYMV.

Breeding for resistance to bean golden mosaic virus in an interracial population of Phaseolus vulgaris L.

SummaryEighty-three F2-derived F8 recombinant inbred lines (RILs) randomly selected from a population (TY 8419) of two common bean genotypes (Pinto UI 114 x ICA Pijao), the two parents, and a control cultivar (Topcrop) were screened for their reaction to bean golden mosaic virus (BGMV) under glasshouse conditions. A randomized complete block design with two replications was used. Data were recorded for disease incidence, foliar yellowing, number of pods and seeds, and yield.Significant (P<0.05) differences among the 83 RILs were recorded for all traits. Of these RILs, 11 lines did not develop any symptom, 24 lines had a mean disease incidence of 8%, 28 lines had a disease incidence of 26.6% and developed intermediate mosaic symptoms, and 20 lines were more susceptible (>50% disease incidence and severe yellowing) than either of the parents, which had 25% to 38% disease incidence and moderate symptoms. Heritability for disease incidence was 0.54±0.01, for foliar yellowing 0.36±0.13, for pods plant-1 0.69±0.01, for seeds plant-1 0.54±0.09, and for yield plant-1 0.49±0.01. Gains from selection (at 20% selection pressure) for these traits were 49.5%, 31.6%, 24%, 0%, and 20%, respectively, over the mean value of all RILs. Correlation coefficients between disease incidence and foliar yellowing and among yield and its components were positive. Disease incidence and foliar yellowing showed negative associations with yield and its components.

Tropical Whitefly IPM Project

The Tropical Whitefly IPM Project (TWFP) is an initiative of the Systemwide IPM Programme of the Consultative Group on International Agricultural Research (CGIAR), financed by the Department for International Development (DFID) of the United Kingdom, the Danish International Development Agency (DANIDA), the United States Department of Agriculture (USDA) and Agency for International Development (USAID), the Australian Centre for International Agricultural Research (ACIAR), and the New Zealand Agency for International Development (NZAID), to manage whitefly pests and whitefly-transmitted viruses in the Tropics. Participating CGIAR and other international centers include the Centre for International Tropical Agriculture (CIAT); the International Institute of Tropical Agriculture (IITA); The International Potato Centre (CIP); the Asian Vegetable Research and Development Centre (AVRDC); and the International Centre of Insect Physiology and Ecology (ICIPE), in close collaboration with the National Resources Institute (NRI-UK); national agricultural research institutions; agricultural universities; and advanced agricultural research laboratories in Africa, Asia, Europe, the Pacific Region, and the Americas. The TWFP was launched in 1996 as five separate but closely linked subprojects targeting: (1) Bemisia tabaci as a vector of viruses affecting cassava and sweet potato in sub-Saharan Africa (IITA, NRI, CIP, CIAT); (2) B. tabaci as a vector of viruses in mixed cropping systems of Mexico, Central America, and the Caribbean (CIAT); (3) B. tabaci as a vector of viruses in mixed cropping systems of eastern and southern Africa (ICIPE, AVRDC); (4) B. tabaci as a vector of viruses in mixed cropping systems of Southeast Asia (AVRDC); (5) Trialeurodes vaporariorum as a pest in mixed cropping systems of the Andean highlands (CIAT); and (6) whiteflies as pests of cassava in South America (CIAT). Diagnostic surveys conducted in Phase I (1997-2000) clearly showed that the two main whitefly pests in the Tropics are B. tabaci and, in the highlands, T. vaporariorum. Other whitefly species investigated by the TWFP included B. afer and Aleurotrachelus socialis. B. tabaci is the main vector of plant viruses inducing African cassava mosaic disease, sweet potato virus disease, bean golden mosaic disease, and many other diseases of horticultural crops, mainly tomato, hot and sweet peppers, squash, melon, and several other cucurbits. Genetic resistance was identified as the most important component of an IPM programme, followed by phytosanitary, cultural and legal measures. The two most important factors contributing to whitefly/virus epidemics were shown to be pesticide abuse and the use of virus-infected planting materials. Biological control is only effective in cropping systems with minimal or rational use of insecticides, and should be considered only as a complementary IPM strategy. Farmer education and technical assistance are considered the most critical steps toward the implementation of sustainable and economically viable IPM strategies in tropical countries affected by whitefly pests and whitefly-transmitted viruses.