Achievements in the epidemiology of begomoviruses and their vector Bemisia tabaci in Costa Rica

Since the early 1990s, and almost simultaneously, unknown diseases started to be observed in many crops, especially in tropical and subtropical regions. These diseases were predominantly caused by begomoviruses, which were poorly known at that time. Their vector, the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae), often reached unprecedented huge populations in agricultural areas. This elicited a serious production crisis worldwide, that caused losses of millions of dollars for farmers in many countries, including the Mesoamerican region. Fortunately, in Costa Rica, some local research centers, with the collaboration of foreign specialists, have been able to study the causes of this phenomenon, in the search for solutions based on solid epidemiological information. In addition to the previously reported native Bemisia tabaci species, New World (NW), two exotic species, -Mediterranean (MED) and Middle East-Asia Minor 1 (MEAM1)- were found. Moreover, native and exotic bipartite begomoviruses have been detected, especially in common bean, cucurbits, tomato and sweet pepper, as well as the worldwide spread monopartite begomovirus Tomato yellow leaf curl virus (TYLCV). Based upon biological and ecological knowledge accumulated to date, this review offers a comprehensive overview of the very dynamic ways in which the interactions of the different whiteflies and begomovirus species have expressed in Costa Rica, with emphasis on vegetable pathosystems. Hopefully, the information provided in this paper may allow farmers, extension agents, and researchers involved in vegetable production to develop sound practical responses to current and unforeseen problems regarding whiteflies and their associated viruses.

Desde inicios del decenio de 1990 y de manera casi simultánea, se empezaron a observar afecciones desconocidas en numerosos cultivos, sobre todo en regiones tropicales y subtropicales. Dichas enfermedades eran causadas por begomovirus hasta entonces poco estudiados. Su vector es la mosca blanca Bemisia tabaci (Hemiptera: Aleyrodidae), de la que no se habían registrado poblaciones tan desmedidas en áreas agrícolas. Ello originó una seria crisis de producción a nivel mundial, con pérdidas millonarias para los agricultores de numerosos países, incluyendo la región de Mesoamérica. Por fortuna, en Costa Rica, algunos centros de investigación, con la colaboración de especialistas extranjeros, han podido profundizar en las causas de este fenómeno, para buscar soluciones fundamentadas en información de carácter epidemiológico. Además de la especie de B. tabaci New World (NW), nativa y previamente conocida, en años recientes han ingresado sendas especies del Mediterráneo (MED) y el Medio Oriente-Asia Menor 1 (MEAM1). Asimismo, se ha detectado la presencia de begomovirus bipartitas, nativos y exóticos, especialmente en frijol común, cucurbitáceas, tomate y chile; además, se detectó el begomovirus monopartito conocido a nivel mundial Tomato yellow leaf curl virus (TYLCV) en tomate. Con base en el conocimiento biológico y ecológico acumulado hasta ahora, en la presente revisión se aporta una panorámica del dinamismo con que se han expresado en Costa Rica las interacciones entre las diferentes especies vectoras y los distintos begomovirus, con énfasis en patosistemas de hortalizas. Se espera que la información aquí presentada permita mejorar el tipo de respuestas prácticas y eficaces de parte de agricultores, extensionistas agrícolas e investigadores involucrados en la producción de hortalizas, frente a problemas actuales o imprevistos.

Phenotypic and Molecular Screening of Okra (Abelmoschus esculentus L. Moench) Genotypes against Okra Leaf Curl Disease

Okra leaf curl disease (OLCD) is a major constraint on okra (Abelmoschus esculentus L. Moench) production in West Africa. The most effective way of managing this disease is through breeding and planting of resistant varieties. In order to identify sources of resistance and or tolerance, 21 okra genotypes were screened against OLCD in field trials which were conducted from May to October, 2015 (rainy season) and November 2015 to March 2016 (dry season). Field resistance was assessed at 2, 6 and 10 weeks after sowing (WAS) based on disease symptoms, and then confirmed by PCR amplification of viral coat protein gene. Populations of whitefly (Bemisia tabaci), the vector of begomoviruses associated with OLCD, as well as fruit yields were also assessed. Both PCR and field trials showed that all the okra genotypes were susceptible to the viral infection. The genotypes varied significantly (P<0.05) among them in terms of severity of OLCD, whitefly infestation, mean fruit yield (t ha-1), and the average fruit weight per plant. Higher cumulative average population of whitefly and mean fruit yield (t ha-1) were recorded in the dry season than in the rainy season. Genotypes GH5332 and GH6105 consistently showed mild symptoms of OLCD and also had very high fruit yields of 11.88 t ha-1 and 9.34 t ha-1 respectively in the rainy season, and 6.108 t ha-1and 4.05 t ha-1 respectively in the dry season, far above the overall mean yields for all the okra genotypes. Both genotypes GH5332 and GH6105 should be evaluated multi-locationally at farmers’ fields prior to their release as varieties or they should be incorporated into breeding lines.

Mutagenesis in ORF AV2 affects viral replication in Mungbean yellow mosaic India virus.

Mungbean yellow mosaic India virus (MYMIV) is a whitefly-transmitted begomovirus with a bipartite genome. We investigate the functions of the MYMIV-AV2 protein, the open reading frame present upstream of the coat protein gene in DNA A component. The ability of MYMIV-AV2 mutants to replicate, spread and cause symptoms in legume hosts, blackgram, cowpea and French bean was analysed. Plants agroinoculated with mutants K73R, C86S and the double mutant C84S,C86S showed increase in severity of symptoms compared with the wild type. However, mutants W2S and H14Q,G15E caused marked attenuation of symptoms. While the double mutants C84S,C86S caused a 50-fold increase in double-stranded supercoiled and single-stranded DNA accumulation, the mutations W2S and H14Q,G15E showed a decrease in double-stranded supercoiled and single-stranded viral DNA accumulation. Because AV2 mutants affect the ratio between open circular and supercoiled DNA forms, we hypothesize that these mutations may modulate the functions of the replication initiation protein.