ABSTRACT
Cassava (Manihot esculenta Crantz) is a vegetatively propagated tropical root crop. Viral infections have been identified as an important biotic component limiting cassava production (2). To survey for virus diseases, symptomatic samples of cassava plants were collected from the Venezuelan cassava production states Amazonas, Aragua, Barinas, Cojedes, Monagas, and Portuguesa. Mechanical transmissions to a differential host (Chenopodium quinoa) and a previously described double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) procedure (3) were used to test for the presence of Cassava virus X (CsVX). The DAS-ELISA procedure used gamma globulin diluted 1/2,000, conjugate diluted 1/2,000, and leaf sap extracts diluted 1/100 (1). CsVX and frog skin disease (detected graft inoculation tests with cultivar Secundina and electron microscopy) were detected in mixed infections in plants collected in Barinas. CsVX was not detected in any of the plants collected in the other surveyed areas. The low incidence of the virus (11.43%; 4 of 35 samples) suggests that CsVX has been introduced only recently via infected planting stock. Transmission of the virus is 100% when infected cuttings are used as propagation material. CsVX caused no symptoms on any of the cultivars examined in the field, and the occurrence of symptomless CsVX in Venezuela may result from the inadvertent use of cuttings from virus-infected plants. This is the first report of CsVX in cassava in Venezuela. References: (1) B. L. Nolt et al. Ann. Appl. Biol. 118:105, 1991. (2) B. L. Nolt et al. Plant Pathol. 41:348, 1992. (3) A. C. Velasco et al. 1994. Prog. Virol. Yuca, CIAT, Cali, Colombia.
ABSTRACT
Frogskin disease (FSD) is a disease of clonally propagated cassava (Manihot esculenta Crantz) and has been reported to reduce cassava yields significantly in South America (1). FSD is caused by an uncharacterized virus that is restricted to South America. The evidence indicates FSD is transmitted by stem cuttings and graft (3). However, little information is available on its distribution and incidence in Venezuela. Eighty-seven samples with virus-like symptoms were collected with the help of technical staff and producers in cassava-producing states: Amazonas (1 sample), Aragua (7 samples), Barinas (35 samples), Cojedes (8 samples), Monagas (19 samples), and Portuguesa (17 samples). In these states, the average daytime temperature was 26°C, but the average was higher (>28°C) during the dry season. Samples were collected during the rainy season because high temperatures and dry field conditions appeared to suppress symptom expression, while cooler conditions tended to favor symptom development (2). Roots of sampled cassava plants were examined for the presence of FSD. A single 70- to 80-cm-long stem cutting was taken from each plant and subdivided into four pieces. Two pieces were used as rootstocks in graft-inoculation tests with Secundina scions for FSD detection, and two pieces were potted in sterilized soil to be used in other tests. All potted and grafted plants were kept in the Vegetable Virology Laboratory of the Faculty of Agronomy (Universidad Central de Venezuela), at an average temperature of 24°C and 80% relative humidity. FSD-infected plants were identified by mosaic symptoms on Secundina scions and the presence of 80-nm spherical viral particles. Most FSD-infected cultivars expressed only root symptoms. However, in the case of Secundina cvs. MCOL 22 and MCOL 113, foliar symptoms were also detected (1). FSD was found in a simple infection in one cassava sample from Aragua State (14.3% incidence, 1 of 7 samples) and in four cassava samples from Barinas State (11.4% incidence, 4 of 35 samples) associated with Cassava virus X (detected by double-antibody sandwich enzyme-linked immunosorbent assay). To our knowledge, this is the first report of FSD detection in Venezuela. References: (1) E. A. Frison et al. Informe Anual. CIAT, Cali, Colombia, 1995. (2) B. L. Nolt et al. Plant Pathol. 41:384, 1992. (3) Technical Guidelines for the Safe Movement of Cassava Rome. FAO/IBPGR. p. 10-27, 1991.
