Chlorotic mottle of bean (Phaseolus vulgaris L.)

For the past years there have been outbreaks of a disease of bean ( Phaseolus vulgaris L.) in Colombia called bean chlorotic mottle. The etiology of bean chlorotic mottle was not known, but the disease was generally believed to be incited by the same whitefly-transmitted virus that causes variegation in malvaceous plants. The aim of this study was to identify and further characterize the causal agent of bean chlorotic mottle.As from the literature it was not clear which symptoms were characteristic of the disease, firstly a symptom had to be established common to all allegedly bean chlorotic mottle-affected bean plants. This symptom, a sharp mosaic in which the discoloured areas ranged from pale greenish-yellow to bright yellow, was of diagnostic value to detect infected plants in the field. In addition to this sharp mosaic, chlorotic mottle-affected bean plants in the field often showed a host of other symptoms, such as mild mottle, rugosity, leaf curling, malformation, dwarfing, proliferation and witches' broom-like growth. In the same affected population some bean plants showed only the sharp mosaic, whereas others showed additional witches' broom-like growth.Mechanical inoculation experiments conducted with crude sap and partially purified preparations from bean plants exhibiting chlorotic mottle in the field, showed three viruses to be present, viz. bean mild mosaic virus, cucumber mosaic virus and southern bean mosaic virus. By means of differential hosts these three viruses could be separated. Isolates of these viruses were designated BMMV-CIAT, CMV-CIAT and SBMV-CIAT, respectively. Experiments conducted with whiteflies did not reveal the presence of any whitefly-transmitted virus in chlorotic mottle-affected bean plants.BMMV-CIAT, is an isometric particle, 32 nm in diameter, with a single-stranded RNA molecule. The virus is readily transmitted mechanically and its host range is restricted to legumes, viz. Glycine max , Macroptilium spp., Phaseolus spp. and Rhynchosia minima . In P. vulgaris the virus usually causes a mild mosaic which is hardly visible, and in certain cultivars with age, the virus becomes latent. Though affected bean plants grew normally producing healthy- looking pods, flowering and pod formation were usually delayed by about a week under glasshouse conditions. The chrysomelid beetles Diabrotica balteata and Cerotoma facialis were efficient vectors of BMMV-CIAT. The virus is seed-transmitted in beans, percentages ranging from 1.2 to 3.6 in the different cultivars tested. In contrast to many other beetle-transmitted viruses, BMMV-CIAT sediments in sucrose gradients as one particle. The physical and chemical properties of BMMV-CIAT are similar or equal to those of BMMV described by Waterworth et al. (1977). The darkly stained granular material in root cells occasionally observed in the light microscope, proved to be aggregates of virus particles.Cross protection and serological tests showed that CMV-CIAT is related to the CMV-type strain. However, many differences exist between them, the most striking being the ability of CMV-CIAT to cause systemic mosaic in P. vulgaris . The host reactions of CMV- CIAT resembled those of CMV-B 32, an isolate from bean in Spain (Bos and Maat, 1974), but the former had a longer longevity in vitro . Generally, CMV-CIAT induced a yellow mosaic in P. vulgaris , but the symptoms varied greatly in different cultivars. In the cultivars Honduras 46 and Porrillo 1 a sharp mosaic, similar to chlorotic mottle, was produced one month after inoculation with CMV-CIAT None of the bean cultivars and CIAT advanced breeding lines tested were found to be resistant or hypersensitive to CMV-CIAT The virus is transmitted by Aphis gossypii and through seeds of P. vulgaris , Vigna radiata and V. unguiculata 'California Blackeye'. It has poor antigenic properties and is serologically related to CMV-B 32, to a CMV isolate from Yucca (Bouwen et al., 1978) and to the Y-strain of CMV (Scott, 1968). Large inclusions in plant tissues infected with CMV-CIAT could easily be detected in the light microscope.The third virus, SBMV-CIAT, is readily transmitted mechanically. Though the host plant range included mostly legumes, the virus also infected Cucumis sativus 'Ashley'. The virus was more harmful to P. acutifolius , in which it caused severe top necrosis, than to P. vulgaris . Among the cultivars and CIAT advanced breeding lines of P. vulgaris , hypersensitive hosts were found. The virus induced a bright yellow mosaic in G. max . Besides P. vulgaris 'Pinto U.I. 650' other suitable assay hosts for SBMV-CIAT were found viz. Mucuna pruriens and M. utilis which reacted with pin-point necrotic local lesions to the virus. SBMV-CIAT is seed transmitted, the percentages ranging from 3.6 to 33.6 depending on the bean cultivar. Thephysical and chemical properties of SBMV-CIAT are similar or equal to the SBMV bean strain, except for the guanine content of the nucleic acid.An experiment conducted to find out the effect of the above- mentioned viruses alone or in combination with each other on bean plants, indicated that symptoms resembling chlorotic mottle could be reproduced in bean plants by mechanical inoculation with CMV-CIAT alone or in combination with the other two viruses. The vast range of other symptoms sometimes exhibited by bean plants with chlorotic mottle in the field, could also be reproduced by mechanically inoculating the viruses in different combinations at different times after planting of seeds. Witches' broom-like symptoms were produced in bean plants inoculated on their primary leaves with a mixture containing SBMV-CIAT with BMMV-CIAT or with CMV-CIATFrom the present study it is clear that bean chlorotic mottle, as described in literature, is a composite disease caused by a combination of BMMV-CIAT, CMV-CIAT and SBMV-CIAT in which CMV-CIAT is responsible for the sharp mosaic and the other two viruses together are responsible for the additional symptoms like rugosity, leaf curling, witches' broom-like growth etc. Efficient vectors of these viruses are present in the bean fields. Though data are not available for the population distribution of aphids, chrysomelid beetles are present throughout the year in the CIAT fields. Seed transmission of the virus is of considerable epidemiological significance. Infected seeds are vehicles for long distance transport and survival from one season to another, especially in case of BMMV-CIAT, and BMMV-CIAT which have a narrow host range. The diseases caused by the three viruses might be controlled by reducing the number of infection sources and by limiting the spread of the viruses.