Role of mixed infection of cassava mosaic viruses in cassava mosaic disease development

Abstract

A study on “Role of mixed infection of Cassava mosaic viruses in cassava mosaic disease development” was conducted at ICAR-Central Tuber Crops Research Institute, Sreekariyam, Thiruvananthapuram, during 2019- 2020. The important findings of the above studies are summarised in this chapter. Fifty samples randomly collected from the five varieties of cassava (Kochangamuttan, Kariyilappothiyan, Karutha gandharipadappan, H226, Manja noorumuttan), showed high score (>4) symptoms scale of very severe mosaic having distorted and misshapen leaves with stunted growth of plants. Multiplex PCR could differentially detect the presence of ICMV and SLCMV from the infected leaves. Among the five varieties of cassava, leaf samples collected from only one variety (Kariyilappothiyan) had ICMV infection as confirmed by PCR. Whereas, the samples from the variety Manja noorumuttan, did not show any PCR amplification. The mosaic symptoms appeared in the leaves of the variety may be due to physiological disorders such as nutritional deficiency. But the rest of the varieties showed amplification and had SLCMV infection. Out of five varieties, Manja noorumuttan variety is free from both ICMV and SLCMV. Out of rest four cassava varieties, Kariyilappothiyan was found infected with ICMV and other three varieties were infected with SLCMV. None of the varieties showed mixed infection of ICMV and SLCMV. Using chip bud grafting, both the SLCMV and ICMV infected plants were used as stock and scion for mixed infection. The grafted cassava plants showed yellow mosaic patterns on newly appeared leaves after two weeks. No substantial changes in the symptoms were observed in the plants grafted with buds from ICMV on SLCMV infected plants. But severe symptoms were observed in ICMV infected plants grafted with buds from SLCMV infected plants. This reveals that SLCMV is more severe than ICMV. Agroinoculation of Nicotiana tabacum was done with a partial dimer of DNA A and DNA B of both SLCMV and ICMV, in order to study the symptom appearance during the mixed infection. The plants inoculated with DNA A components of SLCMV and ICMV individually, showed curling of young leaves and mild mosaic symptoms respectively. The plants infiltrated with both components of SLCMV, showed severe stunting, thickening of mild veins, curling of leaves. However, the plants inoculated with both components of ICMV showed mild mosaic and mild curling of leaves and reduction in size. In the case of mixed infection of DNA A and DNA B components of both SLCMV and ICMV in Nicotiana plants showed symptoms including curling on the newly emerged leaves, yellow mosaic pattern on older leaves and severe stunting, resembling a mild SLCMV infection. Similarly, Agrobacterium strains with infectious clones were injected in the axillary buds of cassava plants to analyse the symptomatology during the mixed infection of SLCMV and ICMV. But the cassava plants did not show any mosaic symptoms in leaves with the infectious clones until 60 days after inoculation. The study reveals that agro- inoculation method is more effective in N. tabacum than cassava.