Impact of the viruses associated with mosaic and yellow leaf disease on varietal degeneration in sugarcane

Abstract

The viruses Sugarcane mosaic virus (SCMV) and Sugarcane streak mosaic virus (SCSMV) causing mosaic disease and Sugarcane yellow leaf virus (SCYLV) causing yellow leaf disease (YLD) are synergistically associated with varietal degeneration of sugarcane in India. The objectives of the study were to understand the degree of degeneration in the sugarcane cultivar CoJ 64 due to the mixed infection of the three viruses. The study was also aimed to quantify the threshold limit of viruses for the plant to express the symptoms. Symptomatic plants exhibited significant reductions in germination and growth as compared to the asymptomatic plants and maintained poor vigour. Such plants recorded significant reductions in different physiological parameters such as photosynthetic rate, stomatal conductance, transpiration rate, chlorophyll fluorescence ratio and leaf chlorophyll content. At the time of harvest a drastic reduction in the juice yield by 30.13–36.04% was found. Diagnosis by qRT-PCR revealed that both asymptomatic and the symptomatic plants showed mixed infections of all the three viruses. The copy numbers in the asymptomatic plants were detected between 102 and 103 for all the three viruses whereas, the copy numbers in the symptomatic plants were in the range of 104 and 106 for all the three viruses. The present study established a threshold limit of three RNA viruses causing varietal degeneration in sugarcane and this information will be used while selecting new varieties to avoid varietal degeneration under field conditions.