Prospecting true ScYLV resistance in Saccharum hybrid parental population in India by symptom phenotyping and viral titre quantification

Abstract

In sugarcane (Saccharum spp. hybrids) cultivation, viral diseases pose a great challenge across the globe. Yellow leaf (YL) disease is one of the important viral diseases caused by Sugarcane yellow leaf virus (ScYLV), a positive-sense ssRNA virus, genus Polerovirus, family Solemoviridae. The disease symptoms appear in later stages of crop growth during grand growth to maturity phase with intense midrib yellowing in the abaxial leaf surface. At present, this disease is managed through tissue (meristem) culture and healthy seed nurseries in India. However, the virus-free plants are infected quickly by secondary inoculum from aphid vectors in the field, which necessitates the importance of developing YL-resistant varieties. We screened about 600?625 sugarcane parental clones to identify true YL resistance based on 0?5 disease rating scale since 2015 and categorised them as resistant, moderately resistant, moderately susceptible, susceptible and highly susceptible. Leaf samples were collected from all these categories of plants during 2018?20 for the viral titre estimation through absolute quantification method (qRT-PCR assay). The viral load was invariably high in all categories of susceptible samples that ranged from 4.40 ? 102 to 8.429 ? 106, whereas in YL-free asymptomatic clones, the viral load ranged from 82.35 ? 5.90 to 5.121 ? 104. The results clearly indicated that highest viral titre of 105?107 copies was present in all the susceptible clones irrespective of their disease severity grades. Our results clearly established that about 22.85% of apparently resistant sugarcane clones remained free from YL symptoms with significantly low ScYLV titre although we could not find a significant correlation between virus titre and symptom expression. The identified resistant parents will serve as sources of YL resistance to develop virus resistant sugarcane varieties.