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Sugarcane yellow leaf disease (YLD) has emerged as a serious threat to sugarcane cultivation worldwide, particularly in India. The aphid, Melanaphis sacchari, has been proved to be the vector for the causative agent sugarcane yellow leaf virus (ScYLV) across the world. However, the population dynamics of the aphid are not established under field conditions. Hence, this study was conducted with the aim of studying the population dynamics of M. saccharifor four consecutive years (2015–2019) under the typical tropical conditions. The sugarcane aphid starts colonizing the crop in April, reaches its maximum in June–July, starts declining in August, and almost vanishes in September. Thus, the 5–7-month-old crop supports higher aphid population irrespective of the varieties, provided the climatic conditions are near-normal. A significant variation in the climatic conditions observed in 2016–2017 significantly altered the population dynamics of the aphid on sugarcane varieties. In 2016–2017, the aphid appeared late in May and extended till December. It was interesting to note that the aphid population was recorded to be higher in the diseased plants as compared with the healthy ones of the same variety. Further, the YLD-susceptible cvs CoC 671, Co 86032, Co 86010, CoPant 84211, and B38192 were recorded with comparatively higher numbers of aphid during the peak season in all the 4 years. The colonization of aphid was observed to be aggregate in nature and not uniform in all the plants of a variety and leaves of an infested plant. Increased levels of aphid infestation was found between + 5 and + 7 leaves in sugarcane plants, across the seasons and varieties, and rainfall was negatively associated with the build-up of aphid colonization. The study provides new findings on the dynamics of M. sacchariunder tropical climate in India for the first time and contributes to the understanding of the disease epidemiology.

In sugarcane (Saccharumspp. 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 × 102to 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 105–107copies 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.