Epidemiological studies on the incidence of papaya ringspot disease under Indian sub-continent conditions

Papaya (Carica papaya L.) is a commercially important fruit crop cultivated worldwide due to its nutritional and medicinal values. Papaya ringspot disease (PRSD), caused by the papaya ringspot virus (PRSV), poses a significant threat to papaya cultivation, resulting in substantial yield losses. In this study, two independent field experiments were conducted at Bagalkote located in the Northern Dry Zone of Karnataka state of India. The first experiment aimed to identify the optimal planting month for papaya to effectively manage PRSV disease. The second experiment was conducted to determine the susceptible papaya growth stage for PRSV infection. The results revealed that early planting of papaya in June or late planting in March were identified as the most optimal planting times across the majority of growth stages, as they exhibited the lowest disease incidence along with superior growth and yield characteristics compared to other planting months. In contrast, planting during the winter season (September to January) resulted in high disease severity due to an increased aphid population. Conversely, planting during periods of low aphid activity (spring season) delayed disease onset until the monsoon. By the time the aphid population increased, the plants had already progressed beyond the flowering and fruit-bearing stages. In the second experiment, the severity and frequency of foliar symptoms on PRSV-inoculated papaya plants were significantly higher in those inoculated at the early growth stage compared to those inoculated at a later growth stage. This indicates that the early growth stage (up to 180 days after transplanting) is a critical period for PRSV infection, necessitating the implementation of effective disease management measures during this time to minimize disease spread and enhance growth and yield. Furthermore, plants inoculated at the early stage exhibited a higher viral titer, more severe symptoms, and a higher percent transmission rate compared to those inoculated at a later stage. These findings were supported by qRT-PCR analysis, which demonstrated a highly significant and positive correlation between early inoculation and disease severity.