The effect of avocado sunblotch disease (ASBD) on tree morphology, fruit maturity, yield and quality of ‘Hass’ avocado in South Africa

Avocado sunblotch viroid (ASBVd) is found in many avocado-growing regions of the world, where it affects fruit yield and quality. The trees develop two types of infections: symptomatic and symptomless infections. Symptoms are most obvious as yellow streaks on leaves, fruit and green stems of symptomatic trees and symptomless carrier trees do not display any such symptoms. Symptomless carrier trees are considered the primary source of disease transmission in orchards. Hence, this study investigated the impact of ASBVd-infected symptomless carrier trees on tree morphology, fruit maturity, yield and quality of ‘Hass’ avocado from 2019 until 2021. Differences were observed in the orchard between infected and healthy trees; trees with medium and high viroid concentrations excessively produced flowers, lost leaves during flowering and ultimately bore few to no fruit at the end of the season. The dry matter content results showed that ASBVd did not affect the rate of maturity of the fruit, as fruit from infected and healthy trees matured at the same time. Yield counts indicated that medium and highly infected trees produced between 83 and 96% lower yields compared to healthy trees. Postharvest studies showed that medium and highly infected fruit significantly lost firmness and coloured up more rapidly than healthy fruit. Infected, non-stored fruit also developed external rots and shrivels, however, these disorders were reduced in fruit stored at 5 °C for 28 days. Therefore, flower overbearing with the shedding of leaves and lower yields can be used as indicators of ASBVd infection in ‘Hass’ orchards but confirmation with molecular testing is required. These observations can be incorporated into an ASBD management strategy in ‘Hass’ orchards.