Juice chemical properties of 24 sorghum cultivars under varying levels of sugarcane aphid (Melanaphis sacchari) infestation

Sugarcane aphids [Melanaphis sacchari (Zehntner)] have become a significant pest of grain, forage, and sweet sorghum [Sorghum bicolor (L.) Moench] in the USA in recent years. However, the effects of sugarcane aphid damage on sweet sorghum juice quality have not been well studied. A three-year (2015–2017) field study was conducted at Tifton, GA to assess planting date effects (April, May, or June planting) and cultivar responses (24 cultivars) to sugarcane aphids in sorghum. Aphid damage ratings were measured in all three years and cumulative aphid days were measured in 2016 and 2017. Cumulative aphid days (ln scale) and damage ratings (relative marginal effect) were correlated in five of the six plantings. Stem juice was collected at maturity from seven plantings for chemical analyses, which included HPLC, fluorescence excitation-emission spectrophotometry with parallel factor analysis (EEM/PARAFAC), and cyclic voltammetry (CV). Aphid damage ratings and cumulative aphid days were negatively correlated with sugar-related traits, particularly brix and total sugars. In four plantings, significant negative correlations (r ≤ −0.493) between trans-aconitic acid concentration and aphid damage were observed. Fluorescence and electrochemical properties related to the presence of polyphenols also showed correlations with aphid damage, particularly in the resistant landrace No. 5 Gambela. These secondary metabolites may play a role in sugarcane aphid resistance or tolerance. Stability analysis revealed that the more tolerant cultivars were able to maintain high concentrations of total sugars and trans-aconitic acid across environments.