Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2016
Publication Date: 10/3/2016
Citation: Zhao, W., Gottwald, T.R., Bai, J., McCollum, T.G., Plotto, A., Baldwin, E.A. 2016. Correlation of Diplodia (Lasiodiplodia theobromae) infection, huanglongbing, ethylene production, fruit removal force and pre-harvest fruit drop. Scientia Horticulturae. 212:162-170. doi:10.1016/j.scienta.2016.09.032.
Interpretive Summary: Citrus greening or Huanglongbing (HLB) disease is devastating the citrus industry and reducing yields, partly through pre-harvest fruit drop. In previous work, it was discovered that a fungus, that usually is only a problem postharvest, was taking advantage of the weakened tree and colonizing fruit tissue while the fruit was on the tree. This study aimed to determine if colonization by that fungus was exacerbating the early fruit drop, by spraying trees with a fungicide. Results showed that the fungicide application reduced fruit drop for three (Midsweet orange, Early Gold orange and Murcott tangor) out of the five citrus cultivar/types evaluated. The fungicide spray had no effect on Navel orange or Ray Ruby Grapefruit.
Technical Abstract: One symptom of citrus huanglongbing (HLB) disease is excessive pre-harvest fruit drop. As the severity of HLB has progressed through commercial citrus groves in Florida, pre-harvest fruit drop has increased, resulting in significant loss of yield. A greater incidence of Diplodia infection was recently found in HLB-symptomatic orange (Citrus sinensis) calyx abscission zones (AZ-C) than in non-symptomatic fruit, and the Diplodia infection was correlated with fruit ethylene production and reduction in fruit detachment force (FDF), suggesting that the pathogen may exacerbate fruit drop in HLB-affected citrus. In this study, the effects of fungicide (Quadris Top) application on incidence of Diplodia infection and fruit drop were determined. The experiment was conducted in a commercial grove with essentially 100% of the trees being HLB-symptomatic, and included five citrus cultivars (‘Early Gold’ orange, ‘Midsweet’ orange, ‘Murcott’ tangor, ‘Navel’ orange and ‘Ray Ruby’ grapefruit). For each cultivar, there were 20 experimental trees, 10 treated with fungicide and 10 not treated. The fruit detachment force (FDF) and Diplodia titer were quantified in samples collected at multiple time points following fungicide application and dropped fruit assessed over time. Diplodia titer was lower and FDF significantly higher in fungicide-treated compared to non-treated ‘Early Gold’, ‘Midsweet’ and ‘Murcott’, but not ‘Navel’ or ‘Ray Ruby’ fruit. Fungicide treatment reduced fruit drop significantly for ‘Early Gold’ (P<0.05), ‘Midsweet’ (P<0.05) and ‘Murcott’ (p<0.001) by 45%, 30% and 46% that of non-sprayed controls, respectively; but did not have an effect on fruit drop for ‘Navel’ orange or ‘Ray Ruby’ grapefruit. Results suggest that fungicide applications may be an effective strategy to reduce pre-harvest fruit drop in citrus.