Visually Asymptomatic Leaf Loss in Xylella fastidiosa-Infected Blueberry Plants

Xylella fastidiosa (Xf), a gram-negative bacterium, is a notorious, world-wide plant pathogen with an extended latent period that presents a challenge for early disease detection and control interventions. We used thermal imaging of tissue-cultured, experimentally Xf-infected blueberry plants to identify visually pre-symptomatic leaves and compared the minimum force required to dislodge symptomatic leaves from infected plants to leaves on uninfected (control) blueberry plants. For two different blueberry cultivars and one pathogenic isolate of X. fastidiosa, we found no statistical difference between the mean downward force for leaf dislodgement, regardless of symptom category, on Xf-infected blueberry plants. That force was about 50% to 30% of the mean force to remove leaves from uninfected blueberry plants depending on the cultivar. These results indicate that visually pre-symptomatic leaves may be just as readily lost under field conditions as visually symptomatic leaves, both of which are important for early disease detection. Second, some thermally symptomatic and visually symptomatic leaves appeared to self-prune (abscise) and this may be an unrecognized early symptom of Xf-caused disease in blueberries. Last, it is possible that the self-pruning of visually asymptomatic leaves may occur in other agriculturally and culturally important plants infected by X. fastidiosa, but this remains an unrecognized early disease symptom.