Disease severity and ecophysiology of rootstock/scion combinations of different avocado (Persea americana Mill.) genotypes in response to laurel wilt

Abstract

Laurel wilt, caused by the fungus Raffaelea lauricola, is a lethal vascular disease that affects many members of the Lauraceae family, including avocado (Persea americana Mill.) trees. Previous studies have shown differences in laurel wilt susceptibility among genotypes, with cultivars of the West Indian (WI) race generally more susceptible to laurel wilt than Guatemalan (G) or Mexican (M) race cultivars. We conducted four experiments to compare susceptibility and ecophysiological responses to laurel wilt among scion/rootstock combinations from the different botanical races. In the first two experiments, clonally propagated ‘Simmonds’ (WI) scion grafted on ‘Waldin’ (WI), ‘Reed’ (G) or ‘Duke 7’ (M) rootstock (Experiment 1), or grafted on ‘Waldin’, ‘Nabal’ (G) or ‘Zutano’ (M) rootstock (Experiment 2) were compared. In the other two experiments, ‘Waldin’, ‘Reed’ or ‘Duke 7’ scion grafted on ‘Duke 7’ rootstock (Experiment 3), or ‘Reed’ rootstock (Experiment 4) were compared. For each scion/rootstock combination, trees were divided into two treatments: inoculated with R. lauricola, or 2) inoculated with deionized water as a control. Disease severity in all trees was determined daily using a rating scale based on the percentage of visible symptoms. Net CO2 assimilation (A), stomatal conductance of water vapor (gs), the daily xylem sap flow rate, and the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm) in leaves were determined three times per week until one or more scion/rootstock combination in the laurel wilt treatment showed severe symptoms. Trees from each treatment were then harvested and stem sections observed for browning of the sapwood (a characteristic laurel wilt symptom), and plated on a selective medium to recover the fungus if present. Raffaelea lauricola was recovered in all trees in the laurel wilt treatment and no trees in the control treatment. Laurel wilt susceptibility varied among the scion/rootstock combinations. Trees in the laurel wilt treatment with ‘Simmonds’ as the scion developed more symptoms and exhibited a greater reduction in physiological variables than the control trees, regardless of the genotype of the rootstock. ‘Duke 7’, either as the scion or rootstock, inoculated with the laurel wilt pathogen had less disease development and less of a reduction in physiological variables than the other genotypes. The results indicate that there are differences in laurel wilt susceptibility among grafted avocado cultivars of different genotypes. These difference may be related to the botanical race of the scion and/or the rootstock.