2018 ASHS Annual Conference

Laurel wilt (LW) is a lethal vascular wilt disease caused by the fungus Raffaelea lauricola that affects members of the laurel family, including avocado (Persea americana Mill.). Since the early 2000s, it has spread across the south Atlantic coastal region and has significantly impacted Florida avocado production. There is concern that it may reach California because it has already spread as far west as Texas. Previous work has demonstrated that LW susceptibility varies among cultivars and that this variation may be related to their physiology. We evaluated the effects of LW on three clonal avocado rootstocks commonly used in California, ‘Toro Canyon’, ‘Dusa’, and ‘Duke-7’. Two-year-old plants were inoculated with R. lauricola; control plants were inoculated with sterile water. Chlorophyll fluorescence [the ratio of variable to maximum fluorescence (Fv/Fm)], leaf chlorophyll index (LCI) net CO₂ assimilation, transpiration, and stomatal conductance were measured on inoculated and non-inoculated plants of each cultivar every 2-3 days. Xylem sap flow was monitored continuously in all plants. External symptoms (leaf wilting and stem dieback) were assessed daily using a visual scale (1 to 10). At the end of the experiment, the amount of vascular staining was assessed using the same scale and stem sections were assayed for the pathogen on potato dextrose agar. External LW symptoms developed 22, 33, or 36 days after inoculation (dai) for ‘Dusa’, ‘Duke-7’, and ‘Toro Canyon’, respectively. The LCI was lower for inoculated than non-inoculated plants 45 dai in ‘Toro Canyon’ and 48 dai in ‘Dusa’. The Fv/Fm was lower for inoculated than non-inoculated plants 27 dai in ‘Toro canyon’ and 31 dai in ‘Dusa’. Inoculation did not affect the LCI or the Fv/Fm in ‘Duke-7’. Net CO₂ assimilation, transpiration, and stomatal conductance decreased as a result of inoculation 34, 41, and 48 dai for ‘Dusa’, ‘Toro Canyon’ and ‘Duke 7’, respectively. We did not observe significant differences in xylem sap flow between inoculated and non-inoculated plants of any cultivar. The amount of vascular staining was low in each cultivar, with inoculated plants exhibiting 20-30% internal stem discoloration. Based on this study and previous results with other cultivars, it appears that these rootstocks are not highly susceptible to LW. However, further studies comparing these rootstocks to susceptible cultivars are needed to confirm this.