Laurel Wilt Differentially Affects Xylem Sap Flow of Three Avocado Cultivars

Laurel wilt, caused by the fungus Raffaelea lauricola, has decimated members of the Lauraceae family in the southeastern United States since the early 2000s.  It was detected in a Florida commercial avocado orchard in 2012, and poses a grave threat to the avocado industry.  Affected trees wilt and usually die due to plugging of the vascular system.  Susceptibility to laurel wilt varies among avocado cultivars.  We assessed the effects of laurel wilt on xylem sap flow in three avocado cultivars that differed in susceptibility.  ‘Russell’, ‘Brogdon’ and ‘Marcus Pumpkin’ trees (clonal scions grafted on ‘Waldin’ seedling rootstocks) were treated with either 750 conidia of R. lauricola at four equidistant sites above the graft union (inoculated; four plants per cultivar) or water (non-inoculated; one plant per cultivar).  Xylem sap flow in each tree was continuously monitored with a Dynagage sap flow system (Dynamax, Inc.) until all of the inoculated trees of ‘Russell’ (most susceptible) completely wilted.  Trees were also rated daily for laurel wilt development, based on an external synoptic scale where 1 = asymptomatic and 10 = entire canopy wilted or dead.  At the end of the experiment, trees were harvested, rated internally for laurel wilt (percentage of sapwood symptomatic, where 1 = asymptomatic and 10 = 100%), and assayed for the pathogen.  R. lauricola was recovered from all inoculated trees but not from the non-inoculated trees.  Mean cumulative sap flow over nine days prior to inoculation was significantly higher in ‘Russell’ than in the other cultivars.  Based on external and internal disease ratings, ‘Russell’ was significantly more susceptible to laurel wilt than ‘Brogdon’ and ‘Marcus Pumpkin’, which were statistically similar.  Beginning one week after inoculation, xylem sap flow for ‘Russell’ declined rapidly until plants died.  Mean sap flow per week was not significantly different between the less susceptible ‘Brogdon’ and ‘Marcus Pumpkin’.  The results indicate that susceptibility to laurel wilt in avocado is associated with the rate of sap flow.  Cultivars with a relatively high rate of sap flow (e.g. ‘Russell’) may be more susceptible due to a corresponding higher rate of conidium transport.  Additional cultivars with a wide range of sap flow rates should be tested to examine this hypothesis.