Current Ph.D. Research Projects

Laurel Wilt Susceptibility Related to Host Physiology and Xylem Anatomy

Raiza Castillo, Ph.D. Student

Laurel wilt, caused by the fungus Raffaela lauricola, is an aggressive disease that spreads rapidly. The primary vector is Xyleborus glabratus, an ambrosia beetle that is not native to the United States. The laurel wilt fungus induces plugging of the xylem of the host tree, thereby causing a reduction or stoppage of water flow resulting in the wilting of leaves and eventual tree death. The disease affects members of the Lauraceae plant family and therefore could have a great impact wherever they are ecologically or economically significant. Since 2003, laurel wilt has caused extensive mortality of redbay trees throughout the southeastern United States.‌

‌‌There are three botanical races of avocado.  The Mexican (M) and Guatemalan (G) races are named for their geographic centers of origin, but the West Indian (WI) race is thought to have originated in lowlands of Central America before being brought to the West Indies. Hybridization occurs freely among the races, which appear to exhibit differential susceptibility to laurel wilt; G and hybrids of G x M cultivars were more tolerant than WI cultivars and WI hybrids. Avocado trees grown in Florida are grafted on ‘Waldin’ (WI) or to a lesser extent ‘Lula’ (WI x G) seedling rootstocks. Therefore, laurel wilt could be a serious threat to Florida’s avocado production because WI cultivars are the most common in the area as both scions and rootstocks.

‌‌Given the economic and ecological importance of Lauraceae species in the United States, a comprehensive investigation of anatomical and physiological differences related to laurel wilt susceptibility among plants in this family may provide insight into the differential susceptibility among species and cultivars. The exact physiological mechanisms that cause some species or cultivars to be more susceptible than others are not known and there is only one study that related the development of laurel wilt symptoms to the physiology of trees (avocado) with different susceptibilities to the disease.‌ 

This project, in collaboration with Dr. Randy Ploetz's plant pathology lab, is aimed at comparing vascular anatomy and physiology among trees in the Lauraceae family that are highly susceptible to laurel wilt with those of less susceptible trees and to relate these anatomical and physiological variables to the progression of the disease in artificially inoculated trees. In one set of experiments, xylem anatomy and physiology as they relate to within tree disease progression will be compared among seedling forest tree species that are highly susceptible to laurel wilt and seedling WI and WI hybrid avocado cultivars commonly used as rootstocks in Florida that are less susceptible to laurel wilt. In another set of experiments, xylem anatomy and physiology and their relationship to within-tree disease progression will be compared among clonally produced avocado trees of the 3 different botanical races (WI, M and G). The results from this study should increase our understanding of the anatomical and physiological bases for laurel wilt susceptibility and may assist in the selection of avocado rootstocks that are most tolerant of the disease.