Searching for laurel wilt resistance in avocados of Mexican and Mexican-Guatemalan ancestry.

Laurel wilt (LW), a lethal vascular disease caused by the ambrosia fungus , has severely reduced avocado ( Mill.) production in Florida and decimated populations of native lauraceous trees across twelve U.S. states. All commercial avocado cultivars evaluated to date succumb to the disease, but the speed at which the tree declines varies. Cultivars with West Indian (WI) genetic background develop severe symptoms faster than those with Mexican (M) and Guatemalan (G) pedigree. Genetic resistance to LW is urgently needed, as management relies on costly cultural practices. We screened non-commercial open-pollinated progenies from 19 Mexican and 6 Mexican x Guatemalan accessions, and 2 Guatemalan x West Indian cultivars recognized as tolerant by growers. From the five disease response parameters evaluated, the final disease intensity index and disease severity on the last evaluation day were used to classify genotypes. A wide variability of responses was observed within and among families. Symptomatic plants were present in all families, while some individuals within 15 families remained asymptomatic. 'Colín V-33' (M×G) family was identified as tolerant, and Libres 3 (M), Bladimiro M-06 (M) 'Colinmex' (M×G), 'Collinson' (G×WI), Libres 5 (M) and Rag-13 (M) families were classified as moderately tolerant. This is the first-time tolerance to laurel wilt in avocado is formally reported, though surviving material needs to be propagated for response validation and field testing. Identifying tolerant accessions can help understand the underlying mechanisms and provide breeders with genetic resources for the future incorporation of resistance genes into commercial cultivars.