The University of California Riverside (UCR) rootstock breeding program has selected five advanced avocado (Persea americana Mill.) rootstocks for their resistance to Phytophthora Root Rot (PRR), a major disease affecting avocados worldwide. Another significant threat to avocado trees is laurel wilt (LW), caused by the fungus Harringtonia lauricola. To date, laurel wilt has been reported in 12 states, but has only infected avocado trees in Florida, where it has killed approximately 290,000 trees worth an estimated $95 million. It is anticipated that laurel wilt will eventually spread to California, the leading avocado production region in the U.S., as well as to Mexico and South America, where the largest world-wide avocado production occurs. For the UCR experimental PRR-resistant rootstocks to be successful in the long term after their commercial release, they should positively influence the LW resistance of ‘Hass’ and other scions. In this study, we tested the LW susceptibility and physiological responses of ‘Donnie,’ ‘Hass,’ ‘Gem,’ and ‘Reed’ scions grafted onto UCR advanced clonal rootstocks PP35, PP40, PP42, PP45, and PP80 under climate-controlled greenhouse conditions. All scions were also grafted and tested on ‘Waldin’ seedling rootstock, which is the primary avocado rootstock used in South Florida. Trees of each scion/rootstock combination were either inoculated with H. lauricola or with deionized water as a control. There were six single-plant replicates per treatment for each scion/rootstock combination, arranged in a completely randomized design. Visual disease symptoms were evaluated on a scale of 1 to10 (1= no symptoms, 10 = severe symptoms) every 3 to 4 days from the time of inoculation until the trees were harvested. Physiological variables, including leaf gas exchange (net CO2 assimilation, stomatal conductance, and transpiration), were also measured. At the end of the experiment, all plants were harvested, and the presence or absence of H. lauricola was confirmed by plating stem sections on CSMA (cycloheximide-streptomycin malt agar) medium. Although all scion/rootstock combinations were susceptible to LW, there were differences in susceptibility among combinations. The combinations ‘Hass’/PP45, ‘Gem’/PP40, ‘Gem’/PP80, ‘Hass’/Waldin’, and ‘Gem’/’Waldin’ exhibited the greatest tolerance to LW, as indicated by fewer external symptoms and less reduction in leaf gas exchange variables compared to the other combinations tested. Therefore, these more LW-tolerant scion/rootstock combinations have potential for planting in areas where both PRR and LW diseases occur. The most tolerant combinations identified in this study continue to be evaluated under field conditions for horticultural characteristics and yields in Florida.
