Screening for Avocado Resistance to Laurel Wilt using Shoot Cuttings

Avocado (Persea americana Mill.) is a high-value specialty crop commercially grown in California and Florida. The Florida avocado industry contributes ~$13 million to the economy, with ~6,800 production acres in Miami–Dade County. However, the avocado industry is newly threatened by the fungal disease laurel wilt (LW; Raffaelea lauricola) vectored by an Asian ambrosia beetle (Xyleborus glabratus), which has devastated native avocado relatives. Injections of trees with propiconazole slow infection, but are not practical for commercial avocado production. Vector management strategies have so far been unsuccessful. However, there are preliminary indications that some avocado genotypes from more freeze-tolerant Mexican (M) and Guatemalan (G) races are more LW-resistant than the West Indian (WI) material that is the foundation of South Florida production. Most effective assessment of LW resistance reportedly requires inoculation of trees > 25 mm in diameter. Screening germplasm using this method requires laborious development of clonally propagated trees or use of heterogeneous own-rooted seedlings; trees are being grown for such field assessments now. However, to facilitate higher throughput screening, use of cuttings for LW resistance screening is being assessed. In preliminary experiments, 20 cm cuttings were placed in test tubes with the lower third immersed in water, inoculated with the LW pathogen, and monitored for disease development and aspects of resistance response. Internal disease symptoms of affected avocado are indicated by reddish brown to bluish grey sapwood.  Disease severity as indicated by sapwood discoloration was recorded from 0.4 up to 13.8 mm from the inoculation point. When tissue was plated onto the semi-selective media CSMA+, the pathogen was recovered from both inoculation point and apex of some cuttings. Lula cuttings started showing external symptoms 12 days post inoculation while Hass x Bacon showed no external symptoms over 6 weeks post infection. Lula is a cultivar with G x WI background recommended for Florida production and has been shown to be more susceptible to LW compared to Hass (G x M) and Bacon (G). The potential application of resistance screening using cuttings to the overall goal of developing excellent avocado selections for East–Central Florida will be discussed.