Impact of laurel wilt, caused by Raffaelea lauricola, on xylem function in avocado, Persea americana

Laurel wilt, caused by Raffaelea lauricola, is a lethal, vascular disease of avocado, Persea americana. Its impact on xylem function was examined in artificially inoculated plants of the Simmonds cultivar. Three, 7, 14, 21 and 42 days after inoculation (dai), plants were rated externally and internally for disease severity on a subjective one (asymptomatic) to 10 (dead or 100% symptomatic) scale. Stems were then cut under water, 15 cm below the inoculation point, and placed in 0.1% aqueous solutions of acid fuschin in a greenhouse. After 48 h, the percentage of functional xylem was estimated 10 and 5 cm above and below the inoculation point by quantifying acid fuschin-stained portions of digitized stem cross-sections. Hydraulic conductivity was determined by placing the proximal end of 5-cm-long stem sections, harvested between 5 and 10 cm above the inoculation point, in water and quantifying the volume of water that was drawn over time through the distal end under partial pressure (350 mm Hg). Functional xylem decreased by 3 dai, well before the development of vascular discoloration (7 dai) and wilting of foliage (14 dai). By 14 dai, extensive vascular discoloration had developed and there was a dramatic reduction in functional xylem; plants with internal disease severities of 7 or greater had <20% functional xylem. Hydraulic conductivity decreased exponentially as non-functional xylem and disease severity increased. In plants with internal severities >7, mean flow rates of water were 0.07 ml-1 min-1 cm-2vs. 42 ml-1 min-1 cm-2 in mock-inoculated plants. The rapid development of these changes suggests that it may be difficult to manage laurel wilt in avocado once plants are infected by R. lauricola. Better understanding of the temporal and spatial development of infection and how the host responds to infection may assist efforts to select laurel wilt-tolerant avocado cultivars.