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

Abstract

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 ÔSimmondsÕ 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 )2 vs. 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.

Destructive Tree Diseases Associated with Ambrosia and Bark Beetles: Black Swan Events in Tree Pathology?

“…In contrast, the xyleborine ambrosia beetles do not produce aggregation pheromones, but rely on semiochemicals and other cues to identify host trees (76,83,110,121). In general, much more information is needed on how these insects recognize suitable host trees and initiate development of the various diseases.…”

Laurel Wilt in Natural and Agricultural Ecosystems: Understanding the Drivers and Scales of Complex Pathosystems

“…Xylem blockage associated with tylose and gel formation appears to be at least partially responsible for the wilting symptoms [122,123]. Xylem function (the ability to conduct water) in avocado is impaired as soon as 3 days after inoculation, before the development of external or internal symptoms of the disease are apparent [123] (Figure 3).…”

“…To date, we have only rudimentary understandings of these processes [33,[121][122][123]. Although natural selection against susceptibility has probably occurred in camphortree and other Asian Lauraceae [1,33], host attributes that are associated with laurel wilt tolerance have been studied only recently [121].…”

“…They proposed that tylose formation induced by infection was responsible for reduced water transport in affected trees; C. smalleyi caused multiple cankers and reduced xylem function, resulting in crown wilt and decline of bitternut hickory. "Simmonds" avocado trees were inoculated with Raffaelea lauricola and xylem function was assessed with an acid fuchsin stain [122]. In (A), burgundy to pink staining in stem sections indicates functional xylem, which was quantified in scanned, digital images.…”

“…[121] observed that less than 1% of the lumena of avocado, swamp bay and camphortree were colonized with a GFP-labelled strain of Raffaelea lauricola, even 30 days after inoculation (dai) (when most plants of avocado and swamp bay had died from laurel wilt). "Simmonds" avocado trees were inoculated with Raffaelea lauricola and xylem function was assessed with an acid fuchsin stain [122]. In (A), burgundy to pink staining in stem sections indicates functional xylem, which was quantified in scanned, digital images.…”