Histological and anatomical responses in avocado, Persea americana, induced by the vascular wilt pathogen, Raffaelea lauricola

Abstract

Raffaelea lauricola causes laurel wilt of avocado, Persea americana. Host × pathogen interactions were examined with light and scanning electron microscopy. The susceptible avocado cultivar 'Simmonds' was inoculated and examined 5 cm above the inoculation site 3, 7, 14, 21, and 42 days after inoculation (dai). No external symptoms were observed at 3 and 7 dai, and there were no anatomical differences when compared with the mock-inoculated plants. By 14 dai, external symptoms were present and dark discoloration had developed in sapwood. Tylose development increased significantly by 14 dai, and was positivity correlated with disease severity (P < 0.05). By 14 dai, gels formed in xylem vessels, fibers, and adjacent parenchyma cells; they were associated with xylem blockage and composed of phenols, pectin, and lipids, as suggested by, respectively, toluidine blue O, ruthenium red, and Sudan III stains. With a chitin-specific stain, fluoresceinconjugated wheat germ agglutinin, infrequent mycelia, and conidia of R. lauricola were visualized within xylem lumena and fibers, regardless of sample date. Understanding how avocado responds to the presence of this pathogen could assist the development of laurel wilt-resistant avocado genotypes and inform efforts to manage this disease with other measures.

Xyleborus glabratus attacks and systemic colonization by Raffaelea lauricola associated with dieback of Cinnamomum camphora in the southeastern United States

“…This results in the plugging of water‐conducting tissues and wilt symptoms, presumably by the fungal tissues as well as by the production of tyloses and gums by the host (Inch and Ploetz ; Inch et al. ). Later generations of beetles make brood galleries in the wilted trees (Harrington and Fraedrich ).…”

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

“…After laurel wilt-susceptible (avocado and swamp bay) and tolerant (camphortree) hosts were inoculated with a green fluorescent protein (GFP)-labelled strain of R. lauricola, Campbell et al [120] reported that the pathogen was scarcely visible in microscopic cross sections, even in dead or dying plants. Although they observed that a maximum of 0.9% of the xylem lumena of avocado were colonized by the GFP-labelled strain 30 days after inoculation (dai) (Figure 4), about 40% of the lumena of avocado were occluded by tyloses 21 dai in another study [123] (Figure 5). Mobility of the pathogen or its metabolites in the xylem seems to be related to susceptibility.…”

“…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].…”

“…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). Tree mortality is associated with vascular functionalities of less than 10%.…”

North American Lauraceae: Terpenoid Emissions, Relative Attraction and Boring Preferences of Redbay Ambrosia Beetle, Xyleborus glabratus (Coleoptera: Curculionidae: Scolytinae)

“…Harr., Fraedrich & Aghayeva (Ophiostamatales: Ophiostomataceae), causes laurel wilt [5]–[6]. The presence of R. lauricola in susceptible hosts elicits a cascade of events, including secretion of resins and formation of extensive parenchymal tyloses that wall off conductive xylem vessels [7]–[8]. This defensive response results in diminished water transport, which initially impedes spread of the mycopathogen, but ultimately leads to systemic wilt and host tree mortality.…”