Possible mechanism of Austrocedrus chilensis decline by the pathogenic action of Phytophthora austrocedrae

VÉLEZ, M.L.; SILVA, P.V.; GRESLEBIN, A. G.

Colonia del Sacramento

Congreso; IUFRO Forest Protection Joint Meeting; 2011

IUFRO, Universidad

Phytophthora austrocedrae, a soil-inhabiting organism, has been found to be the primary pathogen causing extensive mortality of Austrocedrus chilensis in Patagonia. It produces necrosis of the inner bark. However, the mechanistic basis for the disease remains unknown. We studied the action of the pathogen on tree physiology in a four-week study. In the first week stem-inoculated saplings showed a significant decrease in photosynthesis (A, 29%) without alteration of other physiological parameters, and necrotic lesions restricted to the inoculation point. This finding suggested a distant effect from the inoculation area caused by the pathogen. From the second week on, progressive decreases in A (83%, 4° week), stomata conductance (gs, closed stomata from the 3° week) and stem-specific hydraulic conductivity (Ks, 78%, 4° week) concomitantly with significant stem lesions were observed. Water use efficiency (WUE) increased in the second week (78%) and declined progressively from the third week (65%, 4° week). Xylem examination below lesions showed presence of hyphae and resinous materials in tracheids and rays. Necrosis of parenchyma ray cells and blockage of tracheids torus was observed. Healthy xylem showed no resinous materials or tracheid blockage, but abundant starch contents in rays, that were absent in altered xylem. We detected toxin activity in the culture filtrate (CF) of the pathogen, evidenced by changes in extracellular pH and conductivity, and increased necrosis in tissues of leaves and stem challenged with CF in vitro. Similar results were obtained in leaf tissues of the inoculated saplings in vivo, with increasingly alterations along the study. CF injection in xylem of saplings induced a decline in A (33%) and disturbance of leaf tissue integrity, without altering gs, WUE or Ks. The decrease of A correlated with changes in tissue integrity. A possible mechanism of A. chilensis decline induced by P. austrocedrae is discussed.