Histopatology of Phytophthora austrocedri in Austrocedrus chilensis.

TRONCOSO, OSCAR A.; GRESLEBIN ALINA G.; VÉLEZ, MARÍA LAURA

Congreso; 7th Meeting of IUFRO working party 7.02.09 "Phytophthora in Forests and Natural Ecosystems"; 2014

IUFRO, UNPSJB, CIEFAP, FUNDFAEP

Austrocedrus chilensis (D. Don) Pic. Ser. & Bizzarri (mountain cypress), Cupressaceae, suffers a disease caused by Phytophthora austrocedi that leads the trees to death. It has been speculated that trees are killed by extensive death of bark and cambium tissues and by the disruption of phloem transport but the pathogen mechanisms are not totally elucidated yet. The histopathology of A. chilensis disease is being studied to understand the pathogenic mechanisms of P. austrocedri as well as the tree defense mechanisms. Necrotic and healthy tissues, of naturally and artificially infected adult trees, were studied in order to assess the effects of the pathogen on the phloem and xylem. Portions of tissues from the advancing, medium and old areas of necrotic lesions, as well as from healthy areas at least 60-80 cm above lesion, were sliced into transverse, tangential and radial sections of 15 µm using a microtome and observed in a light microscope. Oospores of P. austrocedri were observed in affected phloem, especially in resin pockets. Hyphae were observed in phloem and xylem. In the xylem hyphae grow through rays, and pass from rays to tracheids through the cross-field pitting, and from one tracheid to another through the pits. Crossing hyphae filled the torus of the pit completely and consequently blocked the pit. Affected xylem showed the formation of trabeculae, single or double and frequently aligned, that were absent in healthy xylem. The trabeculae might appear as a response of the tree against the presence of the pathogen and could also contribute to the decrease of hydraulic conductivity observed in affected trees. Formation of traumatic resin ducts in the phloem associated to necrotic lesions was also observed. The resin ducts were much more abundant and bigger than the normal resin ducts of healthy phloem and can fuse to form resin pockets. Thus, these structures are related to the profuse resination produced associated to the advancing zone of the lesion that is assumed as a defense mechanisms of the tree.