Formation of Ligno-suberized tissues in jack pine resistant to the European race of Gremmeniella abietina

A histological study was conducted to provide insights into the defense mec hanisms of Pinus banksiana resistant to the European (EU) race of Gremmenie lla abietina in naturally infected sites. At the time of sampling, the only apparent symptom was a blight induced at the tip of the shoots. The identi ty of G. abietina during microscopic examinations was confirmed by an immun ogold labeling method. Once the fungus had succeeded in penetrating the bra cts through stomata, it invaded the stem cortex and the phloem cells and at tained the vascular cambium. The progression of the pathogen to the pith wa s possible principally through intense colonization of needle traces but al so by the invasion of the rays. Ligno-suberized tissues confining the patho gen within the necrotic area were revealed by histochemical tests. Well-def ined boundaries were initiated at the base of healthy needles and at the va scular cambium level. They regularly formed one continuous suberized barrie r completely crossing the shoot from one needle to the other. A nonlamellar form of suberin was observed in transmission electron microscopy. Restorat ion of cambial activities and tissue regeneration following necrophylactic periderm formation were suggested as essential factors in the defense syste m of R banksiana against the EU race of G. abietina. To our knowledge, this is the first demonstration of an anatomical defense mechanism of a conifer against Scleroderris canker.