Microscopic observations of the early stages of the infection process by Lachnellula willkommii (Hartig) Dennis artificially inoculated on larch (Larix laricina (Du Roi) K. Koch) seedlings

European larch canker (ELC) caused by Lachnellula willkommii (Hartig) Dennis is an introduced disease first reported in North America in eastern Massachusetts in 1927 in European larch (Larix decidua Mill.) plantations (Spaulding and Siggers, 1927). In Canada, the disease was first discovered in the Maritimes in 1980 on native eastern larch (tamarack) (Larix laricina (Du Roi) K. Koch) (Magasi and Pond, 1982). Surveys since then established the distribution of the disease as widespread in southeastern New Brunswick and on mainland Nova Scotia (Ostaff, 1986). A first detection of the disease on Prince Edward Island was reported in 1992 (Simpson and Harrison, 1993).Its perennial attack kills seedlings and young trees or branches of older indigenous larch trees. Annual surveys permitted circumscription of its distribution in the Maritime provinces of Canada and the State of Maine in the USA. This lead to quarantine zone establishment. However, the biological aspects of the infection process by the responsible fungus Lachnellula willkommiiare far from well known.The objectives of this research were to study the initial stages of the disease infection process through microscopic observations of larch seedling stem sections artificially inoculated inside a wound with mycelial cultures of L. willkommii. Comparative qualitative and quantitative observations over three to six months after inoculation during simulated growth and dormant periods of the outermost tissue layers of both 2- and 3-year-old seedling stems were realized. Mycelium invasion showed a differential behaviour for host in growth vs dormant situation. The invasion was slower and concentrated in external tissue layers during growth period, whereas it was faster and in internal tissue layers during dormant period. Moreover, the hyphal invasion involved both direct and indirect modes of fungus penetration: directly, through the walls of adjacent cells (dormant period) and indirectly, through natural openings such as tracheids bordered pits (growth period), radial ray cells (growth period) and resin canals (dormant period). For the first instance, small internal tissue separations (microcracks) in the xylem were also detected, all containing fungal hyphae and exclusively in the inoculated samples from the dormant period, suggesting a possible developmental relationship with the presence of the fungus and a potential implication on wood physical properties. The quantitative observations showed that the inoculation efficiency (x¯ > 80%) confirmed susceptibility of young native larch seedlings to introduced European larch canker disease. Moreover, the infection in combined tissues (epidermis, cortex, phloem and xylem) showed a constant progression over time for both growth and dormant periods. For individual tissues, the fungal infection showed alternate patterns of increase and decrease related with possible host weakeness during its dormant period and resistance during its growth period.