Pathogenicity of A and B morphotypes of Sphaeropsis sapinea confirmed on American larch (tamarack) and European larch

Summary Fungi identified as Sphaeropsis sapinea (or synonyms) have been reported from Larix . Lack of pathogenicity tests, confusion about the identity of isolates mentioned in previous literature, and existence of distinct populations (A and B morphotypes) of the pathogen indicated the need to evaluate the ability of S. sapinea to cause disease of larch. Elongating snoot tips of Larix decidua, Larix laricina , and known hosts of the pathogen, Pinus banksiana and Pinus resinosa , were inoculated with water‐agar plugs colonized with an A or a B isolate, or sterile plugs (controls). Each of five replications included five seedlings for each treatment‐species combination. After 6 weeks, no symptoms had developed on control seedlings, but two‐way analyses of variance revealed significant effects of isolate morpnotype and host on both incidence and severity of disease (values of p ≤ 0.01). The A isolate killed almost all shoot tips, but the B isolate killed from no P. resinosa shoots to 56% of L. laricina shoots. The average length of shoot killed by the A isolate was also greater than that killed by the B isolate. Response to the B isolate again varied among species, with greater average lengths of shoot killed on the larches (compared with the pines). Both A and B morphotypes of 5. sapinea should be considered among the fungi encountered on L. decidua and L. laricina .