1983. Chestnut blight and recovering American chestnut trees in Michigan. Can. J. Bot. 61: 3 164-3171.Stands of American chestnut trees have survived in Michigan in spite of the presence of Endothiaparasitica. Cankered trees were evaluated in eight groves in seven of which two types of abnormal cankers were observed. These abnormal cankers resulted when diseased periderm was replaced by wound periderm. Endothia parasitica isolated from abnormal cankers often differed from E. parasitica isolated from normal cankers in that the isolates had abnormal culture morphology, reduced virulence, and contained double-stranded ribonucleic acid (dsRNA). These characteristics are similar to those of hypovirulent strains of E. parasitica found in Europe, except that four different dsRNA banding patterns were found in Michigan isolates, indicating different dsRNA genomes. Pathogenicity tests showed that host resistance or tolerance probably is not responsible for the prolonged life of infected trees in certain groves. Therefore, native Michigan hypovirulent isolates of E. parasitica may be responsible for the recovering American chestnut groves in Michigan. FULBRIGHT, D. W., W. H. WEIDLICH, K. Z. HAUFLER, C. S. THOMAS et C. P. PAUL. 1983. Chestnut blight and recovering American chestnut trees in Michigan. Can. J. Bot. 61: 3164-3171.Des peuplements de chdtaignier amCricain ont survCcu au Michigan malgr6 la presence de 1'Endothia parasitica. Les arbres atteints ont Ct C inventories dans huit peuplements; dans sept d'entre eux, deux types de chancres anormaux ont Ct C observes. Ces chancres anormaux s'observent lorsque le pCriderme affect6 est remplacC par un pCriderme de cicatrisation. L'E. parasitica isole des chancres anormaux differe souvent de celui is016 des chancres normaux par une morphologie culturale anormale, une virulence attCnuCe et par la prksence d'un acide ribonuclCique bicatenaire (Amb,). Ces caractkristiques sont semblables a celles des souches hypovirulentes de 1'E. parasitica en Europe, sauf que quatre patrons diffkrents de bandes se rencontrent dans 1' ARNb, des isolats du Michigan, rCvClant autant de gCn6mes differents. Des tests de pathogenicit6 ont monk6 que la resistance ou la tolCrance de l'h6te ne sont pas responsables de la survie des arbres infect& dans certains peuplements. Par conskquent, la prCsence de souches hypovirulentes de 1'E. parasitica, indigbnes du Michigan, pourrait expliquer le retablissement des peuplements de chdtaigniers amCricains au Michigan.[Traduit par le journal]

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