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Bronze leaf disease (BLD) of species and hybrids of aspens, grey and white poplars (Genus Populus, section Populus) is a systemic disease incited by the fungus Apioplagiostoma populi (E.K. Cash & A.M. Waterman) M.E. Barr that results in a characteristic bronze to dark brown pigmentation of infected leaves in late summer and early fall. Branches on affected trees die over a period of several years, disfiguring trees and eventually leading to mortality of susceptible trees planted in landscapes. In this study, we describe the histopathology of the disease and fungus development relative to external symptoms on affected trees. A. populi hyphae were observed in leaf vessels, lateral veins and xylem vessels of stems and roots of affected trees. Hyphae were also observed in veins and vessels of asymptomatic leaves. Xylem vessels of growth rings on dead branches were completely occluded with A. populi near the vascular cambium and together with the ray parenchyma were stained a bronze colour. Evidence of putative toxin damage to mesophyll cells, not colonized by hyphae, was observed and may contribute to the characteristic tissue staining observed. The development of the fungus was consistent with previous reports, but in this study, we report additional details including perithecial development and ascospore release from predominantly abaxial leaf surfaces. We also report for the first time the isolation and growth of the pathogen in pure culture and systemic infection of roots. [ABSTRACT FROM AUTHOR]

The life cycles of Septoria and Marssonina were clarified for the north central United States. Spores of S. musiva and M. brunnea were abundant within hybrid poplar plantations beginning with ascospores at the time of leaf flush and extending throughout the season with conidia. Primary Septoria leaf spots were caused by ascopores, hut early Marssonina leaf spots could have been caused by conidia that overwintered in snoot lesions or ascopores discharged from leaf debris. Benomyl applied bimonthly beginning at leaf flush reduced the incidence of Septoria canker of susceptible clones in a nursery to near that of resistant clones. Cultivation to remove inoculum on fallen leaf debris in the spring failed to reduce disease incidence or severity presumably because of the abundance of inoculum coming from adjacent nursery beds. Although rungicidal control may have some application in nurseries, planting resistant clones is the most effective strategy at this time to minimize the impact of Septoria canker in nurseries and plantations, and to minimize the danger of moving this important pathogen to other areas on planting stock. [ABSTRACT FROM AUTHOR]

Open-pollinated progenies from two trembling aspen families with a 30-year field history of 91% Putatively susceptible, PS and 45% putatively resistant, PR mortality, respectively, caused by Hypoxylon marnmatum, were grown in a greenhouse for 3 and 9 months. Seedlings were inoculated with two isolates of H. mammatum, representing high and low aggressiveness, through a stem wound. Beginning 4 days after inoculation, host responses were examined on 312 seedlings per family for a period of 5 weeks on 3 and 9-month-old seedlings. Significant differences in host responses were observed between two isolates, with the more aggressive isolate allowing less callus formation, causing more tissue necrosis and producing longer cankers than the less aggressive isolate on both aspen families. Between-family difference was much greater than within-family difference for tissue necrosis, callus formation and canker length for both seedling ages. Results indicate that artificial inoculation of seedlings in a greenhouse can differentiate two aspen families at the seedling stage, but their correlation to field performance is highly dependent upon the seedling age.