Prediction of long-term field resistance of hybrid poplars to Septoria musiva using a greenhouse screening protocol without wounding
R. QIN (1), G. R. Stanosz (2), J. M. LeBoldus (1). (1) North Dakota State University, Fargo, ND, U.S.A.; (2) University of Wisconsin-Madison, Madison, WI, U.S.A.
<i>Populus</i> species and their hybrids are short-rotation woody crops (SRWC) supplying fiber and biomass to a diversity of industries in North America. Hybrid poplar as a SRWC has been limited in many parts of North America by the fungal pathogen <i>Septoria musiva</i> Peck, the cause of leaf spot and stem canker diseases of <i>Populus</i>. An inoculation protocol that does not rely on stem wounding to achieve infection was recently developed to screen for resistance to Septoria canker. The relationship between this inoculation protocol and long-term field resistance is unknown. Young ramets of 14 poplar genotypes with known levels of field resistance to Septoria canker (Low, Intermediate, and High) were inoculated with a spore suspension of three isolates of <i>S. musiva</i> in a greenhouse. Three weeks after inoculation the following parameters were recorded: lesion number, lesions/cm, lesion area, and proportion necrotic area (PNA). The logistic regression model with lesion number and proportion necrotic area correctly predicted field resistance categories for 11/14 genotypes tested, including the most resistant (NM6, DN164, DN177, DN34, and DN74) and the most susceptible (NC11505) genotypes. These results demonstrate that both canker number and disease severity (PNA) are important for accurate prediction of long-term field resistance. This screening protocol, using young ramets, can be used to focus resources for subsequent long-term field testing on the most resistant genotypes.
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