Prediction of long-term canker disease damage from the responses of juvenile poplar clones to inoculation with Septoria musiva

Stem cankers caused by Septoria musiva severely limit production of susceptible hybrid poplars in eastern North America. A field experiment was conducted to determine whether short-term responses of poplar stems to inoculation with S. musiva were predictive of long-term canker disease damage. Stems of 27 poplar clones were inoculated during their first season of growth by removing the fourth or fifth fully expanded leaf and placing an agar plug colonized by an aggressive isolate of S. musiva over the resulting wound. Four months after inoculation, incidence of cankers, canker length, and percent of stem circumference affected (girdle) were recorded. Clones varied greatly in canker incidence (12 to 98%), mean canker length (10 to 53 mm), and mean girdle (14 to 94%). Logistic regression analysis was used to compare these inoculation responses with canker disease damage categories assigned on the basis of information from longer-term field studies. Incidence, canker length, and girdle data were all informative, but girdle data from this field experiment correctly predicted assigned canker disease damage categories most frequently (24 of 27 clones). Responses of 15 of these clones also were evaluated in a similar greenhouse experiment. Although responses usually were predictive of long-term damage categories, the probability of correct prediction was lower than that obtained in the field experiment for most clones. These results demonstrate the feasibility and potential benefit of screening juvenile poplar clones for responses to inoculation with S. musiva before undertaking field trials for productivity and subsequent release to growers.