The Correlation Between Septoria Leaf Spot and Stem Canker Resistance in Hybrid Poplar

Sphaerulina musiva is an important fungal pathogen that causes a leaf spot and stem canker disease of hybrid poplar. Stem cankers are widely regarded as the greatest threat to hybrid poplar plantations because of their ability to cause tree mortality; thus, the efforts of breeding programs have been focused on stem canker resistance. To explore the relationship between resistance to leaf spot and stem canker in Populus nigra × P. deltoides hybrids, two experiments were conducted. Initially, comparisons among leaves of different ages indicated that younger leaves were more susceptible to leaf spot infection than older leaves. Correlations between leaf spot severity and stem canker severity for both individual leaves and all leaves averaged together indicated that, in 10 of 11 comparisons, there were no significant correlations. The lack of correlation suggests that deploying genotypes resistant to stem canker may not affect the pathogen population causing leaf spot disease. To our knowledge, this is the first study specifically designed to test the correlation between stem canker resistance and leaf spot resistance by inoculating whole trees with a spore suspension in a controlled environment.