Hybrid larch resistance to Lachnellula willkommi.

Two-hundred-and-eighty-three clones of hybrid larch (L. x eurolepis and L. x leptoeuropaea) selected out of 14 full-sib families were inoculated with a pathogenic isolate of L. willkommii during the spring 1990. Each clone wa s comprised of three grafted ramets, 3-4 years old. Each clone received 15 inoculations. Each inoculation consisted of a piece of colonized sorghum st raw introduced into a 1-mm-diameter hole drilled in 2-year-old twigs. A fir st phase of infection developed and resulted in an infection rate of 27.9% after 18 months. A majority of the experimented clones (80-90%) was recepti ve and a broad clonal variability was observed for all symptoms. Neither th e origin (Alpine versus Sudetan) of the European larch genitors in the cros sings nor the sense of the crossings (L. x eurolepis versus L. x leptoeurop aea) seem to influence susceptibility. After 39 months, nearly all infected sites had healed, and the final infection rate was 2.2%. The very low susc eptibility of hybrid larch to L. willkommii canker is confirmed experimenta lly. Host vigour may contribute to resistance. The role of the Japanese lar ch parent in the transmission of resistance is suggested but should be thor oughly examined. ((C) Inra/Elsevier, Paris.).