Reliability of field, greenhouse and cut-shoot screening procedures for evaluating susceptibility of Scots pine to Melampsora pinitorqua

The reliability of field, greenhouse and cut-shoot screening procedures for the assessment of the susceptibility of Scots pine (Pinus sylvestris) geno types to Melampsora pinitorqua, the causal agent of twisting rust, was eval uated. Fourteen genotypes evalEted on half-sib progenies after natural infe ction of 2-year-old plants showed comparable rank for rust susceptibility w ith those evaluated after controlled inoculation of 1-year-old seedlings in a greenhouse. The dynamic of pycnia production was assessed at 2-day inter vals in cut-shoot assays under controlled conditions, giving reliable ranki ngs of Scots pine genotypes between years and being significantly correlate d with rust susceptibility assessed after natural infection of 2-year-old p rogenies. These results underlined the importance of some critical factors such as inoculum load and host phenology in the reliability of susceptibili ty evaluations assessed under different experimental conditions. Artificial inoculation of 1-year-old seedlings in greenhouse experiments could provid e a useful early test for the management of Scots pine breeding programmes and study of inheritance of twisting rust susceptibility. However, in speci fic investigations the cut-shoot assay would constitute a reliable laborato ry test for studying host-pathogen interactions and the variability in path ogenicity of Melampsora pinitorqua populations.