Distribution and frequency of Cronartium flaccidum telia were investigated on Melampyrum spp. growing wild in 355 Scots pine stands in Finland. Telia were found for the first time on M. arvense in Finland and for the first time on M. pratense and M. nemorosum in natural forests anywhere. Cronartium flaccidum telia were found in 22% of all stands with M. sylvaticum, 3% of stands with M. pratense, 12% of stands with M. nemorosum, 100% of stands with M. arvense, and 0% of stands with M. cristatum. Melampyrum spp. with telia were mostly found in northern Finland. The proportions of stands with telia and of plants with telia per stand, the frequency of telia-bearing leaves per plant, and the average number of telia per leaf were greater for M. sylvaticum than for most of the other Melampyrum species. In severely affected pine stands, telia were frequent on M. sylvaticum, but scarce or lacking on M. pratense. On sites with M. sylvaticum, the respective proportion of telia-bearing plants was significantly higher in young development classes and severely diseased stands than in older development classes and slightly damaged stands. The results suggest that the main host of C. flaccidum is M. sylvaticum in northern Finland.

This study provides new information on the infection biology and pathogenicity of an important root-rot fungus, Heterobasidion annosum sensu stricto (Fr.) Bref., through a detailed examination of the vegetative spread of clonal individuals and their capacity to produce fruiting bodies on young pine seedlings. The seedlings were planted in a clear-cutting area (c. 1.2 ha in size) after a pine generation that showed slight external symptoms of Heterobasidion root rot. The first dead seedlings were found five years after planting and during a nine-year monitoring period; nearly 600 seedlings were killed by H. annosum s.s. in 48 individual disease centers. Based on pairing tests of 482 isolates, 117 different H. annosum s.s. genotypes were identified. On average, 2.9 genotypes occurred in a single disease center. The extensive secondary spread of genotypes within root systems (up to 48 pine seedlings infected by the same genotype) resulted in annually expanding disease centers. In addition, more than half of the seedlings killed by H. annosum s.s. produced perennial fruiting bodies thus providing air-borne inoculum. The risk of spore infection should be taken into account in any type of cutting operation in young pine stands. Moreover, new control measures directed towards the secondary spread of H. annosum s.s. in pine regeneration are urgently needed in order to maintain the productivity of the pine forest on infested sites.

The relative susceptibility of Melampyrum arvense, M. pratense, M. sylvaticum and M. nemorosum to Cronartium flaccidum was investigated by artificial inoculations. Uredinia and telia were artificially produced on plants from sown seed or transplanted from the wild in the greenhouse. This is the first report in which C. flaccidum telia were artificially produced on leaves of whole plants of M. arvense and M. pratense, the first report of successful artificial disease establishment on M. arvense and the first report of telia production on the upper surface of Melampyrum leaves. As an earlier unreported observation, telia were occasionally formed on both upper and lower leaf surfaces of M. arvense, M. sylvaticum and M. pratense. Host species, spore source and their interaction each affected some aspects of disease development. Melampyrum arvense and M. sylvaticum were the most susceptible hosts, while M. nemorosum was least susceptible. Melampyrum arvense should be considered a potentially important alternate host of C. flaccidum with its limited area of distribution.