The Finnish Forest Research Institute (Metla) has conducted a study to determine the dispersion potential of rust fungi through alternate hosts. The study involved the cronartium rust and resin top disease that are commonly found in Finland (Cronartium flaccidum, Peridermium pini) and an alien white-pine blister rust (C. ribicola). Based on earlier results, the following new alternate hosts for cronartium rust were found among the cow-wheat family (Melampyrum spp.): common cow-wheat, wood cow-wheat and field cow-wheat (M. pratense, M. nemorosum, M. arvense). The alternate hosts that were already known, Swallow-wort (Vincetoxicum hirundaria) and Small cow-wheat (M. sylvaticum) showed themselves to be highly susceptible to the rust. In contrast, all of the alternate hosts were highly resistant to white-pine blister rust. Rust spore sources from northern Finland infected more tested species than the corresponding rust spore sources from southern Finland.
The recent results indicated that all tested blackcurrant (Ribes nigrum) varieties, of those commonly grown in Finland , are very susceptible to the various spore sources of white-pine blister rust. However, the most commonly grown redcurrant (R. rubru) varieties were relatively resistant to the tested rust spore sources. Of the tested Ribes cultivars, European gooseberry (R. uva-crispa), for example, was very resistant to white-pine blister rust, while most of the ornamental currant cultivars tested were shown to be very susceptible to it. All tested currant cultivars were shown to be resistant to the C. flaccidum species.
The results of the experiments indicate that cow-wheats can serve as significant alternate hosts for the rust and as possible sources of epidemics. Especially Small cow-wheat is a very susceptible alternate host for the rust. The blackcurrant species commonly grown in Finland can also be significant sources for epidemics of white-pine blister rust, which is explained by their great susceptibility to the pathogen. The new information on the susceptibility of these alternate hosts to the pine stem rusts provides basic information on factors relating to rust epidemiology.
Cronartium rust and resin top disease are among the most common fungal diseases of Scots pine in Finland . During recent years, they have caused major epidemics elsewhere, for example, in northern Sweden . White-pine blister rust, which has caused serious damage in five-needle pine (Pinus strobus, P. flexilis, etc.) stands in central Europe, is an alien species in Finland . However, this rust has caused the most significant damages in five-needle pine stands in North America . In the Metla experiments performed over several years, known and potential alternate hosts of cronartium rust and white-pine blister rust have been inoculated in the laboratory and the greenhouse with spores collected from various geographical locations.