In the middle and northern parts of Sweden and Finland Gramineae-planti are infected by a fungus of Sclerotinia-genus which causes injury during the overwintering of the plants. Vleugel in Sweden described the fungus with the name Sclerotinia borealis Bubåk and Vleugel in 1917. In Sweden S. borealis has been met in several years causing injury to winter rye, timothy, cocksfoot and other field grasses. It occurs commonly in Norrland, especially in the country Norrbotten. The southern limit of its spreading area in Sweden is in latitude about 61° N. In Finland S. borealis is met in the middle and northern parts of the country. On the basis of the information hitherto the fungus is more or less general in the years favourable to its occurrence in the provinces of Lapland, Oulu, Kuopio and Mikkeli. When occurring more abundantly as happened during the overwintering 1945—46, S. borealis is in these regions one cause for the poor overwintering of winter rye, timothy and other grasses. The climatical conditions prevailing during autumn, winter and spring are of decisive bearing for the spreading of S. borealis as well as for its abundant occurrence in certain years. The development of the apothecia of the fungus and the spreading of the ascospores is evidently favoured by long, rainy autumn, the damages are promoted by low freezing of the soil and thick cover of snow in the winter as well as by slow melting of snow in the spring. Of the occurrence of S. borealis in different Gramineae-plants, see p. 133—134. Different varieties of winter cereals and species and strains of grasses vary greatly in resistance to S. borealis. It has been observed that, of the varieties of winter rye, especially the Finnish ones resist the disease caused by the fungus very well. Of field grasses, Phleum pratense, Dactylis glomerata, Lolhim perenne, Poa serotina, and P. trivialis are susceptible to S. borealis. Some strains of these species, however, are extremely well resistant to the fungus. The strains of the grasses and varieties of winter rye grown in the north are generally more resistant to the damages by the fungus. In developing new varieties of winter cereals and strains of field grasses for the northern conditions these facts must be duly considered and for the breeding work such material should be selected which has grown in the district where the fungus occurs. Obviously the damages caused by the fungus are less when the plants receive plenty of nutrients, especially phosphates whereby they become stronger to resist infections.
Experiments on the use of some chloronitrobenzene and organic mercury compounds for the control of low-temperature parasitic fungi on winter cereals