This study was carried out on Helminthosporium species found on cereals (Avena sativa L., Hordeum vulgare L., Triticum aestivum L., Secale cereale L.) and couch grass (Agropycon repens (L.)PB.) The objective was to ascertain their distribution and general significance in Finland. The results were obtained from samples of cereals gathered in 2040 fields all over the country during the growing season 1971 1973. The samples of couch grass (approx. 170 samples) were collected infields and the borders of fields. The fungi in all the samples were examined by microscope, using the blotter method and with cultures. Many Helminthosporium species were found to be very common and abundant on cereals, especially in 1972, throughout the country and as far as Lapland. The imperfect stages (conidia) of the fungi occurred dominantly and in abundance, whereas the perfect stages of the fungi were found only occasionally at all times. Pyrenophora tritici-repentis (Died.) Drechs. on Agropyron repens was exceptional in this respect. The most widespread and common Helminthosporium fungi were the following species: H. avenae Eidam was found to be very common and abundant in more than 60 per cent of the oat fields and in nearly 90 per cent of the localities that were studied throughout the country. These figures apply to seedlings as well. H. gramineum Rabenh. ex Schlecht was found frequently in more than 30 per cent of the spring barley fields and in about 55 per cent of the localities that were studied. It was very common and abundant in Ostrobothnia and the northern parts of the country and was frequent on six-rowed barley, too. H. teres Sacc. was found to be common and abundant in nearly 60 per cent of the spring barley fields and in nearly 90 per cent of the localities that were studied. These figures apply to the seedlings as well and are especially representative of observations made in the western and southern parts of the country. The fungus was more common on six-rowed than on two-rowed barley H. tritici-repentis Died, was found to be moderately common in southern and southwestern Finland as follows: in nearly 30 per cent of the spring wheat fields and in 40 per cent of the localities studied; in 17 per cent of the winter wheat fields and in 23 the winter rye fields; for both the latter mentioned moderate abundance was observed in more than 50 per sent of the localities studied. On Agropyron repens the fungus was very common and abundant in 50 per cent of the samples and in nearly 60 per cent of the localities studied. In addition, the perfect stage of the fungi, Pyrenophora triticirepentis (Died.) Drechs. with mature ascoma and ascospores was found in about 1/4 of the samples examined throughout the country. H. sativum Pammel, King and Bakke was found to be moderately common and abundant in about 30 per cent of the spring barley fields and in 55 per cent of the localities studied throughout the country, being most common in southern and southwestern Finland. The fungus was more common on two-rowed than on six-rowed barley. The fungus was observed to be moderately common in 9 per cent of spring wheat fields and in nearly 20 per cent of the localities studied. In all fields H. sativum appeared sporadically also in seedlings on spring barley and spring wheat as well as on the roots of barley. The fungus was found to be moderately common also in oat fields, but only in 1972, whereas on winter wheat and winter rye its occurrence was only rare and sporadic. An undetermined Helminthosporium species, like H. gramineum or H. teres, was found to be sparce but moderately common, in about 10 per cent of the spring wheat fields and in nearly 20 per cent of the localities studied. It occurred only occasionally on winter wheat and on winter rye fields. It is not known how fungi damage affects crop yields. At least H. gramineum, H. teres and H. sativum on spring barley, as well as H. avenae on oats, caused severe damage to plants and occurred commonly throughout the country.
The intensity of competitive interactions between spring wheat (Triticum aestivum L. emend. Fiori et. Paol) and spring barley (Hordeum vulgare L.) under different fertilisation conditions
