Disease Note.

Severe Foot and Root Diseases in Naked and Dwarf Oat Caused by Fusarium spp. J. P. T . Valkonen, Department of Plant Production, P.O. Box 27; FIN-00014 University of Helsinki, Finland . P. Peltonen-Sainio, Department of Plant Production, P.O. Box 27; and H. Koponen, Department of Plant Biology, P.O. Box 28, FIN-00014 University of Helsinki, Finland. Plant Dis. 80:821. Accepted for publication 7 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0821C.

Field plots of naked oat (Avena saliva var. nuda L.) and dwarf oat (A. saliva L.) were affected by a severe disease at the Viikki Experimental Farm of the University of Helsinki in 1994 and 1995, when 60 and 20%, respectively, of the plants of the naked oat cv. Salomon showed grayish brown necrotic areas on lower leaves. Plant growth was retarded but only a few plants died. In 1994, 15% of the plants of the dwarf oat cv. Pal showed similar symptoms. In 1995, 65% of the plot area was killed. Yield from the diseased plots of both oat lines in both years was 50 to 60% of the yield from nondiseased plots in a nearby field. Fifty-seven fungal isolates of Alternaria, Botrytis, Cladosporium, Drechslera, Epi-coccum, Fusarium, and Sclerotinia were obtained from symptomatic leaves of naked and dwarf oat. Seedlings of naked oat were grown from surface-sterilized seeds on moist filter paper in petri dishes in a growth chamber (22°C). The fungal isolates were tested for pathogenicity by placing a water agar plug with fungal mycelium immediately adjacent to the seedling. F. culmorum (W. G. Sm.) Sacc, and F. sambucinum Fuckel killed all inoculated seedlings within 5 to 10 days. No other fungus caused any symptoms during 14 days of observation. F. culmorum killed 75 (o 100% and 0 to 25% of naked and dwarf oat seedlings, respectively, grown in soil inoculated with this fungus. In contrast, F. sambucinum caused no symptoms in seedlings grown in the inoculated soil and the fungus was not reisolated from the seedlings. The oat seed lots used for sowing field plots were free of Fusarium spp. These results suggested that the disease was primarily caused by soilborne F. culmorum. Other fungi isolated from the leaves may have contributed to foliar symptoms observed in the field. Plants grown in the greenhouse did not suffer from drought, which may explain the differences between the symptoms observed in the field and in the greenhouse experiments. F. culmorum is usually not an important pathogen of oat and has not been reported on naked and dwarf oat. In 1994 and 1995, weather conditions deviated from the long-term average in July, when an exceptionally long and severe drought with high temperatures occurred during grain filling. The unusually dry and warm weather was probably favorable for F. culmorum and allowed the fungus to attack the oat roots under drought stress. This was pronounced in dwarf oat, which is particularly sensitive to post-anthesis drought due to its shallow root system