Damping-off of sugar beet with special reference to the fungus Pythium Pringsheim

Abstract

In Finland damping-off of sugar beet can be divided into two distinct phases. The first phase begins with the germination of the seeds and continues until the first true leaves have developed. Under field conditions seedlings usually remain healthy up to about 1 week after emergence. Thereafter a sudden outbreak of damping-off may occur, resulting in rapid wilting and death of seedlings. During the second phase of the disease, when seedlings have one or more pairs of true leaves, disease does not always result in the death of the plant; plants may survive throughout the summer. At the pernicious phase of the disease the soil borne pathogen, Pythium debaryanum auct. non Hesse, is the most common causal agent, accounting in 1979—86 for 53.9 % (variation between years 18.3—90.1 %) of fungal isolations, and Fusarium species for 28.3 % (5.0—58.5 %). At seedling stages with one or more pairs of true leaves Fusarium spp. predominate accounting for 49.4 % (36.1—81.0 %) as compared to 23,9 % (2.9—37.8) for P. debaryanum. The importance of Fusarium species as true damping-off pathogens is, however, doubtful. The seed borne damping-off pathogen Phoma betae Frank was isolated only in 0 to 4 % and was not dependent on the stage of seedling development. Of the factors affecting damping-off, high temperatures were repeatedly shown to increase the disease. This, presumably was an effect especially on P. debaryanum, the aggressiveness of which is strongly increased at high temperatures. Pot experiments showed preceding crops of cereals to have the best disease-decreasing effect, both short-term (one growing period of preceding crop) and long-term (several growing periods of preceding crop) effect. Legumes kept the level of damping-off unchanged or even raised it, especially as a short-term effect. The influence of preceding crops varied in different soil types. Preceding crops also caused considerable fluctuations in inoculum density (0 to 3650 propagules/gram soil) and potential (0.2—16 IPU 50/gram soil) of Pythium. The correlation to damping-off of sugar beet was, however, poor. Seed treatment with the systemic fungicide hymexazol, especially when combined with thiram, prevented satisfactorily the pernicious type of damping-off. In many experiments this seed treatment repeatedly decreased disease incidence significantly, produced denser stands (7100—31200 numbers of beets more/hectare) and increased yield by 5—10 % on average.