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ISSUE 9   JULY 1, 1999


    Producers in areas with large amounts of rainfall over the last six weeks have noticed a patchy appearance in nitrogen and sulfur deficiencies showing up in crops. This patchy appearance is common when water moves through soil and carries the mobile nutrients with it. Water does not move down through the soil in a united wetting front. It tends to move laterally until it finds the least resistance soil to move through. The movement produced is called preferential flow. Preferential flow is important, because whereas a 6 inch rainfall moving through soil with 50% pore space would be expected to fill about 2 feet of soil with 25% water, the actual result is that some of the soil is moistened a little, but other areas where preferential flow is present may be wet several feet deeper than expected. Preferential flow carries nutrients to deep depths with relatively small amounts of water. A number of studies using tracers to mark water movement have shown preferential flow to be the common method of downward water movement in soils. Because the water moves to deep depths in certain areas, these are the areas that leach out nutrients and form the patches of deficiency we see at the surface. With smaller amounts of rainfall, nutrients from areas not experiencing deep water movement may spread out and decrease the variability in smaller areas, however, some variability will exist for some time.

    Another consequence of the heavy rains will be the expansion and appearance of saline seeps later this season and continuing into next year. Around ponds, a ring of salt will develop and retard or inhibit crop growth. At field edges near ditches, or on hillsides, salts will also accumulate due to internal water movement in these landscapes. There is no quick fix for these seeps. There is no amendment useful in neutralizing these areas. Gypsum won’t work because the most common salt in these areas is gypsum. Some cultural practices, such as mowing down weeds instead of discing them in the summer, growing something on the field instead of fallow, and seeding a narrow strip (about 30 feet) of alfalfa along the roadsides to depress the local water table can help to reduce the incidence and severity of salt.

Dr. Dave Franzen
NDSU Extension Soil Specialist



    It is the season for Cercospora leafspot, the most damaging foliar disease of sugarbeet resulting in lower tonnage and reduced sucrose content. The fungus Cercospora beticola causes Cercospora leafspot. The most common source of the Cercospora fungus is infected sugarbeet debris in the field. The fungus is spread mainly by wind and water. Cercospora leafspot develops rapidly in humid, warm and rainy weather. The Cercospora spores are produced at temperatures of 68-79 F and relative humidities (RH) of 90-100 %. Spore release is effected by rain and dew. Optimal spore germination and infection occurs when the temperature is 75-77 F and the RH is 100 % for at least 8 hours. Day temperature of 80-90 F and night temperatures above 60 F favor disease development. Leafspot symptoms may occur about 5-7 days after infection. Cercospora infection produces circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders. Individual spots may coalesce and kill entire leaves, particularly on susceptible varieties. In humid conditions, the spots may become gray and velvety with the production of spores. These spores can further spread the disease, especially within fields, resulting in many infection cycles during the growing season.

    As such, it is important to have early control of Cercospora leafspot. An integrated approach is recommended for controlling Cercospora leafspot, involving cultural practices such as burying infected tops by tillage, planting approved varieties in fields with a minimum rotation interval of three years, and the timely and proper use of recommended fungicides. Eminent 125 SL (tetraconazole) which was very effective in controlling Cercospora leafspot in field trials was approved (Section 18 label) for use on sugarbeet at 13 fluid ounce product per acre at a recommended 14-day interval. Eminent has a 14-day pre-harvest interval. Alternate Eminent 125 SL with other registered compounds such as triphenyl tin hydroxide (TPTH), Mancozebs, and Topsin/Benlate in order to delay the development of resistance to these products. In areas where Cercospora strains are tolerant to TPTH, the higher rate of TPTH may be necessary to have effective control. The benzimidazoles (Topsin/Benlate) should not be used as stand-alone since the Cercospora fungus has developed resistance to this class of fungicide throughout the sugarbeet growing areas in North Dakota and Minnesota. Topsin/Benlate can be used in a tank mix with Mancozeb, but only once in a season. Research conducted by Mark Bredehoeft recently at Southern Minnesota indicates that it is safe to add a grass herbicide, plus oil to Eminent 125 SL when spraying sugarbeets.

Dr. Mohamed F. Khan
NDSU Extension Sugarbeet Specilist

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