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ISSUE 10  JULY 8, 1999

 

CHLOROTIC SOYBEANS AND VARIETY DIFFERENCES

    There has been a fair amount of concern over the last week on the condition of many fields of
soybeans that have been yellow for nearly a month. In particular, Roundup-Ready varieties seem to
have been hit the hardest by this annual phenomena. This is distressing, because at several meetings
last winter with seed people present the question was raised on tolerance of these new varieties being
hurried to market with respect to chlorosis tolerance. The answer was most often that they were as
tolerant as their other premier varieties. This does not always seem to be the case.

    As a reminder to the soybean seed industry, the chlorosis in this region is different than that seen
in my experience in Illinois, and different than that normally seen in Southern Minnesota and Iowa. We
are dealing with very high levels of carbonates, often 10-40% by weight, and also relatively high levels
of salts, from 0.3-2 mmhos/cm. The soils are colder and our drainage is poor compared to tiled regions.
Varietal screening and testing in this region under soil conditions that contain salts and carbonates is in
my opinion necessary to be able to state that a variety is chlorosis tolerant.

    The question from many growers is what to do if a field is chlorotic. Some have added an iron
fertilizer to their herbicide while spraying, but have not seen an improvement. The lack of improvement
is in line with the research conducted in Minnesota in the 1970's which concluded that when salts were
not present foliar iron was more consistent in chlorosis correction than when salts were present. An
older soybean circular from NDSU than the present one suggests that multiple treatments might be
necessary to correct a chlorosis problem. I am not confident that foliar sprays will be effective.

    What has to happen to the field to alleviate the problem if the variety has any tolerance to chlorosis
at all is for the field to dry. As long as the field continues to get rainfall and stays moist, bicarbonate levels
will continue to be high and chlorosis will persist. When fields dry, bicarbonate levels decrease, allowing
the soybeans to overcome their iron deficiency.

    Take this opportunity to travel around the area, especially to farms that have had a history of chlorosis
in the past and ask what they grew this year. If the field is yellow, stay away from those varieties. If the
field is green, consider growing that variety. It’s really too bad that most soybean plot tours are in August
or September. This is really an important time to find varieties with more tolerance to chlorosis, in addition
to the all-important yield information at the season’s end.

Dr. Dave Franzen
NDSU Extension Soil Specialist
dfranzen@ndsuext.nodak.edu

 

MANAGING CERCOSPORA LEAFSPOT IN SUGARBEETS

    Farmers in North Dakota and Minnesota have been faced with low returns for most of their
produce. Sugarbeet was one of the exceptions. However, recently, even sugarbeet growers were
receiving lower prices for their product. As a result, sugarbeet growers need to be aware of all
options available to manage their cost of production in producing an economical sugarbeet crop.

    One major cost of sugarbeet production is the control of Cercospora leafspot. American Crystal
growers spent over $20 M in 1998 in Cercospora control. Growers can save millions of dollars by
scouting their fields and using the Cercospora Spray Advisory to determine the need and timing for
fungicide application. The Cercospora Spray Advisory was developed by Roger K. Jones at St.
Paul, MN, and Carol E. Windels at the Northwest Experiment Station, Crookston, MN. Scouting
should begin at canopy closure, about July 1 (or earlier especially for early planting in Southern
Minnesota) for the ‘Initial Detection’ of Cercospora leafspot and to determine any subsequent
‘Spray Delay’. Cercospora leafspot produces circular spots about 1/8 inch in diameter with ash
gray centers and dark brown or reddish-purple borders. The gray centers of Cercospora leafspot
usually have tiny black spores, visible under magnification.

    The sugarbeet factory districts in North Dakota and Minnesota have developed a daily Cercospora
Spray Advisory available to growers. The Daily Infection Condition Value (DICV) is determined.
When the sum of the DICV’s for the two previous days is less than 6, conditions were unfavorable
for Cercospora infection. Conditions are marginal at a DICV of 6. When the sum of two successive
DICV’s is more than 6, conditions were favorable for Cercospora infection. The Percent Disease
Severity should be determined and the decision to spray should be based on the Cercospora Leafspot
Action Zone and the Cercospora Spray Advisory for that county. Please note that fields need to be
re-scouted, and Cercospora Leafspot Action Zone re-calculated when conditions remain unfavorable
for Cercospora leafspot for seven consecutive days. Production costs can be reduced by with-holding
sprays when the Percent Disease Severity remains in the Safety Zone or if the Percent Disease Severity
enters the Caution Zone and the Cercospora Spray Advisory reports conditions unfavorable for infection.
The Agriculturists will have the most up-dated information on Cercospora in your area.

    Remember, alternate fungicides with different modes of action to prolong the usefulness of these products.

Dr. Mohamed Khan
NDSU Extension Sugarbeet Specialist
mkhan@ndsuext.nodak.edu

 


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