ISSUE 13    July 28, 2005

LOOK FOR SOYBEAN CYST NEMATODES

Soybean cyst nematode (SCN), a microscopic parasitic worm, was found in a number of fields in North Dakota (Richland County) for the first time in 2003. SCN is the most destructive pathogen of soybean world-wide. Once SCN becomes established in a field, it is nearly impossible to eradicate. Therefore, keeping the SCN population levels to a minimum using crop rotation and resistant cultivars is the best way to manage this pathogen. SCN survive as small cysts in the soil. The cyst is the actual body of a female nematode with hundreds of eggs inside her. When protected inside this cyst, the eggs are able to survive in the soil for multiple years. Swollen female nematodes on soybean roots are visible to the naked eye (~1 mm diameter); however, the use of a hand-lens will help in finding them. The swollen female nematodes will be lemon-shaped and can be white, yellow, or brown in color; the nematodes are much smaller than the root nodules. Above-ground symptoms, although not always present, can appear as stunted and chlorotic plants. If SCN is suspected in a field, collect soil samples (6-8 in. deep) from the inside and margin of the affected areas, and send them to a lab (such as the NDSU Plant Diagnostic Lab) to detect the presence of cysts or SCN eggs. Once SCN is found in a field, yearly monitoring of the population is needed. To monitor the SCN population in a field collect soil samples in a zig-zag pattern throughout the field. Approximately 20 soil samples are needed for every 10 to 20 acres. Mix the samples together and send ~1 pt of soil to a lab. The best time to collect the samples is during the Fall of the year. Dry edible beans are also a host of SCN, although more information on susceptibility and yield loss on dry bean is needed. A few resistant soybean varieties adapted to the southern valley are available, and more are in development.


Swollen female soybean cyst nematodes
on the roots of soybean

(Courtesy G. Tylka, Iowa St. Univ.)

Carl A. Bradley
NDSU Extension Plant Pathologist
carl.bradley@ndsu.edu

 

NDSU IPM SURVEY UPDATE, JULY 18-22

NDSU IPM scouts surveyed 110 wheat fields and 20 barley fields during the week of July 18-22. A majority of these crops south of I-94 were in the dough to ripening stages, although a few were still flowering or in the milk stage of kernel development.

Of the wheat fields surveyed, 49% showed symptoms of Fusarium head blight (scab) infection, an increase from the 35% showing symptoms the week before. In wheat fields showing scab symptoms, field severity (incidence of tillers infected x average head severity) ranged from less than 1% up to 43% (field in Sargent county). The highest scab field severities during this third week of July were observed in fields in the southeast and east central crop reporting districts. In barley, only one field of the 20 surveyed exhibited scab symptoms. Many barley fields were nearing physiological maturity.

Other diseases observed in wheat included continual detection of leaf rust, tan spot and Septoria blotch on flag leaves. Bacterial stripe was observed in 25% of the wheat fields scouted in the southeast and east central districts. Ergot infections of grain heads also were observed in 9% of the wheat fields surveyed (see picture). Ergot infections are also favored by wet weather at flowering of the grain crop.

Black sclerotia on wheat head
Black Sclerotia of Ergot Fungus visible in
Maturing Wheat Head

(picture source unknown)

Loose smut was observed in 6% of wheat fields surveyed this week. In barley, the most frequent diseases observed continued to be spot blotch and net blotch on flag leaves.

The field scouts also scouted 16 sunflower fields during the third week of July. Downy mildew infected sunflower plants were observed in 7 of these fields, with incidence ranging from 1 to 11.5%.

 

HARVESTING TIPS FOR DEALING WITH SCAB INFECTED FIELDS

Fusarium head blight, commonly known as scab, has noticeably increased in occurrence and severity in wheat and durum fields over the past week. Severe levels are also starting to show in more northern counties that had excessive rains in June. Growers with severe scab in their fields may take several steps to reduce their risk of harvesting grain with high levels of scabby kernels and DON (deoxynivalenol or vomitoxin). The following questions have been asked, and the answers are based on past experiences with scab epidemics:

* Will I have any insurance or disaster payments?

Before harvest, contact insurance agents and the local Farm Service Agency to determine if any crop loss will be covered, and to learn what insurance contracts require for loss coverage.

* Should I swath or straight cut fields with scab?

Straight combining should be considered to minimize further development of the disease or DON production in the swath. Although swathing is generally used to bring moisture levels of grain down faster than with straight cutting, swathing onto wet ground or getting rains on the swath could cause molding and further grain deterioration.

* Are there ways to minimize scabby kernels and DON in the harvested grain?

Severely infected fields or portions of fields should be harvested and stored separately, if possible or practical. Scab severities and DON levels may be higher in portions of field adjacent to drown-outs or in very low areas. Research at Michigan State University showed that the highest concentrations of DON were along field margins and in headlands in fields that had no problems with drown-outs.

Use mechanical separation of scabby grain. Set combine fan speed to blow out shriveled, diseased kernels that cause problems in marketing and storage. Scabby kernels in the harvested grain may add to yield, but will diminish market price.

Additionally, harvested grain can be further cleaned by gravity tables which may reduce the level of scab enough for achieving top market grade and price in the cleaned sample.

Farm workers and grain handlers should minimize exposure to grain dust in fields with scab, using enclosed cabs with good air filtration systems, and using dust masks when transferring grain.

* What can be done with scabby grain that canít be marketed?

NDSU studies show that cattle and sheep are most tolerant of the DON. Hogs are most sensitive to DON, even at one parts per million contamination of hog feed. The toxin also can cause problems in horses, dogs, and other single stomach animals. With any questionable grain, it is advised to contact a Extension livestock specialists, veterinarian, or feed specialist before feeding grain infected with DON to livestock.

The Food and Drug Administration has guidelines for DON levels in grain:

1 part per million for finished grain products for human consumption

(Many food processors and malt barley companies have stricter requirements)

Cattle over 4 months old: 10 ppm (providing grain at that level doesnít exceed 50 percent of diet)

Swine: 5 ppm (providing grain at that level doesnít exceed 20 percent of diet)

Poultry: 10 ppm (provding grain at that level doesnít exceed 50 percent of diet)

All other animals: 5 ppm (providing grain at that level doesnít exceed 40 percent of diet)

Straw from scab-affected fields should be suitable for cattle and sheep bedding, but is not recommended for horses.

*What about storing affected grain?

Drying wonít reduce scab or DON levels, but drying prevents further fungus development. Moisture content of scabby grain going into storage should be about 12 percent.

Marcia McMullen
NDSU Extension Plant Pathologist
marcia.mcmullen@ndsu.edu


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