NDSU Crop and Pest Report
Plant Pathology


ISSUE 9  July 1, 2004

LEAF SPOT DISEASES ON SOYBEAN

Several reports of spots on soybean leaves have been received in the last week. The cause of the spots in some cases has been Septoria brown spot disease. In other cases, no signs of a biotic pathogen have been found. There are a few biotic diseases of soybean that can cause leaf spots that are known to occur in North Dakota (listed below).

Septoria brown spot. Septoria brown spot, caused by the fungus Septoria glycines, causes brown spots that can vary from a minute speck to 3/16 in. The spots can be found on both the upper and lower leaf surfaces and will usually appear on the unifoliate leaves first. The fungus overwinters in infected plant debris, and the disease is most severe when soybean is grown continuously in the same field. The disease is favored in wet conditions with moderate temperatures (59 to 86 degrees F). Crop rotation with a non-legume crop is the main control tactic used to manage this disease.

Downy mildew. Downy mildew, caused by Peronospora manshurica, causes yellow spots on leaves. The pathogen overwinters in infected debris and on seeds. The disease is favored by high humidity and temperatures of 68 to 72 degrees. Planting disease-free seed and crop rotation are the preferred methods of management.

Bacterial blight. Bacterial blight, caused by Pseudomonas savastanoi pv. glycinea, causes angular lesions that are usually surrounded with a yellow halo. The pathogen overwinters in infected debris and seed. The disease is favored by rainfall and cool temperatures and may be more severe if leaves are damaged by wind or hail. The best control methods are crop rotation and planting disease-free seed.

The above diseases generally do not cause major economic losses in North Dakota. As the weather warms up and dries out, these diseases will not be as severe. Using good management practices such as planting high-quality seed along with crop rotation will minimize the effects of these diseases.


Septoria brown spot on soybean


Downy mildew on soybean


Bacterial blight on soybean


Carl Bradley
Extension Plant Pathologist
carl.bradley@ndsu.nodak.edu

 

FUNGICIDE DECISIONS ON SMALL GRAINS

The recent cool weather and then the switch to relatively warm temperatures and dry conditions have many people wondering about fungicide application. The decision for fungicide application is based on multiple components and include:

Dr. Roger Jones, Emeritus Extension Plant Pathologist at the Univ. of Minnesota, and I developed a fungicide decision aid for wheat fungal leaf diseases several years ago, before the advent of the disease forecasting models, which are strictly based on weather. This system was based on points accumulated, and an example is given below:

1)

yield potential 51-60 bu

3 pts

2)

market price $4.00-$5.00

6 pts

3)

previous crop soybeans

0 pts

4)

presence of tan spot on flag minus one leaf at heading

3 pts

5)

susceptible variety Oxen

3 pts

6)

climatic conditions, avg. moisture, 750 avg. daily temp

2 pts

 

Total pts

17 pts

     

Recommendation if sum total is:

 

< = 13 pts

donít spray

 
 

14-16 pts

spray may be beneficial

 

> = 17 pts

spray

 

Individuals can use different scenarios and adjust points accordingly to determine risk. Greater yield = another point; wetter conditions another point to two points; resistant variety = 0 pt, etc.

 

SMALL GRAIN DISEASE SURVEY, 6/21-6/25

NDSU IPM field scouts surveyed approximately 125 wheat fields and 50 barley fields during the third week of June. Growth stages vary considerably across the state, with crops in the southeast entering heading or flowering stages. The most common disease still observed in wheat is tan spot, while leaf rust became more common, as well.

One of the IPM scouts in the north central region found leaf rust severity of 20% in a wheat field in McHenry county on June 28. Severity of leaf rust has generally averaged less between 1-5%. found little increase in disease development in wheat and barley this past week. Net blotch and spot blotch continue to be observed in barley.

 

DISEASE FORECASTING MODELS - UPDATE

The Fusarium head blight model (FHB = scab) shows low risk of scab across the state, as of June 29, based on environmental factors. The models indicated yes for leaf disease infections across much of the state on June 26 and 27th, but lower risk on June 28 at most, but not all, NDAWN sites. These risks change on a daily basis depending strictly on weather conditions. Leaf rust requires only 6 hours of evening dew, so its risk for rapid increase is always a little higher than the fungal leaf spot disease organisms that require longer dew periods for infection.

Marcia McMullen
Extension Plant Pathologist
mmcmulle@ndsuext.nodak.edu

 


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