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ISSUE 3  May 21, 1998

 

TAN SPOT AND WEATHER PATTERNS

    Reports of tan spot infections are fairly widespread across the state, where wheat is planted into wheat stubble. Infections on older leaves have been reported even in some areas of the state that have been fairly dry. I would not expect continued infection of new leaves to occur in some of these drier areas, unless additional rainfalls occur. On the other hand, in the eastern third of the state, the soils in some areas are so saturated that dews will be present every evening and early morning and continued infections may occur, regardless of additional rainfall in the very near future. The following table uses several sites in North Dakota as examples to show the variability in conditions across the state. It compares total rainfall and potential evapotranspiration (PET), and average dewpoint for May 1-May 19, as downloaded from the NDAWN weather stations.  

 

 

Location

Total
rainfall
(inch)

Total
PET
(inch)

Avg.
Dew
Pt (F)

Watford City

0.41

4.91

36.5

Dickinson

0.96

4.97

36.5

Minot

0.89

4.49

40.3

Cando

1.63

3.73

45.6

Langdon

3.99

3.99

42.7

Dazey

2.20

3.93

47.5

Fargo

5.51

4.23

47.6

Wyndmere

6.00

3.97

50.4

    The locations with higher dewpoint averages and with high rainfall compared to evapotranspiration (PET) are certainly the most likely to see possible continued infections of tan spot. The Dazey site has a high dewpoint average, even though rainfall is below the evapotranspiration rate. Last year the Dazey site had fairly low precipitation, but dews were frequent and leaf spots were common. The driest sites, such as Watford City, are pretty darn dry!

 

UPDATE FROM CEREAL RUST BULLETIN

    The latest edition of the USDA Cereal Rust Bulletin (May 19) indicates the following:

    Wheat leaf rust is increasing in the central Great Plains, with 20% severities on flag leaves in fields in southeastern Kansas in mid-May.

    The first crown rust infections on oat in the St. Paul, MN buckthorn nursery were found on May 14, approximately 2-3 weeks earlier than normal. In South Dakota, the aecial stage of oat crown rust was seen on buckthorn on May 13. The early infections and early appearance on buckthorn indicate a potential for some serious oat crown rust this year.

    Barley leaf rust was observed in Virginia and California so far this year, but none has been reported in the Great Plains. Barley stem rust has not been reported yet this year.

    Surveys of wheat and barley in North Dakota will begin this week.

 

WHEAT STREAK MOSAIC VIRUS

    A sample of winter wheat from Sioux Co. had a severe infestation of wheat curl mites plus the typical symptoms of wheat streak mosaic virus, the chlorotic streaking of the leaves, and a stunting of the plants. This disease is potentially more serious in winter wheat planted where volunteers or weedy grass hosts were present, infection occurred in the fall, and in areas that are drought stressed. Little can be done now with the infected wheat but rain showers and cool temperatures would reduce the activity of the mite and allow the wheat to recover somewhat from the drought stress.

Marcia McMullen
Extension Plant Pathologist

DESTROY VOLUNTEER DRY BEANS TO REDUCE RUST POTENTIAL

    Rust of dry beans has been a serious problem for several years and could be a threat again in 1998 unless growers take positive action. The first step is to destroy volunteer dry beans. Volunteers can be a source of early-season rust infection and buildup, and may host the sexual stage of the rust fungus. Crop consultants report seeing dry bean volunteers in small grain fields; the volunteers are set back by some of the herbicides being used, but remain alive and so may continue as a source of rust.

    The bean rust fungus produces several different spore types on dry bean plants. Brown summer spores are responsible for rust buildup in the summer. In the fall, black winter spores are produced in the same pustules. Both the summer spores and the winter spores can survive the winter in North Dakota. Next spring summer spores may infect volunteer dry bean plants as well as the dry bean crop.

    Winter spores germinate in the spring to form other spore stages that result in sexual reproduction of the rust fungus, followed by the aecial stage. The aecial stage is recognized by yellow leaf spots with tiny aecial cups that release aeciospores. These spores usually serve as an early source of infection to volunteers and the commercial crop. Later, rust is spread by the summer spores.

    Control of volunteers helps reduce early season infection of the commercial crop and also helps limit the sexual stage of the fungus. The latter is very important since the sexual stage may result in the development of new rust races capable of attacking varieties with resistance to the old races.

    Most pinto varieties are susceptible to rust. However, many new pinto varieties are resistant to current rust races, including Apache, Chase, Elizabeth, Focus, Frontier, Maverick, Remington and Winchester. Seed supplies of some of these varieties may be limited. Other classes of dry beans that are susceptible include small reds and pinks.

    Most black turtle and kidney beans are resistant to the current rust races. However, California Early and Sacramento light red kidney are susceptible. Most navy beans are moderately resistant. Resistance in these classes of beans tends to be race-specific.

    The number of identified rust races in North Dakota increased from six in 1981 to 12 in 1994. Dr. Jim Venette, NDSU bean pathologist, identified rust races present in the 1996 rust outbreak. In addition to the 12 races previously identified, there apparently were 2-4 new races. Even though some new races were present, some of the most prevalent races were the same ones that we had several years ago.

    Race shifts can and do occur, however. The pinto variety Olathe was resistant to the rust races present in North Dakota in the early 1980s but is now susceptible to one of the common races now present.

    This emphasizes how important it is to destroy volunteers and to deny the rust fungus this breeding ground for new races. Varietal resistance is still the most effective means of reducing rust losses and we need to do all that we can to preserve the effectiveness of currently available varieties that have resistance to common rust races.

SUGAR BEET WEB SITE

    The Sugarbeet Research and Education Board maintains a web site that contains all of the articles published in the Sugarbeet Research and Extension Reports from 1970-1997. It also contains the current Sugarbeet Production Guide (the pocket guide), a search engine, and numerous brochures. Brochures include Cercospora leafspot, rhizomania, seedling and root diseases, powdery mildew, mode of action of herbicides and herbicide injury symptoms on sugar beet, sugar beet insects, as well as information on production and the impact of the sugar beet industry on the economy of the region. This site is located at: http://www.sbreb.org/

Art Lamey
Extension Plant Pathologist

ASH ANTHRACNOSE

    The week of rain together with the very moderate temperatures experienced in Fargo-Moorhead and the surrounding area created an ideal environment for development of Ash anthracnose. This is a fungal disease that occurs in the spring before temperatures get too warm and when there are periods of prolonged wetness, such as we had last week.

    The symptoms of ash anthracnose occur on succulent, expanding leaves and shoots. They initially appear as water-soaked spots that rapidly coalesce and enlarge. As these lesions dry out, they become greenish brown to tan or dark brown and cause a crinkled look on the leaf. The leaf rachisees may turn brown, and in the case this year, dark brown to black. If environmental conditions persist at just the right time during leaf and shoot development, premature leaf drop results.

    Fortunately, this disease usually doesn’t cause serious injury to ash trees. As the weather warms up, the pathogen shuts down. New, unaffected shoots are produced that help to fill in defoliated areas of the tree as the season progresses. Management of this disease rarely requires chemical control. Sanitation in the form of raking up and getting rid of fallen leaves, both in the spring and in the fall, will eliminate the source of the pathogen and prevent primary infections.

    Other samples submitted to the diagnostic lab in the past week include Cytospora on spruce, several weed identifications, environmental injury on spruce, spider mites on spruce, and eriophyid mites on hawthorn. Wheat samples submitted showed signs of WSMV, tan spot, and roundup injury.

Cheryl Ruby
Plant Pest Diagnostician


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