ISSUE 9   June 30, 2005

NDSU IPM SURVEY RESULTS, 6-20 TO 6-24

NDSU IPM field scouts had a good week of scouting, with fewer storms statewide last week. They surveyed 205 wheat fields, 32 barley fields, and began survey of sunflower fields the week of June 20th.

In wheat, tan spot was again the most common disease observed, in 78% of fields, with severities ranging from 2 % up to 70% (see tan spot severity figure).

Wheat leaf rust was detected in more fields than previously, in 33% of wheat fields, with severities ranging from trace levels to 40% (see wheat leaf rust severity figure).

Other diseases observed in wheat included Septoria leaf spot (14% of fields), spot blotch (15% of fields), stripe rust (6% of fields), and loose smut (3.4%). Grain aphids were observed in 21% of the wheat fields, with a range of 2 to 100% of tillers having at least one aphid present. The highest infestations of grain aphids were observed in a field bordering Richland and Sargent counties in the southeast (see aphid figure).

In barley, fungal leaf spots were commonly observed, either Septoria, Spot blotch, and net blotch. Net blotch was the most common leaf disease in the Northeast, while Septoria and spot blotch were observed in other parts of the state. Barley thrips also were commonly observed in counties in the northcentral and northwest scouting territory, in 61% of the fields scouted in that region.

In sunflower, downy mildew was observed in 27% of the fields surveyed. Carl Bradley had a good description of sunflower downy mildew in Crop and Pest Report # 7, June 16th, 2005 issue, page 7. There are no cures for sunflower downy mildew once infection has occurred and it is noticeable in the field.

Correction on the Wheat Leaf Rust severity map in Crop and Pest Report #8, June 23rd issue. The map for wheat leaf rust severity was dated incorrectly. The fields and severity were correct, but the heading for the map should have read June 13-17th, not June 6-10th.

 

WHEAT LEAF RUST

With wheat leaf rust being common across much of the state, questions about potential yield losses from the disease have come in. Wheat leaf rust losses will depend on variety susceptibility, severity on the flag leaf at different growth stages, and the weather. The causal fungus only requires about 6 hours of dew for infection, and can form new spores within a week under favorable conditions. The NDSU Small Grain Disease forecasting site (http://www.ag.ndsu.nodak.edu/cropdisease/) has predicted favorable weather for leaf rust infection at many NDAWN weather locations in recent days. Growers need to know the risk of leaf rust infection on their varieties, and that information is available at NDSU web sites, in NDSU variety bulletins, and in the ND Crop Production Guide.

Potential yield losses for wheat leaf rust: Information on yield losses (%) predicted with varying levels of rust on the flag leaf at advanced wheat growth stages was given by Kansas State Plant Pathologists for susceptible winter wheat cultivars. These estimates are the best I have, and the estimates and may be a little conservative on estimate of losses, according to Bob Bowden, USDA plant pathologist, Kansas.

Yield losses (%) predicted with different wheat leaf rust severities on susceptible varieties at various wheat growth stages (KSU,’97)

Growth stage

Rust severity on flag leaf

10%

25%

40%

65%

100%

----Estimated % yield loss---

Flowering

10

15

20

30

35

Milk

2

5

8

14

20

Soft dough

1

3

4

7

10

Hard dough

1

1

2

32

5

Leaf rust symptoms are pictured in the following figure, along with a commonly used leaf rust severity scale. In the two photos, leaf rust severities are approximate between 40 and 60%. The leaf rust levels in the two pictures are too high for effective fungicide application. Levels in the 5-10% range would still have enough green area to protect from infection.

Thresholds for wheat leaf rust control in susceptible varieties, also based on information from winter wheat states, are averages of 1-3 pustules of rust on the flag leaf, and 5-10 on the leaf below the flag leaf.

Fungicides: If the crop is showing low levels of rust and is near full head emergence or flowering, the triazole fungicides, Folicur or Tilt, would provide control of rust and also protect against head scab. Full label rates of 4 fl oz/acre should be used, not reduced rates.

 

SCAB SPORE COUNT IN FARGO UPDATE

Dr. Shaukat Ali of the NDSU Plant Pathology department reports that scab spore counts were low to moderate between Friday, June 24th and Tuesday, June 28th. Keep in mind that these counts vary according to weather events, and are only from Fargo. The scab risk maps indicate some high risk of infection in those areas of the state that have saturated soils and have suffered the most from repeated rainfalls. This web site should be checked daily because risk varies each day, depending on previous weather events.

 

FUNGICIDES AND LODGED GRAIN

Recent strong storms in some areas caused grain to lodge. This grain will need to come back up, with heads vertical, before fungicides are applied. There is little benefit from spraying lodged grain.

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

 

CHICKPEA ASCOCHYTA BLIGHT FOUND IN MINOT

Kent McKay reported finding Ascochyta blight on chickpea research plots at Minot last week. Symptoms of Ascochyta blight may be observed on leaves, stems, or pods as tan to light brown lesions with dark margins. Pycnidia (small dark dots) in a ‘target’ formation may occur in the center of the lesions. Large kabuli-type varieties are generally the most susceptible to Asochyta blight. Fungicides registered for Asochyta blight control include chlorothalonil products (Bravo, etc.), Endura, Amistar, Quadris, Quadris Opti, and Headline. In very-susceptible varieties, multiple applications may be needed. If so, it is important that a fungicide resistance management strategy be applied, such as alternating fungicide chemistries for each spray. Data from NDSU Research Extension Centers indicate that chlorothalonil products are most effective if sprayed early (pre-bloom) as a preventative treatment. Any following applications after bloom should be either a strobilurin fungicide (Headline, Amistar, Quadris) or Endura (a carboxamide fungicide). More information about Ascochyta blight of chickpea is available on the NDSU Extension Plant Pathology Website under "Plant Diseases in ND" located at:

http://www.ag.ndsu.nodak.edu/extplantpath/  

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


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