NDSU Crop and Pest Report
Plant Pathology


ISSUE 10  July 3, 2002

 

SMALL GRAIN DISEASE FORECASTING INFORMATION, 7/02

Last week’s Crop and Pest Report indicated some areas of high risk of Fusarium head blight (FHB) and leaf diseases, according to the small grain disease forecasting website at:

http://www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm

This week, after the very hot and dry days of June 29-30, most sites did not have favorable periods for infection of diseases on those two days. For example, the FHB risk map, Model 1, indicates no high risk areas as of July 1. Model 1 is based on the past week’s weather and is a good indicator for risk at the moment. Another link on the web site at the flowering stage goes to Model 2, which predicts FHB for the future, based on past weather conditions. Model 2 indicates some risk along the northern tier of counties, IF rainy weather would return to that area. The risk is based, in part, on available soil moisture.   

model1071.png (7762 bytes)

model2071.png (10075 bytes)

FHB risk Model 1
(current) for 7/1/02

FHB risk Model 2
(future potential if rain occurs)

 

WHEAT LEAF RUST UPDATE, 7/02

Wheat leaf rust is common in many areas and severe in some. The disease is found south of I -94 but at trace levels and pustules are drying up. The disease is more common north of I -94 and in areas of northern Griggs and Steele counties, southern Nelson and Grand Forks counties and in Walsh counties, it is very common and in some case, severe. It has been reported in wheats ranging from the 5 leaf stage to heading stage. Highest levels have been reported on Ingot and Parshall hard red spring wheats, but other varieties also have some rust. See last week’s Crop and Pest Report for information on variety response.

Information on yield losses (%) predicted with varying levels of rust at several advanced wheat growth stages was provided by Dr. Bob Bowden of Kansas State Univ. In personal communication with him, he thinks these numbers are a little conservative.

 

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

Growth stage

Rust severity on flag leaf

10%

25%

40%

65%

100%

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 top pictures are too high for effective fungicide application. Levels in the 5-10% range would still have enough green area to protect from infection.

Growers need to assess their rust pressure and protect the flag leaf, IF yield potentials warrant fungicide use. Tilt, Folicur and Stratego should all provide excellent control of leaf rust for 3 weeks when applied at full label rates; reduced rates would provide shorter duration of control. These products also have systemic and curative activity and would control some very young rust lesions.

wheatrust1.jpg (111791 bytes)   wheatrust2.jpg (43923 bytes)   Scale3.jpg (55769 bytes)

 

STEM RUST OF WHEAT

Dr. Jim Miller, USDA rust pathologist, Fargo, detected stem rust of wheat on Norstar winter wheat on June 28 at Casselton, ND. The severity and incidence levels were trace. The winter wheat was in the 1/4 to ½ berry stage. Norstar is susceptible to stem rust; many winter wheats and most spring wheats are resistant. Dr. Miller also found wheat stem rust on a susceptible experimental line at Fargo that was in the jointing stage.

 

OFF COLOR WHEAT HEADS

Now that wheat crops are in heading stages in many areas, some abnormal colored wheat heads are appearing. White heads were reported by several county agents in the Red River Valley today. Some white heads were due to root rot in wet areas. If root rot is the cause, the whole plant generally has an off color, with a bronzing to greying of the stem area, too, plus the plant easily pulls from the soil. The crowns and roots are discolored.

If wheat stem maggot is the cause of the white head, the head is generally very white and the stem down to the first node also is white (see following figure). The head easily pulls from the stem and evidence of where the wheat stem maggot chewed on the stem is seen.

Another cause of wheat discoloration apparently is a rare and little understood virus which was recently described by the wheat virologist at Kansas State University, Dr. Dallas Seifers. He calls the disease wheat yellow head virus (WYHV), and he believes it to be transmitted by a leafhopper, instead of grain aphids. The wheat head on the right in the following picture is typical of this wheat yellow (yellow-green color) head. The flag leaf also has a yellow-green streak, which is typical. I see this frequently in fields, but it usually constitutes much less than a tenth of a percent of plants with symptoms.

 

SMALL GRAINS SCOUTING REPORT, 7/1

The NDSU IPM field scouts report little damaging leaf or head disease levels in the state. Wheat leaf rust and tan spot are common, but frequently not at severe levels. The barley crop has very little disease pressure in most areas. Information on disease and insect findings by the scouts can be found at the IPM web site:

http://www.ag.ndsu.nodak.edu/aginfo/ndipm/

Marcia McMullen
Extension Plant Pathologist

mmcmulle@ndsuext.nodak.edu

 

SUGARBEET: CERCOSPORA SEASON SLOWLY APPROACHING

In this unique growing season, the only good word to date is that there is no Cercospora alert since symptoms of the disease have not been observed in fields. This is of course due to the fact that most sugarbeet emergence was 2-4 weeks late because of the prolonged cold weather in April and May. Many fields in Southern Minnesota and Minn-Dak factory districts have, or will close their rows in another week. As a result, more scouting will be done in these areas since Cercospora leaf spot usually start appearing in fields after row closure. Day temperatures of 80-90° F and night temperatures above 60° F favor disease development.

The fungus Cercospora beticola causes Cercospora leaf spot. The symptoms of Cercospora leaf spot are circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders on leaves. Bacterial leaf spot, which may be confused with Cercospora leaf spot, has been identified in some sugarbeet fields in Southern Minnesota. Bacterial leaf spot produces irregular to circular spots about 3/16 to ¼ inch in diameter, and they usually appear after a rain. Using a hand lens, spores will be observed as tiny black dots in the Cercospora leaf spot. There are no black dots or spores present in bacterial leaf spot. There is no treatment for bacterial leaf spot since it does not affect sugarbeet yield.

Mohamed Khan
Extension Sugarbeet Specialist

mkhan@ndsuext.nodak.edu

 

FUNGICIDES LABELED FOR CERCOSPORA CONTROL

Fungicides labeled for Cercospora leaf spot control include Eminent (Section 18), Super Tin, Agri Tin, Gem, Topsin M, Mancozeb, and Quadris. None of these products, or products from the same class of chemistry, should be used ‘back-to-back’. Always alternate different chemistry products so as to delay fungicide resistance and thus ensuring prolonged usefulness of these products. The products, chemical classes, and pre-harvest interval are listed below.

Product

Chemical Class

Pre-harvest Interval (days)

Eminent (Tetraconazole)

Sterol biosynthesis inhibitor

14

Agri Tin

Triphenyltin hydroxide

21

Super Tin 4L

Triphenyltin hydroxide

21

Super Tin 80 WP

Triphenyltin hydroxide

7

Gem (Trifloxystrobin)

Strobilurin

21

Topsin M

Benzimidazole

21

Mancozeb

Ethylene-bis-dithio-carbamate

14

Quadris (Azoxystrobin)

Strobilurin

0

Mohamed Khan
Extension Sugarbeet Specialist

mkhan@ndsuext.nodak.edu


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