ISSUE 15    August 11, 2005

FARGO CROPPING SYSTEM DIFFERENCES FOR FHB (SCAB)

Fungicide plots at Fargo were established on last year’s wheat ground, for the purpose of increasing risk of disease. The cultivar planted was Reeder HRSW, a cultivar with susceptibility to leaf spots, leaf rust and Fusarium head blight (FHB = scab). Across the road from this 2 acre plot, a 2 acre planting of Reeder was done the same day, April 29th, on last year’s soybean ground. At the time of evaluation of fungicide plots, we noticed that the field across the road had considerable less scab, although still very apparent. At late soft dough, evaluations of scab symptoms were made on 30 heads at 10 locations in each field, the Reeder on soybean ground, and the Reeder on wheat ground (non-misted and non-inoculated).

The following table indicates the levels of FHB incidence, head severity, and field severity index, according to crop history. The crop history made a big difference in incidence, indicating that many more spores were produced locally on the wheat residue, favoring significantly higher initial infections in that field.

Although average head severity was about the same for both cropping histories, the increased incidence with wheat on wheat, translated into about a 2x increase in overall scab severity, as compared to wheat on soybean ground. The data supports the importance of rotation, even in the presence of weather very favorable for scab infection across a region.

Fusarium head blight levels in Reeder spring wheat with different crop histories, Fargo, 2005

Field history

FHB Incidence (% tillers with symptoms)

FHB head severity (% of head showing symptoms)

FHB field severity index (incidence x head severity)

Reeder on soybean residue

38.6

50.3

19.5

Reeder on Reeder wheat residue

70.0

52.7

42.3

LSD 0.05

8.9

NS

9.9

 

FUSARIUM HEAD BLIGHT (SCAB) RATINGS OF SPRING WHEAT, PROSPER, ND 2005

The hard red spring wheat yield trials at Prosper, ND were rated for Fusarium head blight field severity index by Dr. Bob Stack, NDSU Plant Pathologist. He rated the four replicates at the Prosper yield trials established by Dr. Mohamed Mergoum, NDSU spring wheat breeder. The statistical analysis indicated that differences among cultivars were significant at the 95% confidence level. The following table contains results of Dr. Bob Stack’s evaluations of cultivars at Prosper, each cultivar rated at its respective soft dough stage.

2005 Fusarium head blight (FHB) field severity index on Dr. M. Mergoum’s spring wheat yield trials, Prosper, ND, as rated by Dr. Bob Stack, NDSU Plant Pathologist

Cultivar

FHB Index*

Alsen

17.9

Bigg Red

19.6

Hanna

21.1

Oklee

21.7

Glenn

22.0

Fryer

22.3

Granger

23.3

Ingot

24.1

Steele-ND

29.3

Granite

29.9

Amidon

31.1

Saturn

32.7

Knudson

33.1

Briggs

34.3

Gunner

35.5

Polaris

37.9

Banton

38.3

Russ

38.6

Parshall

41.1

Oxen

41.1

Ulen

43.2

Amazon

47.4

Trooper

47.8

Dapps

51.3

Buck Pronto

58.8

Reeder

61.1

Express

73.0

* FHB Index = (% tillers showing scab symptoms x % head severity)/100

Similar ratings were done at other locations across ND, as well as information being gathered on yield and quality response. The above cultivars vary in susceptibility to leaf rust and leaf spot diseases, which also will affect yields achieved in 2005.

 

NDSU IPM SURVEY, AUG. 1-5

NDSU IPM field scouts are winding down or have completed their survey of small grains. The scouts surveyed 53 wheat and 4 barley fields the first week of August. Wheat and barley fields surveyed were in the milk to hard dough stage of kernel development. Leaf diseases either couldn’t be evaluated or flag leaves were dominated by leaf rust plus Septoria infections. Fusarium head blight (scab) field severities were observed in 70% of the wheat fields surveyed, ranging from <1% to 50% (see map). The average scab field severity index was 11.9%. Of the 4 barley fields surveyed, 2 had symptoms of scab with field severities of 2 and 6%. Grasshopper numbers were increasing in the north central counties, with 2 to 12 grasshoppers/sq ft common.

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

 

NDSU PLANT DIAGNOSTIC LAB

More on Sticky Maples. All summer, you might have noticed your silver maple dripping a constant spray of a sticky, annoying substance. Covering patio chairs and tables, smothering swingsets, and rendering many outside patios useless, the sticky substance, known as honeydew, is produced by at least two insects that feed on maple trees. One of these insects is an aphid. The other is the cottony maple scale, which has been particularly bad this year in Fargo. The cottony maple scale is an oyster shell scale that is usually inconspicuously flattened against the bark of maple trees, blending in very well with its brownish grey coloring.

This insect is inconspicuous until around June, when the females puff up with a white egg sac underneath their posterior. No, your maple tree has not sprung a batch of popcorn, but that’s what the egg sacs of the cottony maple scale look like. Here are some of the questions about this insect that I have received this summer:

What is the sticky stuff?

The insects pierce plant tissue and suck sap, and, they produce copious amounts of sticky honeydew. The honeydew is actually a product of the insect. It is not plant sap, which is what I thought it was at first! The maples on my boulevard are so badly infested that my car’s windshield becomes covered with the honeydew in a matter of hours.

What can I clean it off with?

Fortunately, the residue is easily washed off, unless it has sat long enough for black, sooty molds to grow on it. In that case, a little more elbow grease and warm, soapy water will take care of the problem, at least temporarily.

Why are they so bad this year?

In ‘normal’ years, populations of the maple scale are kept in check by natural predators, such as certain ladybug larvae and tiny parasitic wasps. But this year, at least early summer, I suspect that the weather was not very favorable for these natural predators. And so, as a result, cottony maple scale populations exploded.

Should I spray insecticide?

In fall, the females crawl back onto twigs and bark (after mating with males), and here they overwinter. Another batch of scale insects are hatched the following June and July. Feeding by these voracious insects can cause twig dieback, and severe infestations may eventually kill major limbs. Although one epidemic year will probably cause minimal harm to a mature tree, if you see abundant ‘popcorn’ on your tree in consecutive years, you may want to consider applying an insecticide. Dormant oils can be useful against the mature overwintering scales, but if you use this treatment, be sure to read the label very carefully for timing of application, since some trees can be injured if dormant oil is applied at the wrong time. Insecticides and insecticidal soaps are targeted at the hatching insects in early June, so keep an eye out in early summer next year.

Recent samples. Many soybean samples have been submitted recently, and some show symptoms consistent with Phytophthora. Culturing is currently underway to confirm this. Wheat samples with growth regulator herbicide injury have been evaluated, and so, here is a gentle reminder: growth regulator herbicide injury can occur on wheat if it is applied too late in the growth stage. In general, growth regulator herbicides are safe on wheat up to just prior to boot or to early flag leaf (depends on herbicide; read label and follow its directions carefully to avoid crop injury). Other samples received include sunflower with apparent late-season infection with downy mildew.

Kasia Kinzer
Plant Pest Diagnostician
kasia.kinzer@ndsu.edu

 

SOYBEAN RUST IN SOUTHEASTERN U.S.

Soybean rust has been on the move in the southeastearn U.S. recently with new confirmations in Alabama, Florida, Georgia, and Mississippi. The current total number of counties with confirmed soybean rust is 25 (Alabama - 4, Florida - 12, Georgia - 7, Mississippi - 2) - see map below. Soybean fields in North Dakota are generally between the R3 and R5 growth stages, currently. Research trial results have indicated that once soybean plants reach the R6 growth stage, yield reduction from soybean rust would be minimal. As part of a national effort, North Dakota is participating in a sentinel plot and soybean field scouting program. No soybean rust has been found, but the "look-alike" diseases Septoria brown spot and bacterial blight have been common. To track the movement of soybean rust in the U.S., go to the USDA soybean rust website at: http://www.sbrusa.net

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


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