NDSU Crop and Pest Report
Plant Pathology


ISSUE 3  May 16, 2002

 

GROWTH STAGING IN WHEAT

Several growth staging systems are used to describe development in wheat. Many fungicide labels use the Feekes growth staging system, herbicide labels may use the Haun system, and research papers often follow a European system such as the Zadoks scale. These scales are based on developmental or morphological stages of the crop. Another way to stage crops, without visually inspecting the crops, uses heat units or growing-degree days.

Feekes scale: Divided into 11 developmental or morphological stages and defines critical stages for fungicide application.

Zadoks scale: Two-digit numerical notation system, divided into ten principal plant development stages which are further subdivided into secondary stages, giving even more detailed information.

Haun scale: Divides growth stages into growth units of leaf production, boot, heading and peduncle elongation; does not fully describe the tillering, jointing, or grain development stages.

Growing-degree days (GDD): Determines developmental stage of wheat based on accumulation of heat units. Accumulated GDD relates plant development rate to air temperature. For spring wheat, the base temperature used for calculations is 320F.

Comparison of terminology of various growth stage systems at certain key growth stages, and approximate growing degree day (GDD) accumulation needed to reach that stage in wheat (based on the 320F base temperature)(shaded area = after emergence)

 

Wheat Stage

Staging System

Accumulated GDD Needed

Feekes

Zadoks

Haun

Emergence

----

09

0.0

180 to emerge

1st leaf

1

10

0.5-1

70 after emerge

Tillering

2

21

3.5

360

Jointing

6

31

6.3

715

Boot

10

45

10.5

1360

Anthesis

10.51

65

11.4

1571

Further information about growth staging in wheat is available at the following web sites:

http://www.extension.umn.edu/catalog/item.html?item=2547

or

http://www.ext.nodak.edu/extpubs/plantsci/weeds/w564w.htm

 

EARLY SEASON FUNGICIDES FOR LEAF DISEASE CONTROL IN WHEAT

As wheat begins to emerge, wheat planted into wheat stubble may be at risk of infection by the tan spot fungus. Recent wet conditions will increase the risk of infection, if warmer weather conditions occur. Early season application of fungicides to wheat is recommended IF:

Several fungicides are now available for early season tan spot control:

Tilt (propiconazole): 2 fl oz/acre

Propimax (propiconazole): 2 fl oz/acre

Stratego (propiconazole + trifloxystrobin): 5 fl oz/acre

Dithane, Penncozeb, Manzate, Manex II (various mancozebs): 1 lb/acre or 0.8 qt/acre

Tilt, Propimax, and Stratego have some systemic activity and will be more rain fast, while the mancozebs are protectants and can be washed off the leaf surfaces after heavy rains.

These fungicides can be tank mixed with most herbicides and be applied at the 4-5 leaf stage (before Feekes stage 6 and Zadoks stage 31; at Haun stage 4-5) for both early season disease control and weed control. A spreader-sticker or surfactant addition generally is not required when fungicides are tank mixed with herbicides, unless required for the herbicide. Please see herbicide labels for any tank mix restriction with fungicides for this use.

 

WHEAT LEAF AND STRIPE RUST IN SOUTHERN PLAINS AS OF MAY 8

The latest Cereal Rust Bulletin reports light infections of wheat leaf rust in southern plains states, as of May 8th. Trace levels of leaf rust have been observed in Oklahoma fields and greater than 10% severities were observed in central Texas. Trace levels of wheat stripe rust also have been observed in Texas and Oklahoma. Wheat already is mature in the southern tip of Texas.

 

DIVIDEND EXTREME SEED TREATMENT FOR WHEAT NOW LABELED

A different formulation of Dividend seed treatment is now labeled for wheat. Dividend Extreme contains 7.73% difenoconazole and 1.87% mefenoxam. At these concentrations, the Dividend Extreme label recommends a 1 fl oz/cwt rate for control of common bunt and loose smut, and a 2 fl oz/cwt rate for control of additional diseases such as general seed rots, Pythium damping-off, seedling blight due to Fusarium infection, and common root rot.

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

 

PYTHIUM IS NUMBER ONE SOYBEAN SEEDLING DISEASE IN COOL SOILS

Many questions are still coming up regarding seed treatments for soybean. When soil temperatures are between 50 and 60F, Pythium is typically the main culprit of soybean seedling diseases. Soil must be very wet to saturated for infection to occur. Seeds and seedlings can be protected against Pythium by using a seed treatment containing mefenoxam or metalaxyl. These two products will have activity against Pythium for approximately 10 to 14 days after planting. As soil temperatures rise, Pythium becomes less of a problem and other pathogens may cause disease. See the table for temperature and soil moisture requirements for pathogen infection and seed treatments that have activity against them.

Pathogen

Temp. (F) requirement for infection

Soil moisture

Seed treatment products containing

Pythium

50-60

very wet - saturated

mefenoxam, metalaxyl

Phtyophthora

60-70

very wet - saturated

mefenoxam, metalaxyl

Rhizoctonia

75

moist

azoxystrobin, carboxin, fludioxonil, TBZ, thiram, PCNB

Carl Bradley
Extension Plant Pathologist
cbradley@ndsuext.nodak.edu

 

SPRUCE TREE PROBLEMS

Instead of surf and turf, we have spruce and turf in the lab these days. In addition to spring lawn problems, trees are the next most typical samples in the lab. Spruce trees are showing symptoms of the two predominant diseases we see on conifers about this time of year. Those two diseases are Rhizosphaera needlecast and Cytospora canker. Both cause needles to turn brown and both typically begin in the lower branches of a tree. The similarities end there, however, and differentiating between the two is important to ensure proper treatment.

Rhizosphaera needlecast is caused by the fungus Rhizosphaera kahlkoffii. The needles in the lower branches, inside nearest the trunk, are the ones that turn brownish-purple and fall. The disease gets its name because the tree "casts" off its needles, often in the spring. Confirmation of this disease requires looking at the green needles on branches with some brown needles, or those branches just above the browning branches.

Rhizosphaera Spruce

Microscopically, you can see small black structures that have filled in the stomates which normally appear white. These stomates line up along the length of the needle in two rows. Infected needles show two rows of fairly uniformly spaced black spots. These are the fungal fruiting structures called pycnidia, from which spores are produced that cause new infections.

Spruce needle

Management for Rhizoshpaera needlecast requires two applications of a chlorothalonil fungicide. Timing of the fungicide application is important - the first should be made about the time the new shoots are approximately inch expanded, or roughly when the lilacs are blooming. The second application should be made about 3-4 weeks later. It is critical to do both applications to get the disease under control in a tree. Good disease control will likely require two applications of the fungicide for two consecutive years.

rhizosphaera pycnidia

Cytospora canker is caused by the fungus Cytospora kunzei. When this disease is present in a tree, typically the lower branches are affected first, but the needle browning begins at the end of the branch and proceeds back toward the trunk of the tree. Needles generally turn a light tan to brown and may remain attached to the tree for a year or two. You will often see sap being extruded from infected branches. It appears as a sticky, white or blueish-white ooze on the infected branch or on branches immediately below infected branches. Research shows that this drought-stressed trees are more susceptible to Cytospora canker. This fungal disease is not managed through fungicide application - the fungal fruiting structures produced by this fungus exist just the bark of the infected branches. Rather, management requires judicious pruning out of infected material. All branches with the needle browning symptoms should be removed. It is often prudent to take branches that are immediately above the infected branches as well since they may have already become infected by spores released from symptomatic branches.

Summary recap:

Rhizosphaera needlecast look for purplish-brown needles on the inside of the tree near the trunk. Manage with 2 applications of a chlorothalonil fungicide, timed as new growth is inch in length and again approximately 4 weeks later. Plan to make application 2 consecutive years.

Cytospora canker look for needle browning from the ends of the branches, generally on the lower branches, and sap globs. Manage by pruning out infected branches and those just above. Plan to prune 2-3 consecutive years for good control.

Cheryl Biller
Plant Diagnostician
diaglab@ndsu.nodak.edu


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