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ISSUE 13  July 13, 2001

 

SOYBEAN CYST NEMATODE AND PESTICIDE LAB CORRECTION

The phone number published in the July 12 edition for the Olson Biochem Lab that does pesticide residue testing was incorrect. The correct number for the SDSU Pesticide lab is 605.688.6171. The number has been changed on the web version of the Crop and Pest Report and is correctly listed on the NDSU Plant Diagnostic Lab web page at: http://www.ag.ndsu.nodak.edu/diaglab .

The soybeans coming into the lab are showing varying degrees of iron chlorosis. Symptoms range from pale green interveinal tissue to severely stunted plants showing interveinal yellowing and browning. We have also isolated Pythium, a root rot pathogen, from the roots of several samples that show mild iron chlorosis symptoms but seem more stunted than usual. This is not unexpected given the cool, wet soils many of these crops were planted into. As we move into August, it will be important to scout for symptoms of soybean cyst nematode (SCN). This pathogen has not been documented in ND yet, but it is found in fields in counties along both the SD and MN borders.

SCN is a worm-like organism in the animal kingdom that parasitizes plant root tissue. The host range for this pathogen includes susceptible soybeans, dry and snap beans, peas, and susceptible forage crops such as Sweetclover, Vetch, Lespedeza, and Birdsfoot trefoil. It is called a cyst nematode because the female nematode feeds on the roots, swells, and fills with eggs. As the female matures and dies, she turns a brownish color. This stage is referred to as the cyst, which can survive in the soil and from which eggs are hatched.

Symptoms of SCN usually begin to express themselves later in the season, and only show on above ground parts of the plant after populations of the nematode have sufficiently increased. Symptoms may appear similar to iron chlorosis, compaction, drought stress, other nutrient deficiencies, and herbicide injury. Symptoms of possible nematode problems will show up on oval or circular areas on a field, often in an elongated shape that follows tillage patterns. Unlike iron chlorosis where the whole leaf expresses interveinal yellowing, SCN will cause the outer margin of the leaf to yellow and brown, and the whole leaf is affected, not just the area between veins. Plants are usually stunted and rows may not close in affected areas. Symptoms are usually most severe in the center of the affected area and decrease in severity toward the margins. These affected areas are more common at field entrances, where equipment may enter, and along field edges where windblown soil may accumulate. Cysts move most often in soil and soil-peds in seed, and on equipment that might carry soil from an infested field to a new field. Damage is generally more severe on light, sandy soils, but other soil types can be affected as well. It is not uncommon to have both iron chlorosis and SCN symptoms on the same plants.

While yield loss may be serious with SCN infestations, the problem can be managed, but only if it is identified. Planting resistant soybean varieties will keep SCN populations down, and rotating to non-susceptible crops will minimize disease potential. A corn-soybean rotation is adequate if resistant soybean varieties are used. A better rotation includes a non-host crop in a four year period. Populations of 100 or more eggs and juveniles in a 100 cc soil sample (about ½ cup) will cause yield loss and management strategies should be implemented. This is why it is important to identify the pest.

The NDSU Plant Diagnostic Lab can help with identification of potential SCN infestations. If you suspect an SCN infestation in a field this year, bring in a sample that includes the soils. Gently dig out the plant with the root ball and soil intact. This will enable us to check for cysts on the roots. Soil samples can also be assessed after harvest. The cost for this testing is $20.00. Soil samples should be taken from 0-6 inches, from affected areas, or from several areas in the field following a zig-zag pattern. More detailed sampling instructions can be found on the NDSU Plant Diagnostic Lab web page, http://www.ag.ndsu.nodak.edu/diaglab , or by calling the lab at 701.231.7854. Links to the SD SCN web page and the SCN Coalition web page can be accessed from the diagnostic lab page.

Cheryl Biller
Diagostician
diaglab@ndsuext.nodak.edu

 

WHITE MOLD ON DRY BEANS:  TO SPRAY OR NOT TO SPRAY

The white mold pathogen infects dry bean plants by means of spores released from a mushroom like structure called apothecium. Apothecia are usually tan in color, about ¼ inch in diameter and raises from the ground less than ¼ inch. They are produced by sclerotia located in the upper 2 inches of soil. Apothecium formation starts when this layer of soil remains wet for at least seven consecutive days. Seven to ten days later, the apothecia are ready to release its spores. In a field with a closed canopy, apothecia will be formed wherever there are conditioned sclerotia. If the canopy has not closed completely, some apothecia may be produced in between the plants, but not in the middle of the rows. If the upper layer of soil remains dry, no apothecia will be produced.

Spores can germinate with the morning dew and enter senescent blossoms or leaves. If temperatures are kept in the cool side (<85EF) the fungus will continue to grow and will enter the stem. Diurnal temperatures higher than 85EF will prevent the fungus from growing, but growth will be resumed as soon as the temperature goes down. Dry conditions after infection has occurred will also reduce the growth of the fungus and its impact on yield. The recent rain events during the third week of July will stimulate apothecia formation. You may be able to see them if you look carefully under the crop canopy. The cool temperatures (<85EF) prevailing this week (last week of July) in most of the bean growing areas are conducive for spore dispersal and germination. Infections may occur if your beans started to bloom during the third week of July or are starting to bloom now.

The decision on whether to spray or not will depend partly on your answers to the following questions:

If you answered yes to all these questions, your crop is at high risk of getting white mold, and you should consider the possibility of spraying fungicides.

If you answered yes to all but the last question, because your beans started to flower when the ground was still dry and have finished flowering already, then your crop may no longer be in danger. You may still see some white mold developing in your field but it may not be as serious as to warrant a fungicide application.

The decision on how much product to use will depend on your particular situation, and will vary depending among other factors on the amount of plant tissue to protect, expected disease pressure, price of the chemicals, etc. Spells of hot weather (>85EF) during most of the first week of August could prevent white mold from becoming a problem, even if infection has already taken place in your field. If your beans were at full bloom during the third week of July, your crop is more likely out of risk, although very low levels of with mold could still be observed later on.

The best recommendation for control of white mold continues to be to spray either Benlate or Topsin at a rate of 1 to 1.5 lb per acre. Topsin and Benlate are systemic fungicides. However, better control is usually achieved when drop nozzles are used. Calcium for control of white mold was evaluated at two experimental stations in trials conducted in 1999 and 2000. Results indicated that Calcium alone does not control the white mold. Under heavy disease pressure, plots sprayed with combinations of Calcium and fungicide, were not different from the untreated controls. Additional experiments on the effectiveness of Calcium applied in combination with fungicides for white mold control are being conducted at Oakes and Carrington this season. The impact of calcium as a micronutrient and therefore the impact of calcium sprays on yield of beans were not evaluated in these studies.

Luis del Río
Assistant Professor
Department of Plant Pathology

 

POTATO LATEBLIGHT HOTLINE

Despite heavy and frequent rains in some areas of NE ND, late blight has still not been confirmed in ND, nor in SD or MN. NDSU potato pathologists still recommend applying fungicides on a regular basis, as cooler conditions of this week and irrigation and intermittent thunderstorms will once again increase favorability for infection. The annual potato field day at their irrigated research site near Dawson will be on Tuesday, August 7, from 9 am to noon.

 

DISEASE FORECASTING MODELS

The Sclerotinia risk map, updated on July 23, 2001, indicates some high risk areas of ND for white mold infection in canola. These areas are primarily in Wells county, and in the north central counties of Renville, Bottineau, Rollette, and western Towner county, plus in southern Williams county and in the SW counties of Morton, Sioux, and Emmons.

The cereal disease forecasting information indicates that many sites are still experiencing favorable conditions for infection by tan spot, Septoria blotch, and leaf rust. Certain sites, such as Bottineau, Langdon, Mohall, Cavalier, and Berthold, also have had high spore counts of the head scab fungus in the past week.

 

SMALL GRAIN DISEASE SURVEY UPDATE, 7/25

Leaf rusts:  Wheat leaf rust continues to increase in incidence (see map) and severity.

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In the NE, during the week of July 16-20, Matthew Gregoire found leaf rust in 60% of the spring wheat fields surveyed, with incidence of infected tillers from 12 to 100% and severity on the flag leaf from 1-3%. He is finding higher leaf rust severities (up to 20%) this week in spring wheat in Barnes, Cass, and Ransom counties. Dr. Jim Miller, USDA rust pathologist, reports that SDSU’s cereal pathologist, Dr. Yue Jin, observed 100% severity of leaf rust in Russ wheat near Aberdeen, on soft dough wheat.

On July 23 and 24, Jerry Schneider observed leaf rust in 87% of the spring wheat fields surveyed in counties in the central and south central districts,, with incidences in a field ranging from 0-100% and severities on the flag leaves from 1-30%. Jeanna Jambor observed leaf rust only in 6% of the wheat fields she surveyed in the SW the week of July 16th.

Leaf rust was observed in 54% of wheat fields surveyed in NC counties by NDSU IPM scout Holly Semler, with severities quite low, at 1%. Kelly Novak and Nathan Carlson, surveying in counties in the NW district the week of 7/16, found 22% of the wheat fields to have leaf rust, with severities on the flag leaf ranging from 1-5%.

Leaf rust damage to a crop yield depends on growth stage of the crop when severity levels are reached. The Kansas State University plant pathologist, Bob Bowden, published this table several years ago, to indicate predicted yield losses from leaf rust. Keep in mind that these estimates were made with winter wheat, Dr. Bowden thinks they are conservative, and that other diseases also may be occupying the leaf or affecting the heads or roots which also affect yield.

Yield losses (%) predicted with different rust severities at various wheat growth stages

Growth stage

Rust severity on flag leaf

10%

25%

40%

65%

100%

Flowering

10

15

20

30

40

Milk

3

5

10

15

20

Soft dough

2

3

5

7

10

Hard dough

1

1

2

3

5

Barley leaf rust: Field scouts are not reporting leaf rust observations on their surveys. I saw a trace amount of leaf rust in barley in plots in Fargo.

Stem rust: Dr. Jim Miller, USDA rust pathologist, has observed stem rust in winter wheat plots at Lisbon and in Casselton, with severities up to 40% on susceptible varieties. These crops are in the soft to mid-dough stage. A trace amount of stem rust was observed on Robust barley in plots near Lisbon.

Fungal leaf spots: Tan spot and Septoria fungal leaf spots continue to be commonly observed in wheat by all field scouts across the state.

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Highest severities of tan spot and Septoria are being observed in the central, south central, and southwest counties, plus in McHenry and McLean counties (Septoria on flag leaf up to 45-50% in some fields). Net blotch, spot blotch and Septoria blotch also are common in barley, but severities are generally very low.

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Head diseases: NDSU IPM field scouts are picking up loose smut in wheat and barley, with incidences of 2-12% common, although one spring wheat filed in Renville county had 34% incidence. Low levels (field severities of 1-3%) of head scab are now starting to be observed by field scouts throughout the state where crops are in the soft dough stage. One field of Amidon wheat in McLean county, observed by Jan Knodel, area extension crop protection specialist, Minot, had very severe head scab. Amidon is a very susceptible variety to this disease.

Root rots: Field scouts are also observing scattered plants showing root rot symptoms, with whole plants turning prematurely white and grain heads are empty. My preliminary observations of the cause of these root rots has primarily resulted in isolation of the common root rot fungus or Fusarium graminearum. I also made one observation of take-all symptoms.

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

 

CERCOSPORA IN SUGARBEET

Fields with Cercospora leaf spot have been reported in most of the factory districts. Weather conditions in the last week was favorable for spore germination and penetration. Early planted fields, especially those with the more susceptible varieties, and in areas where the symptoms have been observed should be receiving their first fungicide application. Growers in the northern Red River Valley will need to monitor their fields and have their first application when the symptoms are common in the factory districts, not just in an isolated field. Since most fields were planted late, growers may have one less fungicide application this season.

Dr. Mohamed F. Khan
NDSU Extension Sugarbeet Specialist

mkhan@ndsuext.nodak.edu


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