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ISSUE 5  June 1, 2000

 

TAN SPOT

    Scattered rain showers over Memorial Day weekend will favor continued development of tan spot where infections already occur, and may activate further sporulation by the fungus on wheat residue. Cool temperatures should retard rapid development, however.

 

BARLEY YELLOW DWARF VIRUS

    Barley yellow dwarf virus (BYDV) was confirmed with antibody testing by the Plant Diagnostic Lab in a sample of winter wheat from Towner Co. The plants showed bright yellowing to purpling of the leaves, from the leaf tip downward, and a fairly severe stunting. I believe that this winter wheat crop was infected last fall, when aphids were still active. Late planted spring wheat in the region served as a reservoir of the grain aphids, which then moved into the young winter wheat and transmitted the virus to the winter wheat crop prior to freeze-up. The severity of symptoms certainly suggest fall infection.

    No grain aphids were observed with this winter wheat sample, but scattered observations of low numbers of grain aphids have recently been seen this spring (Phil Glogoza announced grain aphid detection in winter wheat at Carrington in an email to the agdakota list serve on Friday, May 26; Roger Ashley reported grain aphids detection in SW North Dakota earlier that week; and Jan Knodel reports grain aphid detection at Minot on May 29; aphids had not been detected in winter wheat in Cass County a week earlier).

    As Phil stated, wheat and barley fields should be monitored now for aphids, with older wheat to be monitored more closely, as it will be more likely to attract winged aphids. Winged aphids arriving in southerly winds will most likely be carrying BYDV, as infection levels in southern plains states are severe. We will be monitoring closely for aphids in our disease and insect surveys.

 

WHEAT STRIPE RUST

    The latest Cereal Rust Bulletin, #4, May 24, reports that wheat stripe rust is widespread throughout the southern plains. This rust of wheat usually is rare in the southern plains, because it requires very cool, humid conditions, and southern states such as Kansas are usually too hot and dry for it. Stripe rust is a common disease of wheat and barley in the Pacific northwest.

    Stripe rust is very rare in ND; I’ve only seen it once in my 16 years on the job. Stripe rust is distinguished by yellowish orange pustules arranged in a long, narrow, stripe on the leaves.

    Nebraska has an excellent web site on rust diseases of wheat, including comparisons among leaf, stem, and stripe rust, at:

     http://www.ianr.unl.edu/pubs/plantdisease/g1245.HTM.

    Texas has a one page web site describing stripe rust:

    http://cygnus.tamu.edu/Texlab/Grains/Wheat/wsrust.html.

    I don’t expect stripe rust to be a problem in ND, but it may be observed, considering its wide spread occurrence further south. Trace amounts of stripe rust were found in winter wheat breeding plots at Brookings, SD in mid-May.

 

 DISEASE AND INSECT SURVEY

    Six field scouts were trained in survey methods last Thursday at the Carrington Research and Extension Center (see Crop and Pest Report # 4). I need to make a correction in last week’s report: Matthew (not Michael) Gregoire will be the scout operating out of Fargo/Devils Lake, surveying counties in the SE, EC, and NE crop reporting districts. All scouts are starting their field survey efforts in earnest this week.

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

 

WATER MOLDS, SEEDLING DISEASES AND ROOT ROTS

    Scattered rain showers may have saturated soils in some locations across the state. Several fungi, often called water molds, have swimming spores (zoospores) that may infect certain row crops. These water molds are favored by saturated soils.

    Downy Mildew of Sunflower. Downy mildew of sunflower infects when heavy rains occur shortly after planting or emergence. Cool to moderate soil temperatures favor this disease. When heavy rainfall occurs before emergence of the crop, there may be severe seed rot, resulting in poor stands. Many seedlings will be infected early, resulting in systemic infection and death of seedlings or in severely stunted plants that fail to grow. These severely stunted plants may die, or may survive, but not produce seed. The earlier the infection, the greater the damage. If only a low percent of the plants are systemically infected, normal plants can compensate yield. However, yield compensation may result in larger heads, slower dry down and lodging.
Symptoms. Systemic infection occurs through the roots when the soil is saturated. It causes stunted plants with yellow veins on the upper leaf surface and a downy white growth on the lower leaf surface, opposite the yellow veins. Later, secondary spread may occur from wind borne spores, resulting in local lesions, which are yellow spots on the upper leaf surface with a downy white growth on the lower leaf surface opposite the yellow spots. The later infections usually are of no importance.

    Management. Downy mildew was formerly controlled very well by seed treatment with Apron fungicide, but now a substantial portion of the downy mildew population is resistant to Apron. We hope to have a new fungicide available in time for seed treatment of the 2001 sunflower crop. Dr. Tom Gulya, USDA sunflower pathologist, the sunflower seed industry and several pesticide manufacturers have been working to develop a new seed treatment product for 2001. Sources of resistance to all current races of downy mildew are available, but most hybrids are not resistant to all races.

    Phytophthora Root Rot of Soybean. Phytophthora is favored by saturated, compacted and warm soils. There is little infection at soil temperatures below 60F, and optimum infection occurs at soil temperatures of 77-82F. Recent soil temperatures have been pretty cool for Phytophthora, but a few warm sunny days while the soil is wet could provide warm and wet soil, favoring infection.

    Symptoms. Phytophthora can cause a seedling disease and it can also cause a root rot later in the season. Seedling infection may result in seed rot or seedling death. Later infection results in root rot, a sudden wilt and a dark line that proceeds up the stem as far as the second or third node.

    Management. Several races of Phytophthora are present in the area, but race 3 is the most common. Certain varieties have resistance to race 3 and some other races. Resistance is effective in both the seedling and the mature plant.

    Aphanomyces Seedling Disease and Root Rot of Sugarbeet. Aphanomyces infects whenever the soil is wet and soil temperatures are at least 60F; optimum infection occurs at soil temperatures of 72-82F. As with Phytophthora, recent soil temperatures have been too cool for optimum infection, but a couple of warm sunny days could change that.

    Symptoms. Infection in the seedling stage may result in seedling death or unthrifty seedlings with black root tips. Later infection may result in plants that are greenish-yellow and wilt at midday. Infected plants have very little tap root.

    Management. Some varieties are tolerant to Aphanomyces, but none is highly resistant. Seed pelleting with Tachigaren should be used in conjunction with use of a tolerant variety when planting into a field known to have Aphanomyces. Indexing of fields for Aphanomyces potential is available through sugar cooperative agriculturists.

    Sugarbeet Rhizomania, caused by the beet necrotic yellow vein virus, is transmitted by a soil borne water mold, Polymyxa betae. The fungus is active in wet soils when the soil temperature is at least 60F, with an optimum at 77F.

    Symptoms. Early infection results in portions of the field turning a fluorescent yellow-green. Tap roots are often shortened and constricted in a "wine glass" shape, with numerous fibrous roots near the tip. Late infection may result in little or no symptoms and little or no yield loss.

    Management. To avoid spreading rhizomania, harvest infected fields last and power wash equipment before entering other fields. Resistant varieties have been developed for the western US and are being evaluated in southern Minnesota.

Art Lamey
Extension Plant Pathologist
alamey@ndsuext.nodak.edu

 

PLANT DIAGNOSTIC LAB

    As crops begin to emerge, root rot problems begin to manifest themselves. In the lab this past week, we found Aphanomyces in one sugarbeet and Rhizoctonia and Pythium in another 4 samples. Rhizoctonia and Pythium are soil-borne organisms that will cause injury if environmental conditions are appropriate. Rotation has not been shown to be an effective management practice for Pythium root rot in sugarbeets, but it can be effective in managing Rhizoctonia since the specific strain of the fungus is shared by sugarbeets, potatoes, soybeans, and dry beans. Rotation is important in managing Aphanomyces root rot in sugarbeets since it will survive for long periods in the soil. Planting sugarbeets in fields that have Aphanomyces is not recommended more than 4 years apart, or the risk of serious loss from root rot increases. Several fungicidal seed treatments
are available that offer protection from Pythium, and sugarbeet seed treated with Tachigaren is protected for a limited time against Aphanomyces.

    Other samples that have come through the lab in the past week include: turf samples (yellow patch and patch disease complex), winter wheat (barley yellow dwarf virus-BYDV), plant id (false solomanseal - Smilacena), potatoes (soft rot seed piece decay), spring wheat (probable Roundup drift and Pythium root rot), tomato (growth regulator herbicide injury), spruce (spider mites and Rhizosphaera needlecast), shelterbelt trees (growth regulator herbicide injury), juneberry (probable Goal injury), and several insect id’s.

Cheryl Ruby
Plant Diagnostician
diaglab@ndsuext.nodak.edu


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