NDSU Crop and Pest Report
Plant Pathology


ISSUE 7  June 13, 2002

 

WET SOILS AND OOMYCETE PATHOGENS

Plant pathogens known as oomycetes are organisms similar to fungi, but are in the kingdom Stramenopila rather than the kingdom Fungi. These oomycetes are also known as "water molds" because of their ability to thrive under wet conditions. Several oomycetes have spores with flagella, known as zoospores, that can move easily in wet soils by swimming. Oomycetes cause diseases in several of the broadleaf crops grown in North Dakota as seen below.

Aphanomyces root rot and damping-off. Aphanomyces root rot and damping-off can be a major problem in sugarbeet in wet soils with temperatures of at least 60 degrees, with optimum temperatures between 78 and 72 degrees.

*Management: Plant less-susceptible cultivars and treat pelleted seed with Tachigaren. Planting early, cultivating to keep soil dry, enhancing field drainage, and increasing the length of rotation will also help manage the disease.

Downy Mildew. If the disease is present with favorable conditions, downy mildew can infect soybean or sunflower plants systemically, causing stunting and chlorosis. Different organisms cause downy mildew on soybean and sunflower, but both are oomycetes.

*Management: Mefenoxam or metalaxyl seed treatment will work in soybean; however, most of the population of downy mildew pathogens of sunflower have developed resistance to these fungicides in North Dakota. Sources of resistance are available for soybean cultivars and sunflower hybrids, but most are susceptible.

Phytophthora root rot. Phytophthora root rot of soybean can be a major problem. It may be found at any stage of development, and requires wet conditions and soil temperatures of at least 60 degrees. Zoospores are attracted to soybean roots by exudates from the plant.

*Management: Planting resistant varieties in combination with crop rotation is the best way to effectively manage the disease. Treating seeds with metalaxyl or mefenoxam will protect seeds and seedlings for a limited period of time.

Pythium root rot. Pythium root rot can attack dry bean, soybean, and sugarbeet. It is generally considered to be more of a problem in cool (50-60 degrees), wet soil.

*Management: Seed treatment with metalaxyl or mefenoxam can provide protection early. Shallow-planting (0.75 in. depth) sugarbeet seed may encourage maximum emergence and reduce disease.

 

LATE BLIGHT UPDATE

As of June 12, late blight severity values have accumulated at some irrigated production sites as shown below:

Site Severity Value
Hofflund

6

Karlshuhe

5

Mandan

3

Northwood

8

Robinson

4

The recent rainfall produced conditions favorable for seed-borne late blight to develop. If late blight contaminated seed was a concern at planting, then scout fields for presence of late blight in newly emerged plants. An application of Curzate banded over the rows will provide effective control if applied soon after a rain. Curzate has a 2-day kickback action that will eradicate recent infections, and will provide a 3-day protection. Additional protective fungicides should be used if seed-borne late blight is found.

Late blight has been found in the Columbia Basin in southcentral Washington near Pasco. It was found in a field of Ranger Russet, and probably originated from volunteer potato plants nearby. Be sure to call the Bravo/Quadris Blightline at: 1-800-482-7286. The information is updated every Monday, Wednesday, and Friday, and is also available online at:

 http://www.ndsu.nodak.edu./instruct/gudmesta/lateblight/

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

 

DISEASE SURVEY RESULTS, 6/3-7

NDSU IPM field scouts found little in the way of diseases during the first full week of June. Disease was limited to a few cases of tan spot on the lower leaves of wheat. Wheat and barley crops were generally in the 3 leaf stage or tillering stage. Crops surveyed that week were generally under some moisture stress. Recent rains in much of the state will likely initiate some additional tan spot infections.

Roger Ashley, Area Agronomist at the Dickinson Research Extension Center found wheat streak mosaic virus (WSMV) in some winter wheat from the Dickinson area. Presence of the virus was confirmed by ELISA testing by the NDSU Diagnostic Lab. This virus disease infected the winter wheat last fall. Once infection is present in a field or area, WSMV is more common and severe under dry, warm environments, as the vector, the wheat curl mite, is more active in movement and reproduction under these conditions. WSMV currently is the major disease problem in the very arid areas of western Nebraska and eastern Colorado.

 

SMALL GRAINS DISEASE FORECASTING, 6/11

The Small Grains Disease Forecasting site at

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

indicates that for most areas that received rain over the past weekend, favorable infection periods for tan spot and Septoria occurred on June 9 and 10. Additional consecutive wet periods may trigger early season infection by tan spot in wheat on wheat stubble, and make early season fungicide use economic in these areas.

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

 

WATCH FOR APOTHECIA (MUSHROOM BODIES) OF SCLEROTINIA

Watch for fruiting bodies of the Sclerotinia or white mold fungus. Called apothecia, they are extremely small mushroom-like bodies that resemble tiny golf tees about 1/8 inch in diameter. Apothecia liberate millions of spores that are blown about by the wind. These spores initiate infections on the dead flower parts of dry beans, canola and other susceptible broad leaved crops if the canopy stays wet for most of a 40-48 hour period.

Apothecia begin to form after 10 days of saturated soil. Apothecia may begin to form in portions of north central and northeastern North Dakota and northwestern Minnesota by the middle of next week. Based on the Sclerotinia Risk Map for Canola, apothecia might begin to form by the end of this week in portions of northwestern North Dakota.

Apothecia develop from sclerotia, the hard black bodies of Sclerotinia that resemble rat droppings.

Apothecia from soil

Apothecia from soil surface

Apothecia formed in lab

Apothecia attached to sclerotia that have been dug out of the soil. 
(courtesy of Ieuan Evans, Alberta Agriculture Food and Rural Development)
Apothecia as the appear on the soil surface.
(courtesy of the Canola Council of Canada)
Apothecia that have formed in the laboratory.
(courtesy of the NDSU Department of Plant Pathology)

The Sclerotinia Risk Map for Canola: Need for Reports of Apothecia. We need reports of sightings of apothecia for the Risk Map. They provide ground truth for the model. County and area extension personnel, crop consultants and others working on canola are encouraged to report this information to me at the email address shown below. These reports are essential to what is called ground truth for the Risk Map model.

Art Lamey
NDSU Plant Pathologist Emeritus
alamey@worldnet.att.net


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