ISSUE 5   June 12, 2008

MANAGING APHANOMYCES ROOT ROT OF SUGARBEET

Sugarbeet plants in wet and warm soils may be affected by Aphanomyces root rot caused by the pathogen Aphanomyces cochlioides. Optimum conditions for infection occur in wet soils at temperatures of 68 to 86 F. Aphanomyces can be deadly in the seedling stage, and can also cause serious root rot later in the season. Infected seedlings typically have roots and hypocotyls that become black and shrink to a dark, slender thread. Infected plants have light green leaves with older leaves becoming yellow. Plants tend to wilt in the afternoons of hot and sunny days in dry conditions. Assessment of fields should therefore be done in the afternoons of hot sunny days. However, fields of surviving plants with severe root infections have reduced root yield, lower sucrose content, and higher impurities. Diseased roots have much higher respiration rates compared to healthy roots. As a result, the quality of storage piles can be reduced when diseased roots are stored with healthy roots.

Fields with a history of Aphanomyces should be planted with tolerant varieties approved for the grower’s factory district. Many high yielding Aphanomyces tolerant varieties are available. Seeds should be treated with Tachigaren® to provide additional protection, especially since sugarbeet growing areas in North Dakota and Minnesota are in a wet cycle, with conditions favorable for Aphanomyces occurring annually. Planting should be done as early as possible to facilitate early and vigorous growth in conditions unfavorable to the pathogen. Field drainage should be improved since the pathogen needs adequate free moisture to germinate and cause infection. Fields with a history of severe Aphanomyces should be treated with about 10 tons of factory spent lime per acre. Research shows that plots treated with spent lime results in a significant reduction in Aphanomyces disease severity and higher yields.

 

SUGARBEET – LOOKOUT FOR DISEASES

Rhizoctonia solani can infect sugarbeet at all growth stages and results in wilting and death of plants. The fungus causes infection when soil moisture range from somewhat dry to wet and soil temperatures above 68ºF. The fungus typically causes damping-off, and crown and root rot of sugarbeet. Damping-off occurs at the seedling stage when the fungus infects the hypocotyls resulting in rapid collapse of seedlings before soil emergence or post-emergence. Damping-off affects the plant population and ultimately reduces the yield. Crown and root rot infection occurs in older plants when the pathogen infects the petioles or the roots. Characteristic symptoms of Rhizoctonia include sudden wilting of leaves, and petioles of outer leaves are blackened at the point of attachment to the crown. The crown rot extends to the roots causes root rot. Crown and root rots, separately or collectively, are the most damaging phase of the disease and reduces yield significantly. The disease may also produce dry rot cankers on the root surface.

Fields with a known history of heavy Rhizoctonia should be planted to a tolerant variety. There are only a few good Rhizoctonia tolerant varieties available. Crop rotation with non-hosts such as wheat, early planting, avoidance of throwing soil at cultivation into crowns of plants will assist in managing the disease. The use of the fungicides Quadris® or Proline® at the 4 to 6 leaf stage or when the soil temperature at the four inch depth is about 65ºF will also help to control Rhizoctonia crown and root rot.

Mohamed Khan
Extension Sugarbeet Specialist
701-231-8596
Mohamed.khan@ndsu.edu

 

NEW AND REVISED PLANT PATHOLOGY EXTENSION PUBLICATIONS AVAILABLE

Ascochyta Blight of Chickpea. Ascochyta blight, caused by the fungal pathogen Ascochyta rabiei, is the most significant disease on chickpeas (also called garbonzo beans) in North Dakota. In 2005, the Ascochyta blight pathogen became resistant to the QoI class of fungicides (FRAC group 11, also called strobilurins), which include Headline® and Quadris®. Fungicide resistance to these products reduces management options for the disease in chickpeas.

A new extension publication discussing Ascochyta blight of chickpea is now available. Included are numerous pictures and discussions of biology and management techniques. The publication is available online at:

www.ag.ndsu.edu/pubs/plantsci/smgrains/pp1362.pdf,

and is currently being printed. Printed copies should be available at the time of Pulse Days this summer.

Blackleg of Canola. Blackleg is a serious disease of canola in North Dakota. Recently, new pathogenicity groups (PG’s) have been identified in the state, which may challenge some of the resistance currently used in hybrids in the future. A publication of the same name was originally authored by Dr. Art Lamey in the mid 1990’s. This new publication incorporates some of the original information from Dr. Lamey, and includes new information about PG groups, disease prevalence and management, and numerous new photos of symptoms. This publication is available at:

http://www.ag.ndsu.edu/pubs/plantsci/crops/pp1367.pdf,

and is currently in print. Printed copies should be available at Canola Days this summer.

Rust Diseases of Wheat. Three rust diseases have the ability to infect wheat in North Dakota. Leaf rust currently is the most important of the three, but stem rust has been problematic in the past, and stripe rust may appear in cool summers. This publication (PP-1361) is a revision of PP-589 Wheat Leaf Rust, most recently revised in 2002, but the three major wheat rusts are included in the new publication with symptoms, life cycle, and management discussed.

This publication is available at:

www.ag.ndsu.edu/pubs/plantsci/smgrains/pp1361.pdf.

It also is currently being printed and will be available through the NDSU County Extension offices and through the NDSU Ag. Communication’s Distribution Center.

Sam Markell
Extension Plant Pathologist
samuel.markell@ndsu.edu

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

 

RECENT RAINS BRING INCREASED DISEASE RISK

Recent rains have increased the potential for leaf and head diseases of small grains across many areas of the state. The NDSU small grains disease forecasting site indicates a more frequent number of favorable infection periods for tan spot of wheat in the past week at many NDAWN locations, and the Fusarium head blight (scab) risk has also increased. The Fusarium head blight risk should be of concern for producers with winter wheat, as many winter wheat fields are now in the heading stage.

As mentioned in previous NDSU Crop and Pest Reports, the NDSU disease forecasting site provides information on risk of tan spot, Septoria leaf blotch, leaf rust, and Fusarium head blight (scab), and is available at: http://www.ndsu.edu/scabforecast. The leaf disease predictions are obtained with choosing leaf stages of the crop, while the forecast for scab risk is only found by clicking on the flowering stage.

As mentioned in the May 21st NDSU Crop and Pest Report, another scab forecast site, giving information across many states, is found at: http://www.wheatscab.psu.edu/. At this web site, provided through several universities and the US Wheat and Barley Scab Initiative, a person clicks on the spring or winter wheat model, then the state, and the scab risk is provided in a colored map.

The mathematical models used for the NDSU web forecast site and the one provided through Penn State Univ. are the same, but the information sources for the weather that goes into the model are different. The forecast (for most states) provided through the Penn State web site is based on RTMA, a mathematical model that estimates weather from a number of computerized sources. In ND, the model predictions are based on the NDAWN weather station data PLUS the RTMA data.

I find both scab forecasting sites very useful. The NDSU site provides both leaf disease risk and scab risk data plus it is exclusively based on current local weather. The Penn State site is useful because it provides a broad look at multiple states and allows some 24-48 weather forecasting predictions, as well. Both sites allow the user to choose the scab susceptibility of the cultivar for spring wheat.

 

EARLY SEASON FUNGICIDES FOR WHEAT AND BARLEY

Many producers have recently applied fungicides for early season disease control, often in a tank mix with herbicides. In some cases, some injury has occurred with these applications on the very succulent, thin leafed cuticle, wheat and barley. In previous years when this type of symptom was observed, the new growth generally looked good and we did not see yield reductions, but did see yield increases with these treatments. In 2003 at NDSU, we did a number of fungicide + herbicide combination treatments and did not have any injury, a year in which we had a little warmer, sunnier June than we’ve had so far this year.

 

FUNGICIDE UPDATE

The May 15 issue of the 2008 NDSU Crop and Pest Report contained information on fungicides recently registered for wheat and barley producers. Growers contemplating using fungicides for scab control in the near future should be considering using either Folicur®, Orius®, Tilt®, Proline 3+3®, or Caramba®, as these products are triazoles with the best and safest activity against scab. In NDSU field tests, the Proline 3+3® or the Caramba® have consistently given the greatest scab and DON (vomitoxin) reductions.

Bayer CropScience just announced the registration of Prosaro® fungicide, a pre-mix of prothioconazole and tebuconazole (the same active ingredients of Proline® and Folicur®). Although Prosaro® just got registered in wheat and barley, Bayer will continue to market the Proline 3+3 (prothiconazole + tebuconazole, not pre-mixed) program for the 2008 growing season, and Bayer says they will transition to Prosaro® in 2009.

Prosaro® is labeled for wheat and barley for multiple leaf and stem diseases, as well as suppression of Fusarium head blight (scab). The use rate will be from 6.5 to 8.2 fl oz/acre, with 2 applications per season possible, but not to exceed 8.2 fl oz.

 

DISEASE AND INSECT SURVEY-June 6

NDSU IPM Field Scouts scouted some wheat and barley fields the past week, but need a return to sunnier, drier conditions to increase number and area of fields scouted. Of wheat and barley fields surveyed last week, winter wheat was showing some tan spot, up to the leaf below the flag leaf, and some barley fields in the northwest area had a low level of fungal leaf spots. Additional reports of abundant tan spot infections in spring wheat planted into wheat residue have been received. Wheat leaf rust has not yet been detected by the survey scouts.

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


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