ISSUE 9   July 8, 2010

NDSU IPM SURVEY RESULTS, JULY 2

Wheat: NDSU IPM scouts looked at 91 wheat fields the past week, with the average growth stage of these crops at head emergence. As in previous reports, tan spot infection is by far the most common disease observed, with an average severity on the flag leaf about 3%, but higher levels in the canopy. Three commercial wheat fields were observed with stripe rust at very low levels. Only one field was observed with leaf rust. Four of the surveyed fields had some symptoms of Fusarium head blight (scab), but the severity in these fields averaged less than 0.3%.

Barley: Seven barley fields were surveyed and the average growth stage was fully headed and into kernel watery ripe. One field was observed with a very low level of scab infection. Fungal leaf spots of net blotch, spot blotch and/or Septoria were commonly observed in barley, with average severities on the flag leaf of 1-2%. Powdery mildew has been observed in some barley fields this year, with infections mostly still in the canopy, but a few spots observed occasionally on the flag leaf. The symptoms are a white to greyish fuzzy growth on the leaf surface (see picture).


Powdery mildew on barley leaf surface.
Note cottony, raised growth on leaf surface.
Black specks in one lesion are the spore
bearing structures of the powdery mildew fungus.

 

FUSARIUM HEAD BLIGHT (SCAB) RISK

The Fusarium head blight risk maps indicate a only a few areas (in northern tier of ND counties) of moderate risk of infection for moderately susceptible varieties (on July 7).  Scattered infected heads have been seen in winter wheat and a few spring wheat fields in the southeast part of ND in fields that flowered about 2-3 weeks ago (see scouting reports above).

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

 

START PLANNING FOR CERCOSPORA LEAF SPOT IN SUGARBEET

Cercospora leaf spot is the most devastating foliar disease of sugarbeet in Minnesota and North Dakota. The disease is caused by the fungal pathogen Cercospora beticola. The source of the fungus is infected sugarbeet debris in the field. The fungus is disseminated mainly by wind. Cercospora leaf spot develops rapidly in warm, humid and wet conditions. Day temperatures of 80-90 F and night temperatures above 60 F favor disease development. Please note that day temperature above 93 F is unfavorable for disease development. Symptoms may occur about 5-7 days after infection under favorable conditions. Typical foliar symptoms are circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders. Under favorable conditions, the fungus may have several disease cycles during the season, and with each cycle there is a substantial increase in the amount of inoculum. As such, early control is necessary to effectively manage the fungus. Since the fungus damages the leaves, it reduces the photosynthetic capacity of the plants. As a result, Cercospora leaf spot results in lower tonnage, lower sucrose concentration and reduced extractable sucrose compared to healthy plants.

Research done at NDSU and the University of Minnesota shows that application of effective fungicides at first symptoms with subsequent applications based on the presence of leaf spots and favorable environmental conditions (Daily Infection Values for two consecutive days of 7 or higher – information available at http://ndawn.ndsu.nodak.edu) consistently provided the most effective and economical control.

Sugarbeet fields with more susceptible varieties (higher KWS ratings) with closed rows that are close to shelterbelts, waterways, and those close to previously infected fields should be the first to be scouted since they would be the first to become infected.

The following guidelines will help in effective disease control:

  1. The first fungicide application should be made when conditions first favor disease development or at first symptoms. If the first application is late, control will be difficult all season.
  2. Use the recommended rates of fungicides to control Cercospora leaf spot - do not cut rates.
  3. Only one application of a Topsin M in combination with a protectant fungicide (triphenyltin hydroxide) should be used in Hillsboro, East Grand Forks, Crookston, and Drayton factory districts.
  4. Never use the same fungicide or fungicides from the same class ‘back-to-back’.
  5. Avoid using fungicides of a particular class of chemistry as a stand-alone where there is known resistance or tolerance to that chemistry.
  6. Use of high spray pressure (100 psi) and high water volume (15 to 20 gal/ac) will result in better disease control.

The following fungicides are labeled and the most widely used on sugarbeet for Cercospora leaf spot control with re-entry interval (REI) and post harvest interval (PHI).

Product

REI (hr)

PHI (day)

SuperTin 80WP and 4L

48

7

AgriTin 80WP and 4L

48

7

Headline

4

7

Gem

12

21

Inspire XT

12

21

Proline

48

7

Eminent

48

14

Topsin WSB and 4.5F

12

21

Please note that manufacturers of glyphosate do not recommend mixing glyphosate with fungicides – growers make these applications at their own risk. However, research done at NDSU indicated that the fungicides listed above mixed with glyphosate and AMS at labeled rates did not result in any phytotoxicity, and weed and leaf spot control were effective.

Most sugarbeet fields have closed rows and have the potential for record yields. Although the price for sugar has dropped from the record high earlier in the year, sugar price is still relatively high and favorable for growers. As such, growers are encouraged to have their fields scouted for symptoms of Cercospora leaf spot. Growers have done an excellent job of controlling Cercospora leaf spot over the past several years. As such, Cercospora inoculum in growers’ fields is very low. However, current weather conditions are becoming very favorable for disease development and even low C. beticola populations can increase rapidly. As such, scout fields and be prepared to start fungicide applications earlier than previous years, and budget for multiple applications, should this become necessary.

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

 

REVIEW OF CONDITIONS FOR WHITE MOLD

If you are thinking about a fungicide application for white mold, please consider these factors. Conditions favoring white mold include 1-2 inches of rain (or more) within a 1-2 week period before the onset of flowering and temperatures in the 60’s and 70’s (above 85 sclerotinia is really uncomfortable) during bloom. Dense and humid canopies will also favor disease development. Some crops are more susceptible to white mold than others; dry beans are more susceptible than soybeans, for example. Broadleaf crops are susceptible to white mold during the bloom stages. If you determine that a fungicide application is necessary, it is imperative to apply it during early bloom.

 

SOYBEAN RUST AND THE EARLY SEASON HURRICANE

I have received a couple questions from some very astute observers concerning soybean rust. The most likely scenario that would result in North Dakota getting soybean rust is an early season hurricane that landfalls into Texas. Hurricane Alex made landfall as a category 2 hurricane just south of the Texas border in Mexico last week. However, the storm progressed into Mexico instead of taking a northern track up the center of the Great Plains. Although it may help soybean rust develop in Mexico, it will have little bearing on what happens in North Dakota.

 

ASCOCHYTA ON PULSE CROPS

The three pulse crops (peas, lentils and chickpeas) are all susceptible to ascochyta. Although each pathogen is affected by a different species of ascochyta, conditions favoring disease are similar, approximately 60-75 F, and lots of rain events. In field peas, initial symptoms are irregular dark purple specs, located first on the lower leaves. Lentil and chickpea lesions tend to appear as gray to tan lesions, often surrounded with a darker margin, with small black spots (pycnidia) in the center.

On each crop, yield loss is possible. Chickpeas are the most sensitive to the disease, and chickpea growers are well aware of fungicide programs needed to yield a good crop. Lentils are susceptible to the disease, and yield loss under high disease pressure is well documented. In trials conducted at the North Central Research Extension Center during the last decade, a significant yield increase was observed in over half the trials. Yield loss has also been documented in field peas, but yield increase in fungicide trials has not been documented in North Dakota as frequently as in the other pulse crops. If considering a fungicide application, several factors should be considered. First, the more ascochyta you have at early bloom on the lower canopy, the greater the risk. If you see ascochyta in the middle canopy at early bloom, I would call that high risk. Secondly, the more rain events you have in the previous two weeks, the more your risk. One or two might not matter much, five or six increase your risk greatly. Third, how dense is your crop? Dense crops are more at risk. Lastly, look at the forecast, are temperatures in the favorable range and is rain forecast?

Fungicides are available for all the pulse crops, and can be found in the 2010 Field Crop Fungicide Guide.
http://www.ag.ndsu.edu/extplantpath/publications-newsletters/fungicides

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

 

POTATO LATE BLIGHT UPDATE – JULY 7, 2010

Despite the hot, dry weather the last couple of days, late blight has been found in a potato field in northern Walsh County. We now have confirmed cases in southeastern and northeastern North Dakota as well as western and central Manitoba. No late blight has been reported in Minnesota. The symptoms from southeastern North Dakota were observed on the leaflets, similar to those in Figure 1, while stem and petiole lesions were predominant in northeastern North Dakota (Figure 2).


Figure 1. Late blight lesions on leaflets.
 

Figure 2. Late blight lesions on petioles.
 

Figure 3. Cull pile next to shelter belt with
potatoes in bloom.

Figure 4. Volunteer potato in wheat field.

Late blight is a community disease that has the potential to rapidly get out of control if appropriate actions are not taken and information is not shared. It does not discriminate between process, chip, fresh, or seed growers. If a suspicious sample is found, it should be placed in a plastic bag, kept cool, and brought to the Plant Pathology Department at NDSU for immediate confirmation.

Furthermore, as I was driving around last week, I observed cull piles full of flowering potato plants (Figure 3) and volunteer potatoes in wheat fields (Figure 4). These must be destroyed as they are a potential source of inoculum.

As always, fungicide recommendations and current late blight severity values can be found on the Late Blight Hotline at http://www.ndsu.edu/potato_pathology/.

Dr. Nick David
Extension Potato Agronomist NDSU/UM
nicholas.david@ndsu.edu


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