ISSUE 6 June 17, 2010
HEAD SCAB RISK
The ND Small Grain Disease Forecasting website (www.ag.ndsu.nodak.edu/cropdisease) and the US Wheat and Barley Scab Initiative Risk maps (www.wheatscab.psu.edu) indicate a high risk of Fusarium head blight (scab) infections currently (June 15) and for the next few days on moderately susceptible to susceptible flowering wheat crops. Some moderate risk exists on June 15 even for moderately resistant cultivars that might be flowering. The following pictures illustrate what these risk maps looked like on June 15.
ND Small Grain Disease Forecasting Map for Scab
Risk for Moderately Susceptible Varieties, June 15, 2010.
ND maps based on NDAWN weather station data.
ND Small Grain Disease Forecasting Map for
Scab Risk for Susceptible Varieties, June 15, 2010.
US Wheat and Barley Initiative Fusarium Head
Blight Risk Assessment Map for Susceptible
Varieties, June 15, 2010. US Scab Initiative Maps
based on NDAWN station weather plus weather
forecasts from the National Weather Service.
These maps indicate that on June 15th and for the next several days, risk of scab infection for flowering wheat crops (and headed barley crops) was high on susceptible varieties. These forecast maps change daily, according to relative humidity durations across the state, so they should be checked periodically, especially as more spring wheat nears the flowering period. If we are fortunate, sunny and more windy days will return and reduce the head scab risk.
FUNGICIDES FOR FUSARIUM HEAD BLIGHT (SCAB) MANAGEMENT
Extensive university studies across the US have compared Caramba (metconazole) and Prosaro (prothioconazole + tebuconazole) fungicides with Folicur (tebuconazole) for Fusarium head blight reduction in wheat. Results across 12 states showed that Caramba (13.5 fl oz/acre rate) and Prosaro (6.5 fl oz/acre rate) each provided about a 20% better reduction of FHB and a 30% better reduction of vomitoxin (DON) than Folicur (4 fl oz/acre rate), when tested primarily on moderately susceptible to susceptible cultivars of spring, durum, or winter wheat.
How does this compare to results in ND? These products were compared from 2005-2009 in uniform trials at Fargo, Carrington and Langdon on spring wheat, durum and barley, and across various levels of variety susceptibility.
Comparisons from 2005-2008 on spring wheat and durum showed that Caramba or Prosaro (at rates equal to those in national trials) provided an average of 23-36% better reduction of scab severity, a 28 to 58% better reduction of DON (vomitoxin) levels, and a 32 to 44% increase in yield, over the Folicur applications. The yield improvement translates to about a 3 to 5 bu/acre better yield than achieved with Folicur, for a 50 bu /acre yield crop.
It is apparent to me that when head scab is present, the two better products will give enough disease reduction and yield improvements to be very economical, even if priced higher than tebuconazole. All three fungicides also give good control of other diseases, such as the rusts and leaf spots.
In the absence of scab or other disease pressure, the yield differences among these products is diminished. In 2009, scab levels and other disease pressures were very low. Although reductions in disease levels, percentage wise, were similar to previous years, disease ratings were relatively low, even in the untreated check. Yield improvements by Prosaro and Caramba vs Folicur were smaller, averaging from 7-10%. However, at this time in the 2010 growing season, we appear to have a lot more disease pressure (tan spot) and disease risk (scab) than occurred in 2009.
In barley tests from 2005-2008 at Fargo and Langdon, the Prosaro and Caramba products had even better rates of reduction of disease levels and better percentages of yield improvements over Folicur than they did in wheat, primarily because our current barley varieties have high susceptibility to scab.
Tan spot continues to be the most common disease found in wheat by the NDSU IPM field scouts. Of the 101 wheat fields surveyed in the past week, 90% showed some level of tan spot, with severity on the top leaves ranging from 1 to 33%. The disease is more common and severe where wheat stubble is present in the field. Maps of tan spot occurrence, as reported by the NDSU IPM field scouts, may be found under the wheat link at the following ND IPM website:
Since the last pest report, when we reported that no detections of any rust in wheat had been made, stripe rust was detected at low levels - in Adams county in winter wheat by the NDSU IPM scout for that area, Dixie Dennis. Since that detection, some stripe rust has been reported in winter wheat variety trials at Hettinger, a producer north of Dickinson reported some stripe rust occurrence in RB07 spring wheat, and a consultant reported a very low level of stripe rust in winter wheat plots near Berthold.
On June 15, John Lukach of the Langdon REC, reported to me that he has observed stripe rust in winter wheat variety plots on the station. He observed 1-2% severity in Jagalene on the top leaves in all reps, a trace amount on all reps in Boomer, CA9 W07 817, and Hawken, and trace amounts in one or two reps of some other varieties. On June 16th, I found extensive stripe rust on jagalene at Forman, ND, and trace amounts on Hawken winter wheat. (See Crop and Pest Report May 27th issue for picture of stripe rust).
So, the disease is here, but still at only trace levels. It still remains undetected in other parts of the state.
Because this disease is so explosive under favorable environmental conditions, scouts, producers and consultants must keep an eye on the stripe rust disease to see if it develops and spreads in these areas. The fungus prefers night time temps below 60 degrees F, so maybe the coming week’s temperatures will stop development.
At Forman we also detected common wheat rust on jagalene on June 16th. If stripe rust or leaf rust do appear to be present at levels of concern, the triazole fungicides that will be used for head scab control also do an excellent job of controlling rusts, when the rust severity is still low on the flag leaf (1-3% severity).
Extension Plant Pathologist
EARLY SPORE STAGES OF SUNFLOWER RUST BEING OBSERVED
The early spore stages of sunflower rust have been observed by multiple people and in multiple locations throughout the state. The disease has been spotted on volunteers sunflowers near Jamestown, Bismarck, north of Minot, and in Manitoba. It is reasonable to believe that rust will be spotted in many other locations in the coming days or weeks. However, rust has not yet been seen in commercial fields in North Dakota.
The earliest spore stages of rust are the pycnial and aecial stage. The pycnial stage occurs first, usually observed as a relatively non-structural yellow-orange spot on the upper side of a leaf or cotyledon (Figure 1). The aecia will occur immediately opposite the pycnia on the underside of the leaf or cotyledon. Aecia are easier to see and will appear as a cluster of orange cups (Figure 2). When the spores produced in the aecia infect sunflowers they will cause the cinnamon-brown rust pustules we are used to seeing, the uredinia (Figure 3).
Figure 1. Sunflower rust Pycnia (Photo courtesy of Bob Harveson)
Figure 2. Sunflower rust Aecia.
Figure 3. Sunflower rust Uredinia.
Rust is favored by moderate temperatures (55 – 85 F) and dew. Weather conditions will have a great influence on how severe and widespread rust is this year. However, the occurrence of the early spore stages suggests ample inoculum to cause disease will be present. Sunflower rust will take time to develop, but it is advised that growers monitor their fields for rust in the coming weeks. Scouting areas of fields first that have longer dew periods (such as by tree rows), are in close proximity to last year’s flowers, or are by wild sunflower species will give you a good idea if rust is present or not.
Sunflower rust can cause yield reduction in high pressure. Sunflower rust can be managed with fungicides, although it is unlikely that an application in vegetative stages would be necessary. For now, the biggest thing to do is to be aware of the occurrence, and look at your fields in the coming weeks. More information will follow in a future crop and pest report when rust is occurring in commercial fields.
Extension Plant Pathologist
BACTERIAL BLIGHT SHOWING UP ON PEAS
An abundance of rain, mixed with cool temperatures and moderate to high winds, have been very favorable for the development of bacterial blight throughout the pea growing regions in North Dakota.
Bacterial blight is unlikely to cause yield loss. As soon as temperatures warm up and/or the rain subsides bacterial blight development will stop. Lesions will dry and crack and the disease will effectively be stopped.
The biggest potential issue we see with bacterial blight is confusing the symptoms with Mycosphaerella and applying an unnecessary application of a fungicide. Bacterial blight symptoms begin as ‘water soaked’ lesions and/or translucent spots on leaves, often at the leaf tips or leaf edges (Figure 4). Lesions will be nearly clear when holding them to the sky. The margins of these lesions will darken, and the centers quickly turn tan, dry, and the centers crack. Mycosphaerella lesions begin as dark purple specks on the leaves. These lesions enlarge into small irregular dark purple spots (Figure 5). As they mature small black structures (pycnidia) may be produced in the center, but may not be visible without a hand lens. The best way to differentiate the diseases is to look for the early symptoms of bacterial blight, the translucent lesions.
Fungicide applications can manage fungal diseases, but will have little/no effect on bacteria. Further, fungicide applications have been most effective at managing Mycosphaerella/Ascochyta when the peas are in the early bloom stage.
Figure 4. Bacterial blight lesions.
Figure 5. Mycosphaerella lesions.
Extension Plant Pathologist
IPM specialist - North Central Research Extension Center