ISSUE 15   September 18, 2008

SUNFLOWER RUST

This year sunflower rust was a big problem in localized parts of the state. Although not widespread, the disease appeared very early in some locations, and although environmental conditions were not perfect for disease development, they were conducive enough to cause heavy infection. It is likely that rust will overwinter in North Dakota, and if it appears early in the season next year, we will have to deal with this problem again.

Selection of a resistant hybrid may be important for those who had heavy rust infection this year, particularly if next yearís sunflowers will be next to a heavily rust field this year. However, even if a susceptible hybrid is selected, the good news is that rust can be controlled by fungicides. Fungicide trials are currently being conducted in collaboration with the Carrington Research Extension Center, the Langdon Research Extension Center, CHS seeds (trial near Casselton) and Vision Research Park (trial near Mohall). Although we have yet to harvest plots and analyze data, we know that we have low disease pressure in Casselton, low-moderate disease pressure in Langdon, moderate-high disease pressure in Carrington, and high disease pressure in Mohall.

Although we have not yet finished all evaluations and harvest is several weeks away, we are seeing good control with fungicide applications. At three locations, a single fungicide application seems to have limited rust development. At the Mohall location, where infection started very early, one application is inadequate for disease control.

Below is some preliminary data from the sunflower rust trial at the Carrington Research Extension Center. Borders around the trial were infected with rust, and the trial is irrigated. Both measures were taken to try and create a high disease environment. The colored bars represent disease progression over time when four different fungicides were used. Rust was first observed on August 7th, and the fungicides were applied on August 13th, at approximately R5.2-R5.5. The graph below shows disease severity (percent) of the upper four leaves (as a reference, data from overseas suggested a fungicide application needed to be made before 3% severity for yield loss to occur). The preliminary data indicated that the fungicide application reduced rust severity on the upper four leaves. Differences among fungicides appear in the graph, but data from other locations and statistical analysis must be done before any conclusions can be drawn.

I feel comfortable saying that rust is a manageable problem if fungicides are on the table as control options. Selection of a resistant hybrid, crop rotation, and destruction of wild sunflowers will help manage the disease as well.

Sunflower rust graph

 

SOYBEAN DISEASE SURVEY

North Dakota soybean growers have not had to think much about soybean diseases that occur above the ground. Sure we get a little white mold, we know we get some bacterial spots, maybe a little septoria brown spot, but much of the disease we think about is root rots. However, the disease pressure on soybeans will increase as soybeans continue to be grown in ND. Itís not unreasonable to think that we may have disease pressure similar to southern Minnesota or Iowa in the future.

To try and get a handle on what Ďemergingí diseases we have in the state, the North Dakota Soybean Council supported a fairly exhaustive survey in the 12 southeastern counties in North Dakota this summer. We have scouted over 120 fields, focusing on the diseases present on the leaves, stems and pods. Although we are still in the process of culturing and identifying pathogens, entering data, and assessing our disease spectra, it is fair to say we have found high levels of some diseases, and low levels of quite a few other diseases. Without going into numbers or geography, here are some thoughts about what we have found so far.

Virtually every soybean field surveyed had septoria brown spot and bacterial brown spot. In some cases, the amount of disease was quite high, while in others it was fairly limited. Yield loss from these two diseases is somewhat unknown. It is commonly thought that yield loss is minimal.

Also found were downy mildew, alternaria leaf spot, anthracnose, powdery mildew, white mold, charcoal rot, and several other pathogens that need to be confirmed. Some of these diseases, like white mold, were expected, others, although known to occur rarely, were found in a surprising number of fields.

The information gained from this survey will help us assess the disease spectra in North Dakota as it changes. Ultimately, this will lead to management recommendations and research projects that will help growers address diseases issues as they emerge, and help soybeans remain a profitable crop in the state.

 

SOYBEAN RUST UPDATE

No news is good news when it comes to soybean rust, and as you are all aware, there is virtually no news.

Since soybean rust was introduced into the US several years ago, it has spread north during each growing season. Last year rust spread as far north as Iowa. However, the pathogen canít survive freezing temperatures, so soybean rust is killed each fall. The only place the pathogen has been able to overwinter is along the gulf coast states. For soybean rust to cause problems in other states, it would have to spread north from its overwintering locations. Judging from winds currents and the historical spread of other diseases (wheat rusts in particular), spores from Texas or Louisiana would be more likely to reach our area than those from states east of the Mississippi River.

To keep track of soybean rust, the spread is monitored by plant pathologists across the country in Ďsentinel plotsí, which is an area in a soybean field that is scouted once a week for rust, aphids, and other pests. This information is used to generate maps of the disease spread (Figure 1). Additionally, information about other soybean diseases found in North Dakotaís sentinel plots are included in the state commentary section.

soybean rust map

To date, soybean rust has not progressed out of Texas and Louisiana, and spread very little within either state. At this time last year soybean rust had been found much further north than it currently is. Because soybean rust can move so quickly, itís too early to say the disease wonít move into other states. However, most soybeans are at or past R6, so soybean rust is no longer an economic concern for 2008.

For more information about the current spread of soybean rust go to www.sbrusa.net.

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

 

SOIL TESTING FOR SOYBEAN CYST NEMATODE

If you suspect you have soybean cyst nematode (SCN) or if your neighbors have it, you should test your soil for the presence of SCN eggs. Take soil cores (about 8 inch deep) in a zig-zag pattern over the field (about 20 cores per 10 acres) and bulk the samples, mix it thoroughly, and send a pint of the mixed soil to a laboratory that tests for SCN. Thorough mixing of the soil is imperative. Often SCN is found in patches in fields, thus sampling sections of fields and testing the soil from each section is a better way to find out where SCN is most common. It can be difficult to notice SCN damage in fields when populations are low and growing conditions are good, even though there can be yield losses. Soil sampling is the best method to detect the nematode. Early detection allows you to make management decisions before you experience a large yield loss in your soybeans. Resistant varieties and crop rotation to reduce egg levels are the only two useful controls. When SCN populations are high, it is difficult to lower them with crop rotation. The rule in managing SCN is "keep the egg populations low". You can not eliminate SCN, but you can manage it. For more information on SCN consult the following web sites: www.ndsu.nodak.edu/soydiseases/cyst.shtml  and www.plantpath.iastate.edu/dept/labs/tylka/node/125.

Berlin Nelson
NDSU Soybean Pathologist
Berlin.nelson@ndsu.edu

 

WHATíS WRONG WITH MY SOYBEAN LEAVES?

Recently, we have received quite a number of calls and samples of soybeans with bronzing/purpling leaves. Severity ranges from localized spots in the field on just the upper leaves to death of the top four or five leaves (resembling a frost). The symptoms started appearing a couple weeks ago, and have been seen in just about every region in the state. This has puzzled us, and unfortunately we donít have a definitive answer as to what we are seeing, so the best we can offer at this point is a discussion.

leaf bronzing
Bronzing on the leaf surface.
leaf bronzing
Bronzing on the underside of the leaves.

One of the first things many people suspected was disease. We have received numerous samples from different regions of the state, but have not been able to isolate any pathogen. The symptoms donít resemble something that a root rot or stem disease would cause. There is at least one foliar disease that has symptoms similar to what we are seeing, but we have not been able to isolate any pathogen from infected tissue, so we rule that out. We visited with Dr. Berlin Neslon about the symptoms, and he has observed these symptoms in the past (usually late in the season like we are now), but he has never been able to attribute it to any known disease either. So a disease seems unlikely. Make no mistake; we have seen different bacterial and fungal diseases on our leaves (see Sam Markellís article in the plant pathology section), but they are not the culprit.

Sometimes the leaves can show injury of applied herbicides or insecticides. The growth stage, weather conditions, soybean variety, tank mixed chemical components like adjuvants may affect the amount and severity of crop injury. However the symptoms did not show up till later in the season and therefore we assume the injury was not related to herbicides.

As the bronzing is observed on different varieties and over a large geographical area the thought is that the bronzing might have be induced by this yearís growing conditions. The early season was cool and dry. Mid season we had rain and after bloom the plants were under drought stress before receiving rain in August. This year we also have seen soybean aphids in many fields.

Sometimes we do see sunburn of the soybean leaves, a condition that may occur in late summer and causes soybean leaves, especially the undersides, to turn a bronze color. Bronzing due to sunburn usually does not progress over time.

The season is nearly completed and it is not expected that the late season discoloration of the leaves will influence the yield at this time of the year.

Sam Markell, Extension Pathologist
Hans Kandel, Extension Agronomist
Kasia Kinzer, Plant Diagnostician

 

SURVEY OF ROOT ROT PATHOGENS IN PEAS

Root rots have become a problem for pea growers in recent years. Damage from root rots can be caused by numerous pathogens. Consequently, the first step to management of root rots is to understand which pathogens are causing disease. To understand this, a survey of pea fields was supported by the Northern Pulse Growers Association this year. Although all data is not yet analyzed, Fusarium species tended to be recovered most frequently for pea fields. Rhizoctonia and Ascochyta (from the base of the stem) were also recovered. Pythium species were not recovered in this survey, but this is likely due to the extremely dry spring in most of the pea growing areas in the state. Hopefully this information will lead to greater management of root rots in North Dakota.

Rubella Goswami
Pulse Pathologist
Rubella.goswami@ndsu.edu

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

 

WHEAT STREAK MOSAIC MANAGEMENT

Producers wishing to plant winter wheat this fall are concerned about wheat streak mosaic virus (WSMV) infection in that crop. Two key practices reduce the risk of WSMV: control of grassy weeds and volunteers two weeks prior to planting of the new winter wheat crop; and appropriate planting date. Each of these practices is aimed at preventing wheat curl mites from moving from an infected crop or grassy weed into the newly emerged winter wheat crop. If the green bridge allowing mite survival can be broken, the mite is no longer alive and available to transmit the virus into the wheat crop. The following figure illustrates the wheat curl mite cycle and locations where the green bridge can be broken.

WSMV life cycle
WSMV life cycle

Two questions I often get about recommended WSMV management practices:

1) Why do we recommend 14 days between the herbicide control of grassy weeds/volunteers and the planting of a new winter wheat crop?

The wheat curl mite completes its life cycle, from emerging from the egg to development of new eggs, in 10 days. If the grassy weeds are sprayed with herbicide on day 1, and mites are present, these same mites may not lay eggs until several days later and these eggs wonít hatch for another 10 days. (Herbicides donít have any effect on the mites). If a newly emerged winter wheat field was present or just across the road, these newly hatched mites would have a green crop to live on. But if the field and nearby fields donít have a green host present, the newly hatched mites will die. The mites are wingless and donít actively seek new crops; they are carried very short distances by wind currents. So, the extra 14 days recommended for absence of a susceptible crop is to assure that no green plants are around for the newly hatched mites.

volunteer wheat
Volunteer wheat (background) infected with
WSMV adjacent to no-till wheat field (foreground)
prepared for planting. This volunteer should be destroyed.

2) Why is a later planting date recommended, instead of late August or early September?

Wheat curl mites reproduce more rapidly and produce more eggs with warmer temperatures. When temperatures drop, generally in mid-September, mite reproduction slows and there are less mites available for transmission of the virus and a smaller mite population develops in the crop before freeze-up. Data from 11 years of studies in northwestern South Dakota indicated that the optimum time for planting in that location was between September 14 and September 24th, resulting in the highest yields and the lowest infection levels of WSMV. In this study, a Sept. 24th planting date versus a late August planting date resulted in a 50% reduction in WSMV infection and approximately a doubled yield return.

 

UNIFORM FUNGICIDE TRIAL RESULTS ON SPRING WHEAT AND BARLEY, FARGO, 2008

Uniform fungicide trials, sponsored by the US Wheat and Barley Scab Initiative, were conducted at several sites in ND, to evaluate fungicides for Fusarium head blight and leaf disease suppression. The following data are from trials done at Fargo. Some differences among treatments were observed in Fargo although disease pressure was very low because of warm, dry conditions and drying winds in July. The test varieties at Fargo were Alsen spring wheat and Tradition spring barley. DON levels were very low and non-significant.

Alsen Spring Wheat

Treatment
(at flowering)

Rate
fl oz /A

FHB Index %

Leaf rust
%

Leaf spot %

Yield
bu/A

Twt
lbs/bu

Untreated

 

3.3 a

4.2 a

16.7 a

44.8 b

61.7 ab

Folicur

4

0.9 cd

0 b

3.8 c

47.4 ab

61.6 b

Proline

5

0.3 d

0.8 b

3.3 c

50.6 a

62.2 a

Prosaro

6.5

0.4 d

0.1 b

2.4 c

51.5 a

62.0 ab

Caramba

10

1.3 bc

0.2 b

3.3 c

50.9 a

61.8 ab

Caramba

14

0.9 cd

0.1 b

2.6 c

49.6 a

61.8 ab

Topguard*

14

1.5 b

0.1 b

6.9 b

48.9 ab

61.8 ab

Numbers with different letters are significantly different at 95% level
*Topguard = flutriafol; not registered; all other treatments registered

Tradition Spring Barley

Treatment
(at flowering)

Rate
fl oz /A

FHB
Incid. %

FHB Index %

Leaf spot %

Yield
bu/A

Twt
lbs/bu

Untreated

 

91.3 a

1.8 a

9.9 a

75.5 b

48.0 a

Folicur

4

52.5 cd

0.7 c

2.1 b

86.9 a

48.4 a

Proline

5

45.0 d

0.5 c

1.7 b

92.1 a

48.1 a

Prosaro

6.5

48.8 cd

0.6 c

1.2 b

90..0 a

48.2 a

Caramba

10

63.8 bc

0.8 bc

1.0 b

91.1 a

48.2 a

Caramba

14

46.3 d

0.61c

0.3 b

91.3 a

48.2 a

Topguard*

14

71.3 b

1.0 b

2.5 b

86.7 a

48.3 a

Numbers with different letters are significantly different at 95% level
*Topguard = flutriafol; not registered; all other treatments registered

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


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