ISSUE 13 August 7, 2008
NDSU IPM FIELD SCOUTS RESULTS
NDSU field scouts surveyed 81 wheat fields and 9 barley fields into the first week of August. Barley fields were located in the northeast, north central and northwest districts and were in the soft dough stage. Wheat fields were surveyed across the state, although the bulk of the remaining wheat or durum fields surveyed also were located in the northeast, north central and northwest districts of the state.
Because of the advanced maturity of fields surveyed, leaf diseases were more difficult to distinguish. The majority of fields (69.1%) had tan spot symptoms on the flag leaf , but only 3 fields were reported to have leaf rust on the flag leaf.
The frequency of detection of Fusarium head blight (scab) remained low in barley fields surveyed (only one of 9 fields) but increased in wheat/durum during the past week, with 39.5% of surveyed wheat fields showing some symptoms. Often a single floret or single spikelet of infection was observed (see figure).
Single spikelet infection of Fusarium head blight in wheat
The average field severity of scab across the symptomatic wheat fields was very low, less than 1%, but a few fields in the northeast district had field severities averaging 3.3%. Wheat fields in the northeast region may show additional symptoms of scab this week, because of recent rains that have gone through that region.
Fusarium head blight (scab) field severity is determined by multiplying the percent of tillers showing symptoms times the average severity on the grain heads. For example: 10% of tillers x 21% of head area with symptoms (on average in infected heads) = 2.1% field severity. Often, field severity is overestimated by quick glances at the field, because symptomatic heads are striking in appearance and stand out in an otherwise green field.
Also, fields that were treated with fungicides at flowering may show scab symptoms now. Fungicides, if applied for scab control at flowering, were most likely applied three weeks ago and their suppressive activity in the plant has dissipated by now. Unfortunately, the Fusarium fungus that causes scab is capable of colonizing wheat and barley heads until maturity. At maturity, moisture percentages in the grain drops to levels below which the fungus can continue growing. Fungicides in these situations still have reduced scab field severity generally by 50 to 75%, but disease ratings or harvest differences are needed to show these effects. What we need now are dry, sunny days with low dew points, weather that will aid in crop maturation and retard further development of this fungus. Producers should do timely harvest once the crops have matured.
NDSU Extension Plant Pathologist
FOLICUR LABELED FOR MANAGEMENT OF SUNFLOWER RUST
Bayer CropScience has been granted a Section 3 – Full federal registration for the use of Folicur for the management of sunflower rust. This supersedes the ND Section 18 Crisis Exemption Issued on July 24th 2008. To use Folicur on sunflowers you must have the supplemental label in your procession, which is available at http://www.sunflowernsa.com/uploads/Folicur_Fungicide_36F_7-31-08.pdf.
The active ingredient in Folicur is tebuconazole, a demethylation inhibitor (DMI), FRAC group 3 fungicide. The label states that Folicur can be applied at a rate of 4 – 6 oz. per acre with no more than 16 fl. oz. applied per season. An application cannot be made within 50 days of harvest. For more information, the supplemental label is available at the Bayer CropScience website.
Sunflower rust can cause economic losses to sunflower. The disease was first found between late June and early July in several locations in ND. The disease has been found in most regions of the state, although not in every field, and often at very low levels. Research conducted on sunflower rust in other countries suggested a fungicide application before severity on the upper four leaves reaches an average of 3% severity was an appropriate threshold. The same study found that an application 27 days after flowering was no longer beneficial, due to plant maturity. In our environment, I think that a threshold of 1-3% severity on the upper four leaves is probably appropriate. Headline (BASF) and Quadris (Syngenta) also are available for control of the disease. For a more detailed discussion of rust and its management, including pictures and severity diagrams, please refer to the July 17th issue of the Crop and Pest report at http://www.ag.ndsu.nodak.edu/aginfo/entomology/ndsucpr/index.htm, or a similar article published by the National Sunflower Association (which has a link the supplemental label for Folicur) at http://www.sunflowernsa.com/growers/default.asp?contentID=367.
BACTERIAL BLIGHT SHOWING UP ON DRY BEANS
In the last week, bacterial blights have shown up in dry beans. Three different bacterial blights are found on dry beans; common bacterial blight, halo blight, and bacterial brown spot. Each disease is spread by rain splash, and the diseases are frequently worse when the plants are injured by wind or hail. Several strong storms have moved through some of the state in the last two weeks, and this has contributed to the development of the diseases.
Although multiple diseases can occur at the same time and on the same plants, each disease has different symptoms.
Leaf symptoms: Halo blight appears as small water soaked lesions and often produces a large yellow-green halo. When temperatures are hot, the halo’s may be smaller. Early symptoms of bacterial brown spot (see photograph on page 9) also appear as a small water soaked lesion but have a small yellow-green halo, often with a very defined border. As bacterial brown spot develops, the lesions turn brown and the centers fall out. Heavily infected plants look like they were shot by a shotgun. Common bacterial blight lesions begin on the edge of the leaf as a scalded area with water-soaked spots. Common bacterial blight lesions enlarge quickly and are bigger (up to an inch) than the other two bacterial diseases, and often have a narrow yellow border.
Pod Symptoms: Symptoms of halo blight and common blight are similar. Both diseases begin as water-soaked spots or streaks on the pod surface. As the lesions develop, they may be surrounded by a reddish-brown zone. Under humid conditions, a cream-colored exudate may be present in halo blight lesions, whereas a yellow exudate may be present in common bacterial blight lesions. Bacterial brown spot lesions begin as small water-soaked lesions, but become sunken brown spots. Pods with bacterial brown spot infections may be bent or twisted at the site of infection.
The biology and management of each disease is similar. Bacterial blights can survive in seed, bean residue, and sometimes weed hosts. Once the diseases are present, they can spread quickly if heavy rains and adverse weather conditions (high winds, hail, etc.) occur. Unfortunately, little can be done this season to manage bacterial diseases. Although copper fungicides are sometimes used in other regions of the country (dryer western states like Colorado), little success has been observed in North Dakota. Management for future years is important if you have bacterial blights in your fields. Clean seed, rotation, and good sanitation are all recommended. Although these practices are not a guarantee you will not have bacterial blights, they minimize the likelihood of getting the diseases. If you have bacterial blights in your field now, it is critical not to keep that seed for planting in the future.
Bacterial brown spot
Common bacterial blight
(All photos courtesy Dr. Howard Schwartz, Colorado St.)
Extension Plant Pathologist
CERCOSPORA LEAFSPOT IN SUGAR BEET
It is the season for Cercospora leafspot, the most damaging foliar disease of sugarbeet which reduces tonnage and sucrose content. The fungus Cercospora beticola causes Cercospora leafspot. The most common source of the Cercospora fungus is infected sugarbeet debris in the field. Cercospora leafspot develops rapidly in warm, humid, and rainy weather. The Cercospora spores are produced at temperatures of 68 to 79 F and relative humidities (RH) of 90-100 %. Spore release is effected by rain and dew. Optimal spore germination and infection occurs when the temperature is 75-77° F and the RH is 100 % for at least 8 hours. Day temperature of 80-90 F and night temperatures above 60 F favor disease development. Leafspot symptoms may occur about 5-21 days after infection depending on prevailing weather conditions. Cercospora infection produces circular spots about 1/8 of an inch in diameter with ash gray centers and dark brown or reddish-purple borders. As the disease progresses, individual spots coalesce and kill entire leaves, particularly on susceptible varieties. In humid conditions, the spots become gray and velvety with the production of spores. These spores further spread the disease, especially within fields, resulting in many infection cycles during the growing season.
As such, it is important to have early control of Cercospora leafspot. The best manner to control leafspot is by using an integrated approach that include cultural practices such as burying infected tops by tillage, planting tolerant varieties in fields with a minimum rotation interval of three years, and the timely and proper use of recommended fungicides.
There are a number of fungicides available for controlling Cercospora leaf spot. These fungicides are Inspire, Proline, Eminent, Enable, Headline, Gem, Super Tin, Agri Tin, and Topsin. Fungicides with different modes of action and which are most effective in research trials should be used in rotation. Always use recommended fungicide rates. Growers using ground rigs should apply fungicides in 15 to 20 gal of water per acre at 100 psi and aerial applicators should use 5 to 7 gallons of water per acre for best coverage. The benzimidazole (Topsin) should not be used as stand-alone application. Topsin can be used in a tank mix with a protectant such as Super Tin, but only once in a season.
At this time growers should scout fields for disease symptoms. Growers have done an excellent job of combining tolerant varieties, crop rotation and timely fungicide applications over the past years resulting in very, very low inoculum levels. As such, disease symptoms are now more difficult to find and appears later in the season compared to 10 years ago. First symptoms will be observed in fields with less tolerant varieties that are close to windbreaks, in sheltered areas, and those close to rivers. The first fungicide application should be made when symptoms are first observed. Growers who are not scouting should consult their agriculturists or consultants to determine when symptoms are observed in their factory district. It is very important that the first fungicide application be applied in a timely manner or effective disease control will be difficult to achieve for the remainder of the season. Subsequent fungicide applications should be based on the presence of disease and favorable conditions for disease development. Please check the NDAWN website (http://ndawn.ndsu.nodak.edu/) for information on the favorability of environmental conditions for development of Cercospora leaf spot.
Extension Sugarbeet Specialist
NDSU & University of Minnesota