ISSUE 9 July 2, 1998
SMALL GRAINS DISEASE UPDATE
The wheat disease surveyors have found the following this past week: In McHenry, Mountrail, Divide, and Williams counties in the NC and NW districts, leaf rust has not been observed, and tan spot and Septoria infections are primarily still confined to the bottom leaves of the crops in the jointing stages. Crops in these counties in the boot or later stages are showing trace amounts of tan spot or Septoria on the middle leaves.
In Stutsman, Sheridan and Burleigh counties in the C and SC districts, one field of spring wheat in Stutsman county had 100% incidence of leaf rust and 10% severity on the middle leaves, and 3 % severity on the flag leaf. Other fields in these counties had only trace levels of leaf rust. Fields in these counties also had only low levels of tan spot and Septoria, primarily on the bottom and middle leaves.
In Barnes and Griggs county of the EC district, leaf rust was detected but at very low severities; tan spot and Septoria severities were low, primarily still on bottom leaves.
Crop consultants have reported a scattering of scab symptoms on spring wheat fields in Cass county, plus some scab infection has been seen in winter wheat variety trials at Casselton. In barley fields in Cass county, we had to look hard to find a few heads showing the characteristic symptoms of scab, a brown, watersoaked kernel or spikelet. These crops are in the 1/4 to 3/4 berry stage of kernel formation. In the wheat survey on June 30 in Cass county, 2 of 12 wheat fields were found to show scab symptoms, with incidence in one field at 10% and in the other at 25%. Thus, scab incidence is low in wheat and barley fields so far, and we hope for bright, sunny, and windy days to keep infections and spread at a minimum.
LEAF RUST RATINGS OF HRS WHEAT FROM 1997
The following table illustrates leaf rust ratings taken on some HRS wheat varieties in 1997. The value given is coefficient of infection, averaged over three locations in ND, as rated by Drs. Jim Miller and Jack Rasmussen last year. The values indicate that McNeal is a very susceptible variety, but this variety is generally grown out west where leaf rust is seldom a problem. Among commonly grown varieties, 2375 had the highest level of leaf rust, closely followed by Bacup and 2371. These values are field ratings and race identifications on the varieties were not made and are not known.
Why is leaf rust appearing on varieties that are listed as resistant in our variety trial publications? All of these commonly grown varieties were developed and released several years ago. They were screened for rust reaction in the field when the prevalent leaf rust races fell into a group called M races, as defined by rust pathologists and determined by the reaction of a set of differential lines to the rust isolates. The most predominate races of leaf rust in the past decade have been the M group of races. In very recent years, the Cereal Rust Lab has identified some other races occurring somewhat commonly in the Southern Plains, a group of races identified as T races, based on the reaction of differential lines to infection by these isolates. As described in the latest Cereal Rust Bulletin, June 16, 1998, of 80 isolates submitted from Texas this year and tested on the set of differentials, 70% are still of the M group, while about 30% are identified as a T group. Thus, the predominate races are still those to which our varieties were screened against, but some of the T group isolates may also be making their way northward on prevailing winds, and if spore showers of these races occur, we may be seeing more rust in some fields. In 1997, of the isolates identified from ND, only 15% had the T group of isolates.
LATE APPLICATION OF FUNGICIDES TO SUPPRESS SCAB
Research at various NDSU research sites in recent years indicated that application of fungicides for scab suppression is optimal in wheat at early to mid-flowering, and application at watery ripe to early milk stage of kernel development gave very little control. Application of fungicides after the flowering period may not adequately control scab, but some late season leaf disease control could occur. However, keep in mind the pre-harvest interval of the fungicides registered for wheat:
Tilt - 40 days PHI
Folicur - 30 days PHI
Mancozebs - 26 days PHI
Benlate - 21 days PHI
STERILITY IN BARLEY HEADS
Barley heads in a variety plot at Carrington, plus a sample of barley sent to the Diagnostic Lab from Logan Co., exhibited some strange kernel sterility, with some sterility at the tip of the head, some in the middle, and other barley heads with sterility at the base of the head. The florets were white, empty, and the awns were white and curled or frizzled. Various possible suggestions were made for causing these symptoms, including genetic causes, frost, herbicide injury, or hail. Dr. Jerry Franckowiak, 2-row barley breeder at NDSU, said that he has observed this type of kernel sterility before, and has associated it with late infections of the oat blue dwarf virus. This virus is carried and transmitted by leafhoppers. Early infections of the virus may cause stunting and a deep green-blueish color of the leaves, but late infections generally do little economic damage, as long as affected plants are scattered and not widespread. The Diagnostic Lab is asking Dr. Mike Edwards, USDA Virologist, to test samples for this virus.
Extension Plant Pathologist
SECTION 18 UPDATE FOR CANOLA
Benlate Denied. The Environmental Protection Agency (EPA) denied North Dakota and Minnesota's request for a section 18 for Benlate to suppress Sclerotinia on canola. EPA was aware that canola growers faced a potentially severe problem with Sclerotinia this year. Intense pressure from the canola industry and legislative delegations helped convince EPA to provide some other product.
Quadris Substituted. On Saturday, June 27 EPA granted a section 18 for the use of Quadris to suppress Sclerotinia on canola. Quadris, manufactured by Zeneca, is a strobilurin, a new class of fungicide with a new mode of action. Quadris has unusually broad spectrum activity. It is locally systemic and has several days post-infection activity. In Canadian trials it has provided Sclerotinia suppression at about the same level as Benlate.
Quadris is to be used at 15 fl oz/A. Application should be made no later than 30% bloom, which is when there are 20 open blossoms on the main stem. Cost of 15 fl oz of Quadris is about $32.
Research is underway in North Dakota and Western Canada examining earlier applications of Quadris and different rates. The objective is to determine the most effective timing and rate. It is possible that an earlier application may be more effective, and that research may demonstrate that a lower rate may be possible if the application is earlier. There is not sufficient data to support lower rates at this time, however.
Crisis Exemption for Ronilin. When the section 18 for Quadris was granted, many fields were at more than 30% bloom. The industry again approached EPA, indicating that it was too late to use Quadris in many fields, and petitioning for Ronilan, which could be applied up to 50% bloom. A crisis exemption for Ronilan was declared by the North Dakota and Minnesota Departments of Agriculture on July 1 for the use of Ronilan to suppress Sclerotinia. I have not seen the label yet, but the use rate will around 1 lb/A. Be sure to check the label for actual rates. Ronilan can be applied up to 50% bloom. Cost will be about $19/lb. Ronilan has provided effective Sclerotinia suppression in Canada.
Late Application is not advised. Do not apply later than 30% bloom (20 open flowers on the main stem) for Quadris or 50% bloom (pods beginning to elongate on lower portion of main stem) for Ronilan. Late application provides reduced disease suppression.
A Hearty Thank You to Beth Nelson, Minnesota Canola Council, who would not give up on a section 18 for canola and kept pursuing every possible avenue. Also a big thank you to the legislative delegations from both states for their support.
POTATO LATE BLIGHT
Dr. Marty Draper, extension plant pathologist, South Dakota State University, confirmed late blight on tomato the latter part of last week. The late blight was in Aberdeen, SD.
Dr. Gary Secor, NDSU plant pathologist, confirmed late blight on potato on June 29 from a sample at East Grand Forks, MN.
The late blight hot line is available at 1-800-482-7286 or at the web site:
The late blight severity values for 1998 are running about two weeks ahead of the severity values for 1997.
CERCOSPORA LEAFSPOT ON SUGARBEET
Dr. Al Cattanach reported that Cercospora has been confirmed on sugarbeet near Foxhome and Campbell, MN on June 26 and near Climax and Crookston on June 29. Weather conditions have been favorable for Cercospora and rows are closing in many fields. It is likely that Cercospora will be identified in additional fields by early next week. Scouting for Cercospora should be intensified.
Identification. Cercospora produces circular ash-gray spots with a dark border. The spots are 1/8-3/16 inch in diameter. After several days the spots develop tiny black fruiting bodies (the spore bearing structures). These bodies resemble specks of finely ground pepper and are best recognized with the aid of a hand lens. Bacterial leafspot is common in many fields, so it is important to distinguish it from Cercospora. Bacterial leafspot lesions are larger, more irregular, and often a darker gray. Leaves may tear across the lesions. No fruiting bodies develop in the bacterial leafspot lesions.
Resistance Management. Since it is going to be a long spraying season for Cercospora, resistance management practices should be used throughout the season. Suggestions can be found in the 1998 Sugarbeet Production Guide, better known as the Pocket Guide. Rotating Super Tin with mancozeb or Quadris should help to retard the development of tin tolerance. Either could be used as the second or third spray, and again as the fourth or fifth spray in southern areas where numerous applications are needed. The next fungicide application should be made 7 days after an application of mancozeb or 14 days after an application of Quadris.
Extension Plant Pathologist