ISSUE 15 August 9, 2001
SMALL GRAIN DISEASE UPDATE, 8/09
NDSU IPM scouts are winding down their field surveys in the next few weeks, as they are helping with field notes and harvest, and heading back to school soon. They have observed considerable increases in leaf and head diseases as the crops approach maturity.
Leaf rust: The highest incidences and severities of leaf rust were observed in the south central and central counties, where 89% of the 30 wheat fields surveyed by Jerry Schneider had leaf rust, with severities on the flag leaves ranging from 2-30%.
In McLean, McKenzie, Williams, and Mountrail counties, 75% of the spring wheat fields surveyed by Nathan Carlson showed leaf rust, with severities on the flag leaf ranging from 1-11%. In Ward, Burke, Divide, and Renville counties, leaf rust was found in two-thirds of the spring wheat fields surveyed by Kelly Novak, with severities ranging from 2-25% on the flag leaves. Durum wheats are slow rusting and rust detections were primarily on hard red spring wheats. In Bottineau, Pierce, McHenry, and Rollete counties, Holly Semler found 80% of spring wheat fields with some level of rust, with severities on the flag leaf from 2 to 6%.
Leaf rust was found in most spring wheat fields surveyed by Matt Gregoire in the northeast counties. Infections were particularly severe on Russ wheat this year. Leaf samples were collected from different varieties in plots in the northeast and sent to the USDA Cereal Rust Lab for race identification.
Jeanna Jambor found leaf rust in 50% of the spring wheat fields surveyed in the southwest and west central districts, and severities ranged from 4-10% on the flag leaves. She found oat crown rust in all oat fields surveyed, with severities on the flag leaf from 5-20%.
Leaf spots: The fungal leaf spot diseases of tan spot and Septoria blotch were found in 100% of the spring wheat and durum wheat fields surveyed the past week. Severities on flag leaves ranged from 1-59%, with highest severities in the south central, central and north central counties. Jerry Schneider observed glume blotch, the head infection phase of Septoria, on 17% of the wheat fields he surveyed the past week.
Scab (Fusarium head blight): Head scab was observed in 86.7% of wheat fields surveyed in the south central and central counties, with incidences in a field from 4-64% and head severities from 7-97%. Individual field severities (incidence x head severity) ranged from <1% to 34.2%.
In the north central and northwest counties surveyed, head scab was observed in 72.1% of spring wheat and durum fields surveyed. Incidence of plants infected ranged from 2-100%, and severity ranged from 3-67%. The field severity ranged from <1% to 34.3%.
In the southwest, scab incidence was less than 2% and was not picked up in the random examination of 50 heads/field. During the wheat quality tour, Mike Peel did observe some high incidences of scab in two spring wheat fields in Stark county.
In the northeast, scab was found in every field. Head severity levels in spring wheat fields ranged from 2-15%, with an average around 5% severity, but some durum fields had severity levels up to 50%.
OTHER DISEASE OBSERVATIONS
Keith Brown, Divide County Extension agent, Pat Carpentier, McLean County Extension Agent, Neil Riveland, Williston Research Extension Center Agronomist, Kent McKay, Area Extension Specialist, North Central Research and Extension Center, and Terry Gregoire, Area Extension Specialist, Devils Lake, have all reported on crop condition to me in recent days. Wheat and durum disease levels, including head scab, are variable in their regions, depending on variety grown, date of planting, and amount of rainfall per location. In many locations, head scab is very common and may cause significant yield loss. Kent McKay and I, on 8/8/01, observed durum fields with head scab field severities of 80% or more in Renville and Burke counties. Keith Brown reported more root rot than head scab in durum at Fortuna, but scab levels were high in spring wheat at that particular location. In all of these areas, the experiment station and off-station variety and fungicide trials are being evaluated for disease severity and differences among cultivars and treatments. Similar work is being done at Carrington and Langdon. Terry Gregoire reports that barley scab in the northeast counties is generally light, with most infection occurring on nearly mature seed. He also reports that Sclerotinia infections in canola appear to have occurred late, with more infections on individual branches and main stems.
POTATO LATE BLIGHT HOT LINE
Late blight favorability values continue to be positive in all locations. However, late blight has still not been found in North Dakota or Minnesota. The hot weather will slow sporulation and spread of the late blight fungus, but will not kill any late blight inside the plant. NDSU potato plant pathologists continue to recommend scouting fields carefully, watching for changes in the weather, and continuing a regular fungicide application program.
Heavy rain, particularly in the northern growing areas, caused water damage to potatoes in low spots. Because of the high soft rot potential in these damaged potatoes, it is recommended to destroy them so they will not be harvested and stored. Abandoned potatoes should be sprayed with fungicide so they will not act as source of late blight for the neighborhood.
Extension Plant Pathologist
BLOSSOM END ROT AND BACTERIAL BLIGHT ON DRY BEAN
There have been several calls in the last week describing a condition on tomatoes called Blossom end rot. This is a nutrient deficiency brought on by environmental conditions, and not an infectious disease, despite the appearance. Blossom end rot looks like a rot on the bottom, the end opposite the stem. The disorder begins as a light tan colored spot on the blossom end of immature tomatoes. As the tomato matures and ripens, the spot will become sunken and leathery in appearance and turn brown to black in color.
Blossom end rot is caused by a calcium deficiency resulting from fluctuating soil moisture, especially moisture stress. The condition can be exacerbated by excessive nitrogen fertilization and root pruning. Subsequent rot by storage rot fungi may occur as a secondary problem since the lesion caused by blossom end rot provides an opening for these rot organisms to enter.
Try to keep soil moist but not saturated. Regular watering between rain episodes is important to avoid flucuations in soil moisture or drought stress. Mulch can be helpful in keeping soil from drying out between waterings. If plastic is put down, be sure to cut an opening wide enough that the plants stems will not come in contact with it, causing heat injury to the stems. If you cultivate for weed control, stay shallow around the roots once the plants are established. This can cause root damage that may induce moisture stress. Excessive nitrogen will result in rapid growth that is sometimes too much for the calcium and water to keep up with. A nitrate form of nitrogen is best. Fertilizer formulations such as 4-12-4 or 5-20-5 work well for tomatoes. Foliar calcium sprays may provide some relief from the condition in later tomatoes. The one spray I was able to find in Fargo is called Yield Boost.
CANOLA DISEASE SURVEY AND THE SCLEROTINIA RISK MAP
Canola Disease Survey. The hot weather is pushing canola maturity, and swathing is taking place in various parts of North Dakota and Minnesota. The canola disease survey was initiated last week and will be getting into full gear by the end of this week. The survey involves various area specialists and county agents/educators from both states, and Dr. Martin Draper, the extension plant pathologist in South Dakota for that portion of the survey. We will be assessing the incidence (percent infected plants) of Sclerotinia stem rot, black leg and aster yellows and will be determining the severity (percent of pod area infected) for Alternaria black spot on the pods.
Sclerotinia Risk Map. In addition to the data listed above, we will attempt to get information on fields that were near NDAWN (North Dakota Agricultural Weather Network) weather stations, which provided the weather data for the Sclerotinia Risk Map. For selected fields near NDAWN weather stations, we will attempt to get information on planting date, crop rotation and whether or not the crop was sprayed with a fungicide. Dr. Gary Platford, who directs the Risk Map for Western Canada and also for North Dakota and Minnesota, wants to correlate Sclerotinia data from fields near NDAWN sites with the Risk Map data. This information will help to make any fine tuning necessary for providing the most accurate Risk Map information possible next year.
Sclerotinia Risk Map Assessment. We will have an assessment of the Risk Map after the canola survey has been completed and we have had a chance to evaluate the Sclerotinia data. There were a few concerns this year, and these will be addressed. On the whole, I believe that the Risk Map provided good information for producers.
Great people donít win; Great teams win Ė Jeffery Bezos, CEO, Amazon.com. This comment, made when Mr. Bezos spoke at the Technology conference in Fargo this spring, struck home with me. The Sclerotinia Risk Map was a team effort in every sense of the word. I wish to thank all of the county agents/educators, area specialists, Dr. John Enz and the NDAWN personnel, branch station personnel, Dr. Bruce Seelig for NDAWN soil type data, and Dr. Gary Platford and many other persons in Manitoba who organized and ran the model for us. I also wish to thank BASF, the Northern Canola Growers Association and the Minnesota Canola Council for the funding that was needed to support the Risk Map.
Plant Pathologist Emeritus
BACTERIAL BLIGHT IN DRY EDIBLE BEANS
Bacterial blight in dry beans in ND is typically not a serious problem, however, the relatively higher humidity and frequent rains have combined to make the environment favorable for disease development off and on for most of the season. Disease begins from infected seed or bacteria that survive in crop debris. There are three bacterial pathogens that cause disease, Pseudomonas syringae pv. syringae (bacterial brown spot), Pseudomonas syringae pv. phaseolicola (halo blight), and Xanthomonas campestris pv. phaseoli (common blight). Severe defoliation and serious losses are not typical in ND, but due to the highly infectious nature of the pathogens, losses exceeding 75 percent of potential yield have been reported.
Bacterial blight symptoms start as greasy-looking, water-soaked spots on the leaves, stems, or pods. Under favorable conditions, the leaf spots will enlarge and rapidly dry causing brown spots that may fall out of the leaf. With halo blight, the lesions may show a bright yellow halo around the brown spot as long as temperatures are below about 70 degrees F. Above 70, halo blight will look like bacterial brown spot. Common blight may show a distinctive yellow band separating infected and uninfected tissue. As pod lesions age, they may become sunken with a reddish margin. In the field, masses of bacteria may exude from the lesions appearing yellowish or cream colored. As these dry, they may become flaky.
Once infection begins in the field, there is little to be done. Management should begin with clean, high quality, certified seed. Streptomycin seed treatments eliminate surface bacterial contaminants from infecting seed but does not prevent internal, seed-borne bacteria. Crop rotation of 3-4 years will allow time for breakdown of crop debris. Minimizing wounding, such as occurs with cultivation will cut down on the points of entry for bacteria. Do not cultivate when the leaves are wet or after rows close. Clean equipment (by steam or noncorrosive disinfectant) before going from one field to another. Bacteria can also move on people as they brush up against infected plants and move into healthier material. Copper fungicides are registered for control of bacterial blight, but these are protectant products and need to be applied before infection occurs. They will not protect against seed-borne infection. These have not proved economically effective in ND.
NDSU Plant Diagnostic Lab
ALTERNARIA LEAF AND POD SPOT OF DRY BEANS
Warm and humid diurnal conditions with cooler nights favor dew formation. When abundant humidity is present, infections caused by opportunistic organisms are exacerbated. This is usually the case of Alternaria leaf spot, which is caused by a number of species of the fungus Alternaria. This fungus usually enters the plant tissues through wounds or following bacterial infections, although some species can enter the leaves through natural openings. Alternaria leaf spots are more common during the filling of the pods than during active growth periods. Typical foliar lesions are small and have irregular-shapes. They are gray to reddish brown in color, and could be surrounded by a darker border. Lesions showing this symptom can be easily confused with the bacterial brown spot caused by Pseudomonas syringae pv. syringae. When Alternaria leaf spot lesions enlarge, they may develop concentric rings that usually become brittle and fall out. Alternaria leaf spot rarely causes economical yield reductions in this region and therefore a fungicide application for its control is seldom justified. However, under very favorable conditions, it could infect the pods and damage developing seeds. In Ontario, where Alternaria leaf spot has been considered an important problem, applications of iprodione at flowering time have produced better results than applications of benomyl. Iprodione is registered for control of white mold in North Dakota.
Luis del Rio
Department of Plant Pathology