ISSUE 13 July 25, 2002
FIELD SCOUTING UPDATE FOR WHEAT AND BARLEY DISEASES
A look at growth stages of wheat scouted during the week of July 12-19 indicate that some wheat crops in north central and northwest counties are in early heading to flowering stages that may still require some fungicide decisions (see following figure), while other locations of the state have fast maturing crops.
Wheat growth stage 7/15-7/19
NDSU IPM field scouts are starting to detect Septoria leaf spots now on crops in heading stages, although tan spot is still most common in wheat and spot blotch in barley. Leaf rust levels detected the past week ranged from 0 to 50% severity on the flag leaf. Barley stem rust was detected in one barley field in Steele County.
Bacterial leaf blight was detected in field plots of wheat and barley at the Carrington Research Extension Center, in fields near Casselton, near Grand Forks and in Traill county. Bacterial leaf blight is characterized by brown necrotic streaking running lengthwise on flag leaves; frequently the shiny appearance of the dried bacteria also is seen. These bacterial infections were a result of recent winds and rains that provided wounds and moisture for bacterial infections to become established. Some varieties appear to be more susceptible to bacterial blight. Fungicides used to control leaf rust, tan spot or scab will not prevent bacterial blight.
Fusarium head blight (scab) also has recently been detected. Low incidences of plants/field (1-15%) have generally been observed (see figure below) . Head severities on infected plants has generally been from 7 - 21%.
NDSU field scouts have observed some wheat and barley fields with considerable loose smut. Some wheat fields have had as much as 34% incidence of loose smut, while a barley field had a 16% incidence of this disease. These percentages of plants with infection almost directly reflect potential yield loss. Certain seed treatments with activity against the loose smut fungi are very effective in controlling this disease and are inexpensive ways to assure that these large losses donít occur.
Detection of the disease in barley seed can easily be done prior to planting. The ND State Seed Department can test for percent loose smut infection in the embryos of barley seed. If levels are above 1%, an appropriate seed treatment is recommended. The embryo loose smut test is NOT reliable for wheat so the State Seed Department does not offer a wheat test for loose smut. Growers with considerable loose smut in their wheat this year should consider seed treatment of this grain next year if it is to be used for seed.
Extension Plant Pathologist
SOYBEAN MOSAIC VIRUS
Soybeans, sunflowers, and sugarbeets are the majority of samples in the lab recently. One of the diseases that soybean growers should be on the look out for as they are scouting for soybean aphid is Soybean Mosaic Virus (SMV). It has not previously been reported in ND but it is in both South Dakota and Minnesota. SMV is seed borne, which provides the primary inoculum source. There are several perennial weed species hosts that overwinter the virus, and this virus is transmitted by soybean aphids in the field. According to NDSU Extension entomologist, Dr. Phil Glogoza, soybean aphids have been reported in the following counties in ND: Richland, Sargent, Ransom, Cass, Traill, and Grand Forks, with numbers high enough to consider treatment in Richland, Cass, and southern Traill counties.
Symptoms of the disease may vary with cultivar, plant age, virus strain, and the environment. Some of the symptoms include stunting, reduced pod set and pods that may be flattened, dwarfed, and lacking hairs and seed, and a mosaic look to the trifoliate leaves. The mosaic appearance of leaves begins as a mottled light and dark green. These areas may become raised or blistered, this is called rugose. This rugosity is most severe in plants grown in cooler temperatures (about 64 degrees F). These symptoms are less severe at about 77 degrees F, and they are mostly masked at temperatures above 80 degrees F. Some strains of the virus have overcome the resistance induced in certain cultivars by resistance genes (Rsv1 in particular). This condition may result in symptoms of severe stunting, systemic necrosis, leaf yellowing, petiole and stem necrosis, terminal necrosis, and defoliation.
Seeds infected with SMV may be mottled brown or black, and may be smaller than seeds from non-infected plants. In addition, germination levels are often reduced in seed from SMV-infected plants.
Soybeans infected with SMV may also be infected with BPMV (see crop and pest report for July 10). The effect of this may be synergistic, so plants show severe dwarfing, foliar distortion, necrosis, and leaf mottling. Plants infected with both viruses may have fewer root nodules and yield loss may be as high as 85%.
The best management strategy for SMV is to plant disease-free seed and avoid planting late so the crop is more mature by the time the aphids may infest.
The NDSU Plant Diagnostic Lab can verify this virus in soybeans by means of a serologic test. Leaves and stems of symptomatic plants should be sent to the diagnostic lab for evaluation as soon after sampling from the field as possible.
FOLIAR DISEASES OF SOYBEAN
With the recent wet conditions in the major soybean-producing areas, several foliar diseases of soybean are beginning to show up. Listed are a few foliar diseases of soybean that are likely to appear on soybean in North Dakota.
-Bacterial Blight - Caused by the bacteria Pseudomonas savastanoi, bacterial blight is commonly found on soybean leaves in North Dakota. Symptoms first appear as water-soaked spots on the leaves, and eventually turn brown to black with a yellowish-green halo. Spots will usually appear on lower leaves first, and will move to newer leaves if weather conditions are cool and rainy. Yield reductions due to bacterial blight are typically negligible; however under the right environmental conditions and on an extremely susceptible variety, yield can be reduced significantly. There are resistant genes available, but may not be available in many varieties grown in North Dakota. Avoiding cultivation while foliage is wet may help reduced the spread of disease from plant to plant.
-Brown Spot- Caused by the fungus, Septoria glycines, brown spot symptoms occur as irregular, dark brown spots. Leaves may eventually turn yellow and drop. The optimum conditions for disease development is 75 to 80 degrees F, and wet leaves. Dry weather will halt the progression of disease. Lower leaves will generally be infected first as the inoculum is moved from the ground by wind and splashing rain. Yield reductions up to 15% have been reported in the field. There is no known resistant genes, but varieties vary in susceptibility. Disease tends to be more severe when soybean is planted 2 consecutive years on the same land. Crop rotation with a non-legume crop for at least 1 year will help manage the disease. There are some fungicides labeled for control; however, disease severity is generally not severe enough to realize an economic return with use of a fungicide in North Dakota.
-White Mold- Also known as Sclerotinia stem rot, white mold symptoms appear as wilting plants with bleached stems. A characteristic cottony growth is sometimes seen on the stems or branches. Yields can be significantly reduced when conditions are favorable for disease development. No variety has complete resistance, but susceptibility to the disease will vary among varieties. Fungicides are labeled for control; however, essential coverage of the blossoms can be extremely difficult and may limit the effectiveness of the fungicide. Use of wider row spacings and lower plant populations may help manage the disease.
Extension Plant Pathologist