ISSUE 12 July 20, 2006
SOYBEAN VIRUSES TRANSMITTED BY APHID
Presently, no viruses have been confirmed on soybean in North Dakota. However, with all of the soybean aphids present in the state, it is possible that viruses could show up. Soybean aphids are able to transmit several viruses including soybean mosaic virus and alfalfa mosaic virus. Foliar symptoms of soybean viruses can vary, but mottling and mosaic patterns and rugose (rough) leaves may be present. Leaf cupping and rolling (as seen with growth regulator herbicides) are also symptoms that can be associated with viruses. Stems that remain green after maturity can also be attributed to viruses. Seeds that come from plants infected with a virus may show mottling or irregular hilums. If you suspect that a virus is present in your soybean field, some labs (such as the NDSU Plant Diagnostic Lab) can test for the presence of some viruses.
Foliar symptoms caused by a soybean virus
(Courtesy Loren Giesler, Univ. of Nebraska)
Carl A. Bradley
Extension Plant Pathologist
WHEAT AND BARLEY OBSERVATIONS, July 10-14
Wheat: NDSU IPMís summer field scouts surveyed 92 wheat fields during the week of July 10-14. The average growth stage of these crops during that week was Zadoks 72 = kernel watery ripe to early milk stage.
Diseases did not advance from the previous week, due to hot, dry conditions in most areas of the state. Leaf rust was observed in 29.3% of the surveyed fields, and average severity of leaf rust in infected fields was 5.1%, slightly higher than the 2.2% of the previous week.
Grain aphids were detected in 65% of the wheat fields surveyed. Some leaves and heads are sticky and shiny from the abundant aphids present. Grain aphids can transmit Barley Yellow Dwarf Virus (BYDV). BYDV symptoms were not observed in all of the fields with aphids detected, but BYDV symptoms were common in some fields in the SE, with distinct golden yellow flag leaves. Some "firing" of leaves may initially have been caused by BYDV infection, followed by heat stress.
Barley: Twelve barley fields were surveyed across the state the second week of July. Grain aphids and thrips were commonly found in these barley fields. Where grain aphids transmitted BYDV, pockets or patches of BYDV were commonly seen in any un-ripe barley fields.
Thrip damage was noticeable as a white cast of the leaf sheath below the grain head, and some kernels were turning a tan/white from thrip damage. A trace level of barley stripe rust was observed in non-fungicide treated barley plots at Fargo on July 14.
Fusarium head blight was detected in one surveyed barley field, in Grand Forks county, and in five wheat fields in SE counties. Field severities were 2% or lower.
SOME SMALL GRAIN DISEASES MORE COMMON IN DROUGHT YEARS
A plant disease organism probably exists for every environmental condition above freezing. For small grains, some disease organisms, or their vectors (carriers and transmitting agents) actually do more harm to crops under dry conditions.
The drought-enhanced small grain diseases that are most frequently seen in North Dakota are common and Fusarium root rot and wheat streak mosaic virus. In addition, this year the drier conditions have also enhanced grain aphid reproduction and numbers, aphids which carry barley yellow dwarf virus = BYDV (see previous section). Wheat streak mosaic virus (WSMV) has already been described this year in Crop and Pest Reports of May 11 and June 22. All of these diseases add stress to a small grain plant already severely stressed by lack of moisture.
I just started noticing some root rot symptoms this past week on wheat crops in the east, crops which were "hanging in there", but heat stress over the weekend caused some plants with infected root systems to turn white.
Root rot disease interferes with water uptake and subsequent grain fill. Root rot infected plants can be distinguished from those with wheat stem maggot or scab in that with root rot, the whole plant, including the head, generally becomes prematurely white or light tan in color, and the affected plant is easily pulled from the soil.
Recent heat stress will make root rot infected plants more visible. A plant under heat stress transpires rapidly, and a diseased root system canít absorb water fast enough to replenish water lost through leaves.
Root rot management: Crop rotation is the most effective method of reducing the risk of root rot. Fewer root rot fungus spores are found in soils left fallow or planted to non-host crops. Seed treatment products also are available that suppress the common and Fusarium root rot disease in the early growth stages. Variety differences among spring wheat and durum wheat have been documented for common root rot. The latest information on spring wheat variety susceptibility can be obtained from your county agent, area agronomist or branch station personnel or from the NDSU Extension publications on spring wheat varieties (NDSU Extension Publication A-574).
Seed treatment. Currently, several seed treatments are registered for suppression of common and/or Fusarium root rot in wheat and barley. Products registered in ND are listed in NDSU Extension Circular PP-622 "Field Crops Fungicide Guide". Additional information about root rot may be found in the NDSU Extension Circular PP-785 (rev) "Root and Crown Rots of Small Grain."
Extension Plant Pathologist