ISSUE 9 July 9, 2009
NDSU IPM FIELD SURVEY UPDATE - JULY 8
NDSU IPM field scouts surveyed 191 wheat fields and 44 barley fields across the state for the week ending July 3rd. Average growth stage over all wheat fields surveyed advanced to the second node and in barley to the boot stage.
Wheat: The severity of tan spot in wheat had slight increase from the previous week, with an average severity of 11% versus 9.4% of the previous week. The wheat disease forecasting site had indicated favorable days for infection for tan spot at many NDAWN sites over the past week. Leaf rust, at trace levels, was found in one commercial field, in Emmons County. Low severities of head scab were observed in two winter wheat fields in the southwest region.
Wheat streak mosaic virus (WSMV) symptoms were observed in 10% of the fields surveyed, with many of those fields in the northwest and northcentral counties, but also in some east central counties that were surveyed. Severity generally wasn’t high, except in some late planted spring grains, but symptoms were noticeable. See accompanying report that follows.
Cereal grain aphids were observed in 7% of the surveyed wheat fields; these aphids may vector barley yellow dwarf virus (BYDV), but BYDV symptoms were only recorded in 3% of the fields.
Barley: Net blotch and or spot blotch were again the most frequent disease symptoms observed. Four of the 44 fields had grain aphids present and two of these had BYDV symptoms. Ten fields showed symptoms of barley thrip damage. No scab symptoms were observed in barley.
WHEAT STREAK MOSAIC VIRUS (WSMV)
As mentioned above, field scouts are seeing quite a bit of WSMV symptoms this year in wheat (see May 28th Crop and Pest Report for pictures of symptoms). In addition, Kasia Kinzer and Monty Botschner of the NDSU Plant Diagnostic Lab have tested 41 wheat samples to date, and 23 of those have been positive for WSMV. The samples have come from 9 different ND counties across the whole state.
Virus symptoms have ranged from light infection on scattered plants, to fairly severe infection in very late planted spring cereals. Leaves from all wheat classes - winter wheat, spring wheat and durum - have tested positive. We have no data on differences in variety susceptibility to WSMV for our commonly grown wheat varieties.
WSMV is transmitted from plant to plant by the wheat curl mite, a tiny mite that is only 1/100 of an inch long. The occurrence and severity of WSMV is related to the survival and reproduction of the wheat curl mite.
What led to increased or more noticeable WSMV detections this year? Several factors are possibilities:
1) a good snow cover over much of the state may have resulted in a greater overwintering survival of the vector, the wheat curl mite;
2) a late, killing frost allowed corn (which is a host for the virus and mite) to stay green into late October or early November, and mites moved from that green corn into volunteer wheat or grassy weed hosts;
3) fall rains favored flushes of volunteers and grassy weeds that served as good "green bridges" for the mite and virus;
4) cool conditions at the time of "burndown" herbicide application may have resulted in slower or poorer weed control, and these weeds and or volunteers remained as sources of the mite and virus.
The wheat curl mite must have a green food host year round to survive. The abundance of green material well into late last fall and good overwintering conditions for wheat volunteers or grassy weeds and thus the mites, probably resulted in greater mite numbers this spring in winter wheat volunteers or grassy weed hosts.
The "green bridge" for mite survival was not adequately broken, and poor spring weed control conditions may have increased the problem. WSMV management must be aimed at breaking the "green bridge", through careful weed control and appropriate planting dates.
The following diagram depicts the life cycle of the virus and mite. Please note that perennial grassy weed hosts also could be a site of overwintering, although not depicted on the diagram.
Grass hosts: In addition to wheat crops and corn that harbor the mite and virus, perennial and annual grasses may be hosts to the mite and or the virus. Only a few perennial grasses are known hosts to the virus and allow increase of the mites: they include bluegrass, Canada wild-rye, and ricegrass.
Many annual grasses may be hosts to the mite and or virus: they include barnyard grass, cheat grass, downy brome, Japanese brome, field sandbur, and green foxtail (not yellow foxtail).
More information about wheat streak mosaic virus may be found in the NDSU Extension publication PP- 646 (revised) "Wheat Streak Mosaic Virus", found on-line at:
NDSU Extension Plant Pathologist
SOYBEAN RUST/APHID SCOUTING BEGINS IN SENTINEL PLOTS THIS WEEK
Soybean rust has the potential to be the most destructive soybean disease in any given year. However, the pathogen is only able to overwinter in areas of the United States where it is not killed by freezing temperatures, basically, the gulf coast. In attempt to stay one step ahead of soybean rust, plant pathologists monitor the spread of the disease as it progresses northward. Monitoring is done by scouting ‘sentinel plots’, which are designated soybean fields that we scout and assay leaf samples from once a week. Sentinel plots are scouted nationwide, 9 of which are located in North Dakota. Additionally, mobile scouting (randomly sampled fields) is done on a bi-weekly basis. Data is available to the public at www.sbrusa.net. Maps are available to monitor the spread of rust, commentary is provide by state specialists, and management information is available. The effort is part of the national ipmPIPE program and the sentinel plots, mobile scouting, and public interface are supported by the North Dakota Soybean Council and The North Central Soybean Research Board.
DRY BEAN DISEASE/INSECT SURVEY WILL BEGIN THIS WEEK
In addition to scouting soybean sentinel plots for soybean rust, ten dry-bean sentinel will begin being scouted at the end of this week. The presence and severity of diseases and insects will be monitored for and reported on a weekly to biweekly basis. The ten plots will be located from Pembina to Cass county in the Red River Valley, with several plots in the Wells/Foster area. As information is generated it will be upload to the website http://legume.ipmpipe.org/ North Dakota information should be available by next Monday (scouting will be done Thursday and Friday of this week). In addition to disease and insect occurrences; management information, photographs, and commentary from state specialists will be available.
SUNFLOWER RUST UPDATE
After speaking to multiple crop consultants, extension personal, and growers, it appears that sunflower rust is developing very slowly. The early occurrence of the disease is still alarming, but the slow progression is good news. My recommendation is to neither panic nor let your guard down, just scout your fields. Management options are available if a fungicide application is necessary. More information is available in the previous issue of the crop and pest report, and on the National Sunflower Association website www.sunflowernsa.com.
Extension Plant Pathologist