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Field and Forage Crops |
The major bollworm moth flight, although late this season, has moved rapidly through much of the state this past week. Many light trap counts are now available on-line on the same day as the traps are checked. See the following web site for these light trap data: http://ipm.ncsu.edu/cotton/insectcorner/blacklight/index.html. Some light traps reported 3-day levels on Monday, August 1, in the 200 to 300 range, with Cumberland County coming in at 534, and Lenoir County registering a one-night capture of 178 bollworm moths on July 31. So far, this looks like about a medium-plus fight compared to previous years. One can safely assume that this flight be in progress up to the Virginia border by this weekend, or early next week. For producers growing conventional cotton, this typically means that the bollworm egg threshold has either already been met or will shortly be met in the southern and northern part of the state, respectively. In Bollgard, BG II, or in Widestrike cotton varieties, the triggering of the bollworm larval threshold is far less predictable. In most cases, these Bollgard cotton lines reach the larval threshold approximately 1 to 2 weeks after the egg or larval threshold has been met on conventional cotton. However, in some cases, the bollworm larval threshold is not met on Bt cotton, and in other situations the intensity of the moth flight and the attractiveness and susceptibility of the cotton dictates two insecticide applications. Complicating things further, stink bug and/or plant bug damage to bolls can figure prominently into bollworm spray decisions in many, or in most, situations in Bt cotton.
So far this season, the combination of stink bug and plant bug internal damage to quarter-sized bolls has resulted in a significant number of treated acres. Many of these have been close calls; that is, close to the 10 percent internal damage level used at this time of year. In general, producers seem to be much more aware of boll damaging bugs this year than last. As indicated by our annual damaged boll survey, potential bug damage to bolls does not appear to have a strong regional aspect, so all areas of the state potentially may experience significant damage to stink bugs and/or plant bugs in unmanaged cotton fields.
Because plant and stink bug damage to quarter-sized bolls is often recent, this damage may not be obvious (or even show) on the outside of the boll. Therefore, be sure to open randomly selected, quarter-sized bolls, whether or not they have external spotting. We have seen more than our share of internal stained lint and/or warts on the insides of bolls without the associated external boll spotting.
In cases where green stink bugs appear to be the most prominent bug species and where threshold levels of bollworms are also present, pyrethroid insecticides at medium to high rates are the materials of choice. Pyrethroid insecticides are only fair against brown stink bugs, however, and generally similar to one another in their activity at high labeled rates. A material such as Bidrin is the insecticide of choice if brown stink bugs constitute part or most of the stink bug population. In tests conducted here and elsewhere, Orthene and Vydate appear intermediate in their brown stink bug activity. If one has partial thresholds of bollworms and stink bugs that combined add up to a complete threshold (for example a 60 percent stink bug threshold and a 60 percent bollworm threshold) and brown stink bugs are present, a tank mix of a pyrethroid plus Bidrin is suggested. In the case of a stink bug-only spray with either Bidrin or Orthene without bollworms, expect the possibility of increased bollworm survival in both conventional and Bollgard cotton during the coming week. Be sure to re-scout this cotton within about five days (in the case of conventional cotton, one can almost count on needing a pyrethroid for bollworms by the time of the next check following a stink bug spray with Bidrin or Orthene). As of August 5, bollworm establishment on Bt cotton has been minimal.
On the plus side, cotton aphids and spider mites appear to have given up the ghost, at least for the time being. Good moisture levels in many areas of the state appear to have helped reduce levels of the two above pests.
There is quite a variety of insect activity in peanuts now. The most frequent question I've received over the past week is associated with rootworm control. The Peanut Production Guide states that applying rootworm insecticides after August 5 may not be effective. However, many growers found themselves in a situation this year where the field was dry until about two weeks ago. While dry weather was in place, there was no concern over rootworms, so no application was made. Suddenly conditions seem to be more favorable and the question of treatment is much more difficult this time of year. There are no absolutes in nature, but I think the benefits of an application will be reduced for any treatments made from this point forward. However, if your field is in the higher range of the rootworm index, it might be wise to treat. Keep in mind that if it turns dry again, a late season rootworm treatment can really set off a spider mite outbreak.
Various caterpillars are showing up in many fields. Keep the 4 worm per row foot threshold in mind when checking fields.
Also watch for spider mites. Some areas had two to three days of cool, wet weather about a week ago and these conditions quite often will initiate a fungal disease of the mites that causes the populations to rapidly decline.
Sentinel plots continue to be monitored weekly and several samples from kudzu, collected in the southern part of the state, were taken to the lab for examination. All were negative for Asiatic soybean rust and rust has not been found in North Carolina thus far. Brown spot caused by Septoria glycines has been detected in numerous locations and in some instances has resulted in defoliation of the lower portion of the plant. Similarly, soybean downy mildew is also quite common this year. Neither disease ordinarily requires management. Target spot caused by Corynespora cassicola has been identified in Wilson, Perquimans, Beaufort, and Alamance counties and this disease does warrant attention.
A fungal disease, target spot of soybean caused by Corynespora cassicola, was identified on three samples sent to the Plant Disease and Insect Clinic at North Carolina State University. In 2004, this disease caused early defoliation of certain soybean varieties and resulted in yield losses of as much as 50 percent. At this time we suspect that only a few varieties are susceptible to this disease. When submitting samples, please specify the variety, as this helps us keep track of which varieties are susceptible. The use of a foliar fungicide may well be warranted on susceptible varieties. Please consult the on-line version of the North Carolina Agricultural Chemicals Manual for fungicide recommendations.
Soybean downy mildew, caused by the fungus Peronsopora manshurica, is a common early season disease of soybeans. In June and July, yellow spots are visible on the upper surface of the leaves. On the underside of these leaves, especially early in the morning following heavy dew, white to gray to blue green tufts of fungus can be seen with a hand lens or even with the naked eye. Usually starting in August, these yellow spots may become brown with a dark brown border. This can be confused with frogeye leaf spot, but the border is much less distinct than that of frogeye. Downy mildew is generally not serious enough to warrant fungicide treatment. Most fungicides are not effective against downy mildew, although fungicides containing strobilurin fungicides (Headline, Quadris, Headline SBR, Quilt and Stratego) or Chlorothalonil-containing products may have some limited activity.
Recent weather associated with hurricanes may have transported Asiatic soybean rust to North Carolina, but the numbers of spores would be very low. For more information see the following web site: http://www.ces.ncsu.edu/depts/pp/soybeanrust/index.php. Fields should continue to be scouted for foliar diseases, including rust, frogeye leaf spot, and target spot. Scouting efforts should be intensified in about 2 to 3 weeks as more soybeans enter the reproductive stage and infections that may have occurred this past week may then be evident.
It seems likely that soybean and kudzu have latent infections of soybean rust in the mid-South and Delta. The amount of spores that may have been transported to the mid-South with hurricane Dennis was estimated to be very low. It will probably be 2 to 3 weeks before rust is evident in those areas, and can begin serving as a source for North Carolina.
A commercial field in Baldwin County, Alabama has fairly severe rust infections at this time and was sprayed with fungicides recently. Soybean rust has also been found near Savannah, Georgia, at sentinel plots in Lee and Elmore counties, Alabama. So, Asiatic soybean rust is still about 200 miles from the Charlotte area. There were reports of Asiatic soybean rust-like spores being found in spore traps in Tennessee and Kentucky several weeks ago, but subsequent scouting has failed to detect soybean rust in these areas. Four Asiatic soybean rust-like spores were found in a spore trap in Illinois last week. It is not possible at this time to say that these spores are Asiatic soybean rust. There are hundreds of rust fungi in North America and spores look very similar. Also, it is not possible to evaluate the viability of these spores. We have received reports on spore traps located in North Carolina, and these have been negative thus far. You can get up-to-date on distribution of soybean rust at the following web site: http://www.usda.gov/soybeanrust/. See what specialists in other states are recommending in terms of whether to spray or not.
In the past ten years or so, foliar fungicides have not been recommended routinely for use in soybeans in North Carolina. The exceptions to this rule have been in situations where frogeye leaf spot was found on a susceptible variety, or in instances where a grower received a premium for edible beans or seed beans. Fungicide trials typically showed a one to two bushel yield increase with the use of materials like Benlate. With soybean prices generally in the $5.00 per bushel range, this expense could hardly be justified. More recent work with Quadris at 6 ounces per acre showed yield increases of 1.5 bushels per acre in North Carolina, although some locations in Virginia and elsewhere showed yield increases on the order of three to five bushels per acre or more with strobilurin-type fungicides (Headline or Quadris). With soybean prices approaching $7.00 per bushel, the chances of recouping chemical costs from fungicide application at the R3 stage certainly improve, and may allow producers to sleep better at night. Also, consider the rate per acre of the fungicide you choose to use. In general, higher rates will provide longer residual control, so the possibility exists that one application at the higher rate may eliminate or reduce the need for a second application. Remember though, that soybean should not be sprayed beyond the R5 stage and much of our full season soybean crop, especially early maturing varieties, are nearly out of the woods as far as soybean rust is concerned. Approximately 5,000 copies of the book "Using Foliar Fungicides to Manage Soybean Rust" have been delivered to county extension offices and it is also available on-line at: http://www.oardc.ohio-state.edu/SoyRust/index.htm.
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Last modified on August 8, 2005 by Stephen J. Toth, Jr.