FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina and may not apply in other areas.
As mid-May approaches, producers may be tempted to omit an at-planting insecticide and rely solely on foliar treatments for thrips as we get further into better germination, stand emergence and seedling growth conditions. This option is not recommended. As a general rule of thumb, producers can probably cut back on the rate of their at-planting insecticide to the tune of 40 percent (e.g., 3 versus 5 lb. of Temik 15G) after about May 15 due to the anticipated shorter time period in which seedlings must be protected (the time between expanded cotyledon to 6 to 7 true leaf stage), and the typical decline of migrating adult thrips populations after the 4-week or so at-planting insecticide protection window. However, because late-planted cotton can not afford to be further delayed in its development, protection from thrips damage, though for a shorter time interval, is extremely important. Therefore, at least a 60 percent rate of an at-planting insecticide is recommended for all late-planted cotton.
Scouting for Thrips
Plants should be checked approximately weekly for live thrips and their damage beginning around May 10, or perhaps a little later due to our cool recent weather. Until roughly mid-May, thrips are usually associated with their wild hosts and have not begun their migratory flights onto cotton and other crops. Examine the newly emerging first true leaf (in younger plants) or the terminals of 25 to 50 plants throughout the field. In the case of the first true leaf showing through the fully-expanded cotyledons, look for silvery patches on the cotyledon leaves and thrips in the growing point area. Remember that the cotyledon leaves can be pretty weather beaten without this damage being due to thrips injury. This early diagnosis is somewhat difficult, but the timing of a needed treatment at this stage offers protection at a critical time in the young seedling's growth. In the more common case of seedlings with two or more true leaves, the presence of both crinkled or possum-eared leaves and the presence of immature thrips should be confirmed prior to remedial treatment. Magnification via a 15 to 30x hand lens may be required to find the active, off-yellow immature thrips. Emphasis should be directed toward the newer unfurling leaves -- older leaves remain crinkled and thus reveal little, if any, recent information. Alternatively, several seedlings may carefully picked and rapped sharply against either a white or a dark flat surface (an index card or a shallow box will work). A treatment threshold of an average of about 2 immature thrips per plant is suggested. The presence of brownish or blackened buds along with thrips is often an indication of moderate or severe stunting.
As a general rule of thumb, earlier planted fields require a more extended period of protection, and may show thrips symptoms first. Also, Temik at times provides more extended thrips control than Thimet, Di-Syston, Payload and certainly Gaucho-treated seed. Finally, hot, dry windy weather often generates higher adult thrips populations flying from drying alternate hosts into cotton. Try to respond to these potential differences in thrips levels and injury by grouping fields -- a foliar treatment may be required in some situations and not others on the same farm. Remember, that as June approaches, treatments with any of the foliar insecticides recommended for thrips can destroy beneficial insects to the point that the odds of having to deal with June to early July tobacco budworms (except on Bollgard cotton) and cotton aphids may increase greatly. Good pesticide stewardship and economics dictates that cotton be treated for thrips only when needed, especially in the southern half of the state. Automatic foliar treatments for either thrips or plant bugs should be strongly discouraged. However, thrips should be regarded as a serious maturity-delaying, yield-reducing pest.
Growers and their advisors should be alert to flea beetles on tobacco for the first three or four weeks after transplanting. The threshold for treatment at this stage is four beetles (some states use three) per plant. This level of infestation is, thankfully, relatively uncommon. Special care must be taken in evaluating flea beetles if Admire has been used. Flea beetles affected by this material may not die quickly but are made very lethargic. Since these beetles do not jump away as readily, they are very obvious and counts may be artificially high. However, these affected beetles are not actively feeding. Thus, when Admire has been used, decisions about remedial treatment should be based on the level of damage and whether new damage is occurring.
Hessian flies are present in many wheat fields. Richard Rhodes, Tyrrell County Chairman, investigated a poor, unevenly growing stand of wheat and found plants to be commonly infested with the Hessian fly puparia (the resting stage). I observed about 75 percent of the stems taken from this site to be infested with one to six puparia. Visually the plants had many unthrifty stems with small heads. There was a lot of variation in stem and head size between infested and uninfested stems. Although our recommendation for Hessian fly is to avoid early (October) planted wheat this infested field was planted in November planted and shows that the fly free date concept does not always fit in North Carolina, especially in years of warm fall weather.
Hessian fly infestation is not obvious. The larvae (maggots) and puparia are located between the lower leaf sheath and stem, just above a lower stem node (usually the first and/or second node). Pulling the leaf sheath away with the fingers does not work well and is tedious. Using a sharp scissors to cut across the stem plus sheath just above (ca. 1/4 inch) the node to expose the larvae or puparia is an easy way to tell if stems are infested. Cross-cutting stems may sever the puparia or larvae if the stem is cut less than 1/4 inch above the node but the insect may not be visible if cut much more than 1/4 inch. Larvae are white maggots and puparia (called flaxseeds due to their resemblance to the plant seed) are dark, shiny mahogany colored resting stage (pupal stage) of the insect.
Typically we do not consider the Hessian fly a very significant pest. However, a survey about 10 years ago showed it was generally spread across North Carolina and was above threshold in many fields. During my tenure at Plymouth I have seen few instances where the Hessian fly has obviously injured wheat. In general, wheat growers pay little concern to the Hessian fly and perhaps this is okay. However, the Hessian fly can be very damaging under conditions of high infestation and our increasing wheat/no-till soybean rotation may be increasing the abundance of this pest. The Hessian fly has a short life cycle and as long as adequate hosts are available during warmer weather it can continue to reproduce and build-up. In the wheat/no-till soybean culture volunteer wheat can serve as a host to bridge one wheat crop to the next. This may be leading to a general increase in the Hessian fly.
Hessian fly management follows three strategies: 1) avoiding early planted wheat; 2) using Hessian fly resistant cultivars; and 3) Gaucho seed treatment. As mentioned earlier the planting after the so-called fly free date of November first does not always work although it probably is almost always partially effective. Varietal resistance is also a on and off situation because North Carolina Hessian fly biotypes can attack many so called resistant varieties. Lastly, Gaucho seed treatment is reported to give good to moderate results against the Hessian fly. There may be limiting plant-back restrictions with Gaucho.
Few issues have frustrated farmers more than the lack of effective fungicides for the control of Sclerotinia blight. Growers are asking: Why can't we use some of the compounds we know will work against this disease? and What is taking so long? I am writing this article to help shed some light on this important issue.
Sclerotinia blight has been in North Carolina for about 25 years. For several years it appeared to be restricted to Virginia production area. However, in time this devastating disease moved south and is now found in every peanut county in North Carolina. Long rotations and avoidance of susceptible cultivars are the primary methods of Sclerotinia blight management. Breeding resistant cultivars for this disease has been difficult, however, promising lines are now being evaluated. Unfortunately, resistant varieties currently available, and rotation are not enough to prevent significant yield losses in fields where there is a history of severe disease.
Years of disappointing results with the performance of Rovral, its high price, and the continued spread of Sclerotinia blight has led to much unhappiness. If that was not enough, Ronilan, a fairly effective fungicide for Sclerotinia control, received a registration for use on strawberries but not on peanuts. This seemed to be either a cruel joke or another example of illogical policies. To add insult to injury, fluazinam, a promising new chemistry that had been tested for many years, seems to never quite make it through the registration process. And to polish it off, Oklahoma is requesting a Section 18 (an emergency exemption from label requirements) for Botran, whereas North Carolina is not. I will attempt to make sense of these issues by discussing the strengths and weaknesses of each compound, the registration status of each, and the reasons for the approach I have taken to address the need for effective fungicides.
Fluazinam
This experimental compound is the most effective sclerotinia blight fungicide I have tested. However, it can fail. I have seen it do poorly in very wet conditions. However, if applied prior to infection, this compound will out perform all other materials tested to date. While registration has been anticipated for some time, the process came to a halt while the basic manufacturer merged with a new marketing firm. I understand that the registration process is back on track. Too many unknowns remain in this process to guess when it will make it to market (definitely not this year). The limiting factor in the registration process is the data package that must be submitted to the EPA by the company to fulfill the requirements for labeling the product. I have offered to submit a Section 18 to allow our growers to use the compound in 1998. However, the company can not support a Section 18 at this time due to insufficient data that must be generated by the company. All university field trials have been concluded that are necessary. Therefore, we are waiting on the company to finish this work.
Ronilan
Ronilan is currently registered for use on strawberries for another disease. While common sense would argue that this would be a compound that is safe to humans and should be registered, the fact is it will never be registered for use on peanuts. There are real safety considerations due to residues found in the nutmeat that the company is not going to resolve to the satisfaction of the EPA. Ronilan residues in peanuts are viewed as a risk to children. The company that makes this product is sensitive to the use and misuse of this product and will not support any type of label or Section 18 exemption for use on peanuts. Without their support, the EPA will not grant any form of labeling or exemptions.
Rovral
Rovral became registered because it controlled approximately 30-45 percent of the disease when used in carefully controlled experimental plots, and because there was no other registered compound. Rovral is expensive, however, it often pays for itself when used properly. It is important to understand that Rovral, like all fungicides that I have tested, works best when applied prior to symptom development. Proper timing Rovral or other fungicides requires that fields with a history of severe disease be treated when plants begin cover the ground between the rows and weather conditions turn favorable for the disease. Rovral will continue to be our only registered product through the summer of 1998.
Botran
Botran was one of the first compounds to show some activity against Sclerotinia blight. In the 1970s, North Carolina State University petitioned the EPA for an emergency exemption to the registration process (Section 18) and was granted temporary permission to use the compound in certain areas of the state that were considered to have an emergency. The company that produced this compound either did not try, or was not able get registration. Yearly exemptions were received from the EPA to use Botran until the early 1980s when Ronilan was shown to be a more effective compound. This compound was substituted for Botran in future Section 18 applications until Rovral received a federal label for Sclerotinia blight. At this time the EPA quit issuing Section 18 exemptions for Ronilan as the emergency was over.
Recently, Botran was acquired by Gowan Company. Gowan is submitting for a full federal label for the peanut market. It is not clear when, or if, they will receive it. Oklahoma will be submitting a Section 18 for use of Botran for the 1998 growing season.
Rovral Versus Botran
Field tests conducted in North Carolina from 1983 to 1990 compared Rovral and Botran activity against Sclerotinia blight. These tests showed that Rovral was as good as Botran at improving yields in fields with a history of disease. The results from North Carolina are in agreement with considerable work done in Virginia with the same compounds. There is no advantage to the use of Botran economically. North Carolina will not benefit from a Section 18 for this product in 1998 and there is no scientific merit for submitting an application.
Conclusions
Sclerotinia blight represents a serious threat to many of our fields. There is intense research activity at the university exploring every conceivable approach to the control of this disease. We are cooperating with our colleagues at other universities where Sclerotinia blight research programs are conducted. Whenever there is a new approach that is effective, we do what it takes to get this information to you. However, no mater how much we want a good Sclerotinia blight fungicide for this season, there will not be one. Consequently, it is to our advantage to make the most of what we can with the tools we have. Recommendations for the 1998 season are outlined in "1998 Peanut Information" found at your county Extension office.
Sclerotinia blight control practices for 1998 include:
Web page last updated on May 11, 1998 by Stephen J. Toth, Jr..