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By J. P. Gilreath, J. W. Noling and P. R. Gilreath Research conducted here in Florida over the past 6 years has demonstrated that few of the products currently available are likely replacements for methyl bromide in most vegetable crops. Few provide reliable control of soilborne diseases and nematodes and none are that effective against weeds, especially purple nutsedge. While Vapam can control many weeds, and has controlled nutsedge in some other regions of the USA, it has not performed well in Florida, other than in a few limited instances. Of those products which offer some degree of control for nematodes and soilborne disease pathogens, statewide research has demonstrated that the most consistent performance has been obtained with a combination of Telone and chloropicrin, in varying proportions. This combination, sold under the trade names Telone C-17 (17% chloropicrin) and Telone C-35 (35% chloropicrin), appears to be what most growers will be using in a few years. It provides nematode and disease control similar to that obtained with methyl bromide, but weed control is sadly lacking. Thus, a herbicide will be required in most instances. This sounds simple enough, but the choice of herbicides for most vegetable crops is rather limited and it is even more limited when you select for only those herbicides which will give acceptable control of nutsedge. What is acceptable control? Well, today we think in terms of no less than 80 to 90% control because that is what we can expect with methyl bromide, but 5 years from now our expectations may change as we realize that even some control is better than none at all. Tomato growers are perhaps the most fortunate of all vegetable growers because they have a wider choice of herbicides and that list includes Tillam which will provide good nutsedge control if applied correctly under proper conditions. Notice there are a few "ifs" in there. The ifs are crucial to good performance with Tillam. At this stage, the luck of tomato growers is not quite as clear cut as indicated because the Tillam label currently restricts its use to only mechanically transplanted tomatoes. Strictly speaking, most Florida tomatoes are hand transplanted and that means Tillam. is not an option, unless you are willing to purchase and use a mechanical transplanter or the label is modified to allow hand transplanting. Presently, the manufacturer, Zeneca Ag Products, is working with FFVA, the state of Florida and the EPA to address that problem. Hopefully the Tillam label will be resolved soon. But how good is Tillam compared to methyl bromide? Tillam has been combined with Telone C- 17 and Telone C-35 in many experiments on University of Florida research centers and on commercial farms and, in general, has provided nutsedge control comparable to that obtained with methyl bromide. Control of other weeds has been as good as or better than methyl bromide in most cases; however, there have been some examples of less than ideal performance. There are a number of do's and don't's with Tillam and we hope to address them here. Tillam is a very volatile herbicide and must be incorporated as soon as it is applied; not 5 minutes later, but immediately. This means it must be applied and incorporated in the same pass. One of the easiest ways to achieve this is to place the spray boom on the implement frame so the herbicide is applied just in front of the implement. This reduces the time between application and incorporation to less than a second. Thoroughness of incorporation and proper depth of incorporation are as critical as timeliness of incorporation. Also, it must be applied to a soil which has been well prepared, not one that has a lot of trash in it or clods. The soil should be just as you would want it to be for a good seedbed. Small weeds present generally are not a problem, but large clumps of grass and nutsedge 6 inches tall or taller will reduce performance greatly. Tillam can be incorporated with a disk, a field cultivator or a rototiller, but whatever means you choose, you must be certain to assure uniform mixing throughout the soil profile for that soil which will end up in the bed. Since most bedders pull soil from about 8 inches deep, this means you must achieve incorporation to that depth. In some cases, 6 inches in adequate, but that is usually where bed formation will not use soil from below that depth. A rototiller and a field cultivator will mix to the depth at which the implement is positioned, but it is generally felt that a disk will not. A rule of thumb for a disk is to run it 2x the depth to which you want uniform incorporation. That means that if you want Tillam mixed to 8 inches, you need to run the blades at 16 inches. That may be a bit deep, but 12 inches deep should be acceptable. One of the problems with disk incorporation is that the label states that you should double disk the field at right angles which we cannot do due to our field ditches. A second pass across the field in the same direction but offset a bit from the first pass would be a worthwhile investment, if you are going to use a disk. Speed of disking and field cultivating is important for good mixing. Slower speeds do not mix that well; you need to run at least 6 mph to really move the soil and work it well. Speed with a rototiller takes the opposite approach, you must go slow enough to thoroughly work the herbicide into the soil. Too fast and you do a poor job of mixing. Soil moisture is very important. Although the label states apply to dry soil, our experience has been just the opposite. A dry sand tends to be warmer than a moist one and this is a very volatile product. The speed of volatilization will increase with increasing temperature. Some of the worst performance we have obtained was when Tillam was applied to a soil that was on the dry side and the cultivator fluffed it up and allowed it to dry even more. Bed formation was delayed by 2 days in order to bring the moisture back up so firm beds could be formed and the resultant nutsedge control was greatly reduced. Although a field cultivator is preferable for incorporating Tillam, this is one time when a disk might have been a better choice of implements. If applying Tillam more than a day ahead of bed formation, consider packing the soil with a roller after application to decrease aeration of the soil and to improve moisture retention and movement up from undisturbed depths for those growers who use subsurface irrigation. If incorporating with a disk, a drag can help level the soil surface and lightly seal it so that moist loss is diminished. Drags have not proven that helpful with a field cultivator. Whatever you do, make CERTAIN that you have accurately calibrated your herbicide applicator so you are applying the exact rate you need. Be sure that you are using the same size nozzles all the way along the boom. Differences in nozzle sizes can produce hot spots or streaks in the field and this can result in crop damage and/or poor weed control. Calibrate in the field, not on the field edge. Soil conditions in the field are often quite different from those on the firmer ground associated with field borders and that can change ground speed and the delivered volume of spray mix per acre. Pepper growers are less fortunate than tomato growers; they have few herbicides from which to choose and none of them control nutsedge very well. However, remember our concept of what constitutes good control may change with time and this may be one of those cases. Of the products registered for weed control in pepper, only Devrinol is likely to give much control of nutsedge and the level it will provide can range anywhere from about 0 to 70%. Most of the do's and don'ts associated with Tillam apply to Devrinol as well. But let's make one thing clear, Devrinol absolutely must be incorporated thoroughly to provide any measurable degree of nutsedge control. Applying Devrinol to the surface of the bed prior to laying plastic may give you control of many weeds, but it will not provide any control of nutsedge because nutsedge tubers can sprout and emerge from any depth in the bed. Most of the nutsedge in beds emerges from the top 4 inches, but deeper tubers can emerge if conditions are right. Devrinol is not, volatile like Tillam, but it is rapidly destroyed by sunlight and can be completely degraded if left on the soil surface for more than a day. just as with Tillam, it is best if Devrinol is incorporated at the time of application. Telone formulations have been the most effective fumigants for pepper. Combinations of Telone C- 17 and other fumigants with Devrinol, Dual and Tillam have not produced deleterious effects on pepper except some occasional injury from Tillam and Dual. Although Tillam seldom has been injurious in experiments at Bradenton, it has produced damage in some trials at other locations and is not labeled for use in pepper. Tillam is not likely to be labeled in pepper due to this issue. Dual has a third party registration in pepper row middles in Florida, but the manufacturer has not expressed interest in full registration in the bed until recently. Peppers are very rate sensitive. Dual at I pint per acre is safe but 2 pints produce injury sometimes. Unfortunately, the I pint rate does not give very good control of nutsedge. Dual needs to be incorporated in the bed, just like Tillam and Devrinol. In fact, all of our work indicates that whatever the product, if it is not incorporated to the full depth of the bed, nutsedge control will be reduced. One interesting aspect of all of this work has been the effect these alternatives have on double cropping. The economics of tomato production, and pepper to a lesser degree, are such that many growers have found it necessary to double-crop some other crop in order to make the farm profitable. Along with residual fertilizer and mulch comes residual pest control as a result of the use of methyl bromide. The use of alternative fumigant/herbicide programs adds a certain degree of uncertainty to this practice. If the alternative is not as effective against the existing pests, the second or double crop may suffer. Experiments have been conducted to address this issue with varying results. Of all of the alternatives, only Telone C- 17 or C-35 combined with the appropriate herbicide allowed double cropping without serious yield reductions. In some experiments, even Telone C-17 with herbicide did not control soilborne pests for a long enough time to allow the double crop to produce as well as methyl bromide. The duration of control is not really a result of some residual level of Telone and chloropicrin in the soil, but rather is an indication of just how well it controlled the existing pests to start with. Herbicides are different in this regard in that residual control is a factor as well as the initial level of control. For example, in one recent experiment, Telone C-17 controlled rootknot nematodes as well as methyl bromide and tomato yield was not different; however, a cucumber double cropped experienced slightly reduced yields and the incidence of root knot galling was much higher than with methyl bromide. So what happened? Well, while there was no statistical difference in numbers of rootknot nematodes at the end of the tomato crop, there were a few more in the Telone plots. These few more nematodes allowed the population to grow faster and the extent of galling was greater because the nematodes were there in higher numbers to inflict damage earlier. But what about the residues of the herbicides themselves, will they not cause damage to the double crop? Based on work with cucumbers and squash, there does not appear to be enough residual herbicide left at time of planting to injure these crops where Vapam, Basamid, chloropicrin or Telone C- 17 has been applied the previous season. However, in some experiments, the level of herbicide has been sufficient to induce damage where the herbicide was used in combination with methyl bromide. Experience is limited with other possible double crops, so growers are advised to test it in a small area on their own farm. In fact, testing any new practice on a small basis is a good idea, making certain you compare it to your standard practice. Mistakes made on a small basis are affordable, but big ones can be devastating. This is why growers need to begin trying Telone C-17 or C-35 and herbicide on their farms. just as there are tricks to the proper application of methyl bromide, there also are tricks to the proper application of Telone C-17 or C-35 and herbicide. Waiting until you have to use an alternative on everything can be disastrous. You need to be learning now, not at the eleventh hour. The level of residual pest control with any of the alternatives may not be good enough for a grower to afford to double crop. This decision will have to be based on the individual's situation and the level of risk he is willing to assume. What about solarization and double-cropping? We have an experiment that just finished its first year of a three year study which includes solarization and double-cropping. Although we have seen solarization work well for nutsedge control, we have seen it not work more times than work. Solarization has possibilities but a lot more work needs to be done to identify the limitations and solutions for them and to clearly determine where it will fit in. In the study in question, double-cropped cucumbers had approximately one-half the yield with methyl bromide. Rootknot nematodes and nutsedge were serious problems for the staked cucumbers and obviously played a significant role in the yield reduction The future of solarization remains a bit cloudy at this time. So far we have mentioned tomato and pepper; crops for which we have some options for weed control that will give some degree of nutsedge control, but what about the myriad vegetable crops with no options? What about watermelon, another one of the larger users of methyl bromide in Florida? There is the possibility that we may get a postemergence herbicide labeled for use in watermelon, tomato and pepper which will control nutsedge. Permit is a product of Monsanto which is not labeled at this time. At least one company has been discussing with Monsanto the possibility of acquiring marketing rights for Permit in vegetable crops. If this happens, tomato, pepper, and perhaps melon growers, will have a very useful herbicide. This would allow a grower to use some other herbicide in the bed to control other weeds then, if nutsedge became a problem, the grower could use Permit to control the nutsedge. Hopefully we will know something in the not too distant future. What if we don't have a herbicide to control nutsedge or for some reason we do not want to use one? What if the chance of a yield reduction for double-cropped cucumbers with Telone C- 17 or C-35 is not a chance we can afford to take? We need to remember that soilborne pest control is not so much control as it is management. Methyl bromide never completely eradicates nutsedge or nematodes. If it did, we would only have to treat once and keep the pest from being re-introduced. Fumigants and other pesticides allow us to reduce the pest to manageable levels. In the future we need to start thinking in terms of a soilborne pest management system for farms. We need to know what pests are problems in the fields we farm - this means start writing it down, so you can go back and refresh your memory. None of us remember as well as we think we do, especially after the passage of considerable time. This information will help you decide which blocks would be suitable for a given crop and avoid those with known problems for which you have no solution. For example, if you are going to grow tomato and pepper and you have a block that is heavy in nutsedge, you might be well advised to grow tomato in that block so you could control the nutsedge with Tillam. Or if you have a block with a history of rootknot nematode problems, you might farm it one season, but double-cropping it would not be a wise choice. There are ways to manage some pests and we need to think in those terms as we plan our production operations. Soil pH management can have an influence on some organisms. For example, raising the pH to above 7 can greatly reduce the incidence of Fusarium wilt because the causal organism does not thrive very well above that pH. Years ago growers allowed water to stand on their fields during the offseason. This can aid greatly in managing a nematode population. Most growers want to maintain good drainage so they can leach out the residual fertilizer salts, but the advantage of raising the water table can be a compelling reason to do so. Earlier researchers like Overman and Jones noticed that water that was pumped into a field was not as good as rain water for this activity, presumably due to the dissolved oxygen in the flowing water providing the nematodes with needed oxygen. Fluctuations in temperature and moisture can have a great impact on biological activity. Varying the temperature often can cause more damage than holding the temperature constant at the peak temperature. Allowing water to pond then drain may be beneficial by fluctuating the redox potential of the soil. It can stimulate weed seed, nematode eggs and disease spores to initiate growth and make them susceptible to control agents. Dormant nutsedge tubers are very difficult to control; whereas, tubers which are actively sprouting are much easier to kill. The same is true for spores of many diseases and nematode eggs. Some crops and cover crops affect nematode populations by being poor hosts or by not favoring development of a large population. Rootknot nematodes have been reported to produce fewer eggs on squash than on tomato. Some cover crops do not support rootknot but do support other nematodes. You need to consider the cover crop and what crops you grow and how they will fit together. Growing a cover crop which serves as a good host for rootknot nematode, then planting tomato in that field, is not the wisest decision. Offseason weed management can include disking to reduce weed growth and seed production, but clean fallow carries a price -it burns up your already limited organic matter. Some cover crops favor nutsedge growth and development while others do not. If you have purslane, there may be a cover crop which is better suited for it than sorghum. Roundup can be used in an offseason program of spray, wait two weeks and disk then allow it to come up again and repeat. There may be other herbicides with a short residual which could allow you to reduce the population during the offseason and not be present in sufficient quantity to inflict damage on your next cash crop. Methyl bromide has allowed us to forget a lot about soilborne pest management. Now it is time for us to relearn and expand our knowledge. J. P. Gilreath, associate professor, Gulf Coast Research and Education Center, University of Florida, Institute of Food & Agricultural Sciences, Bradenton; J. W. Noling, associate professor, Citrus Research and Education Center, University of Florida, Institute of Food & Agriculture Sciences. |
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