The use of foliar fungicides in soybeans is affected by time of planting, crop history, variety disease package, and weather (current and future). Some soybean acreage will benefit from a foliar fungicide, but other acreage will not.
Weather over the last several weeks has us set up for problems. Aerial blight, downy mildew, SDS, and frogeye have increased rapidly because of the recent weather. In some fields, the diseases are as severe as any I have ever encountered.
Foliar fungicides can provide excellent benefits. It all depends on when you use them and what you are using them to control. The best benefits will come from use before diseases get out of hand. I like the preventive approach; unfortunately, control is often delayed until symptoms are visible. Foliar fungicides are insurance. If you knew you were never going to have an accident, you would not need automobile insurance. The same is true for fungicides.
A large portion of this crop is extremely early. In fact, some of the Mississippi soybean crop on July 11 was about two weeks from being at the R6 (complete seed fill) growth stage. This crop needs little help, and some acreage will be a candidate for a fungicide application.
There are some basic strategies for fungicide applications. A late shot (R5-R6) can aid in controlling late-season cercospora and/or improve seed quality. There are two options, Quadris or Topsin M. Thus far, Quadris has not controlled late-season cercospora — at least not at the rates or timings we are using. Seed quality can be improved with Quadris at the 6-ounce rate, but I believe 4 ounces will do the job.
A well-timed application of Topsin M can control late-season cercospora and benefit seed quality. If it is applied too early, it might not have the staying power to help late in the season. Cercospora appears to have gotten worse the last few years, particularly on early-planted Group 4 varieties. If you want to control cercospora, do not wait too late.
Topsin M has a generic counterpart called Thiophanate-methyl (TM 85). The rates are 0.75 pound of formulated product for Topsin and 0.6 pound for TM 85.
Plantings after mid-April to mid-May have excellent yield potential. The exception may be where flooding or prolonged wet feet have affected yields. If you think you have a good crop and have not had any major problems, consider a fungicide and Dimilin combination. We are not applying Dimilin to the early crop. However, our plans are to apply Quadris (4-ounce rate) or Topsin M (0.75 pound) or TM 85 (0.6 pound) plus Dimilin (2 ounces per acre) to beans planted in this window.
Applications to late plantings should be based on yield potential, cropping history, and the outlook for future weather.
We are not making blanket applications of fungicides. On early-planted beans (prior to mid-April) we are considering a quality shot. We are trying to avoid what happened in 2001.
How much you treat will depend on your number of acres, your crop's growth stage and your combine capacity.
Although elevators want this crop early, they do not want problems. We know what options are available, but there remain two questions: “Is the disease situation going to get worse?” and “Is August going to be dry?”
Conditions over the last few weeks have the potential to greatly enhance disease development, but dry conditions could halt development. These decisions are not cut and dried, but year after year, even if a fungicide application does not allow you to achieve a big increase in yield, the increase will be sufficient to pay for the application.
Do not forget to pay attention to irrigation needs and insects, primarily stink bugs.
Alan Blaine is the Mississippi Extension soybean specialist. e-mail: email@example.com