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Apply as warranted: Fungicides: to spray or not to spray?

The dry weather we have experienced in the last weeks has not been very conducive for disease development. However, the recent rains in some areas could contribute to an increased incidence.

The report of spores in Louisiana should not be misinterpreted. There were not enough spores for a positive identification, but I hope the announcement heightened awareness. We have 10 spore traps out and have not had any reports yet. Thus far Asian soybean rust has not been observed in the Mid-South. Given the absence of rust, we plan to apply fungicides as warranted.

The decision to use a fungicide in the absence of rust is not cut and dried. Response to a fungicide on early-planted Group 4s is not always positive. Early-planted, early-maturing soybeans avoid a lot of late-season disease pressure. Once rust enters the picture, that might change, but for now, it has been inconsistent.

As you plant later or use a fuller-season variety, the potential for a positive response increases.

Yield increase is somewhat of a guessing game. In addition to the points I mentioned above, crop rotation plays a major role. This will not be a factor if rust becomes prevalent, but it is today.

Another factor is yield potential and whether the field is irrigated or dryland. Some soils have a greater, more consistent yield potential than others. Irrigated beans are a prime candidate for a yield enhancement application because of yield potential and microclimate. Non-irrigated soybeans can benefit, depending on yield potential, planting date and crop rotation.

Since this crop has grown off slower than crops in years past due to the long, cool, dry growing season, we are going to delay fungicide timing slightly. Our plans are to allow the early-planted crop to achieve as much vegetative growth as possible prior to spraying. This will provide protection on most of the foliage.

Because of this delay, we will probably be spraying closer to R4 than R3, which should afford us the opportunity to spray once instead of twice. This is only being considered because no rust has been found. In the future, based on what we know today, it may require two well-timed sprays, but only time will tell.

Some growers sprayed too early due to concern about rust or in an attempt to piggyback fungicides with Roundup. Regardless, without rust present, sprays prior to R3 are too early and will not have the staying power to protect soybeans through the entire pod-fill period. Spraying between R3 and R4 will get you closer to season-long protection, but probably will not carry us all the way through.

Some growers are spraying just as insurance. This is an option, but when you are intensively monitoring a crop, numerous scenarios will result in top yields without a fungicide. These fields will more likely be soybeans that are rotated with another crop instead of soybeans following soybeans, and most likely they will be varieties planted early.

Plans must evolve around controlling other diseases, not just rust. Preventive disease control is the ideal way. Waiting until you see symptoms and then trying to control a disease will cost you yield.

Another factor that makes this somewhat of a guessing game is dryland beans. If you spray today and it does not rain anymore for six weeks, it is doubtful any increase will be gained. You must consider all these factors when making a fungicide decision.

Year-in and year-out, fungicides will pay — some years better than others. What you use, what you want to control, the rate, and when it is applied will determine the degree of response.

Do not misunderstand me. Fungicides are integral tools in crop management and most of the time they will make you money. In the instances they don't, they will almost always pay for themselves, but I like to do better than break even. Consider the points I have mentioned above and you will improve your odds.

Supplies of certain materials are extremely limited. In Mississippi, we all agree that rust or no rust, at R3 our base fungicide program will be built around a strobilurin. These may be used alone or in a mix, but this chemistry has given us our best yield increases. With supplies of products such as Quadris and Headline limited, the mixes will be used in greater quantities. There are numerous options, pre-mixed or mix your own. The biggest difference between a straight strobilurin and the mixes will be broader spectrum of control and additional cost per acre.

Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected]

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