The lack of any positive finding of Asian soybean rust in the Mid-South makes us feel good about our earlier predictions.
We may be dealing with such low inoculum potential that it will require another weather event for concern about rust to increase.
This is not surprising, but all the information circulating may have led to a lot of confusion and some unnecessary spraying.
Current research has shown no yield benefit from spraying fungicides prior to R1. It is understandable why.
When the plant begins blooming, its physiology changes. As fruiting structures begin to form, nutrients move from leaves to developing pods, increasing the susceptibility of the plant to some diseases. Many diseases, therefore, do not appear until after R1; rust appears to be no different.
Don't forget other diseases. Look at the needs of the entire crop and what you need to accomplish with a fungicide program. Base plans on what we already know and what has made us money in the absence of rust.
If you consider that a soybean plant is going to double in height (indeterminate) from bloom on, you can see the concern of spraying too soon, particularly when rust is not present.
We have all heard that if you see rust it is too late. That is Brazil, not the United States. We and surrounding states have crews looking twice a week. If we find rust in our sentinel fields, we should have enough of a warning to develop spray plans.
Due to the absence of rust at this time, we are going to hold off spraying our Group 4 varieties until they have canopied or achieved most of their growth. Based on this decision, we are trying to accomplish a one-spray program on our early-planted Group 4s.
Another reason for our decision is that in the previous years the likelihood of increasing yields on early-planted Group 4s has been inconsistent. Numerous factors figure into this equation, but this is what we are going to do on all our SMART fields and other fields where we are advising growers. If conditions change, our plans may change, but not as of today.
We are not saying don't spray. Take advantage of the earliness factor (early maturity, higher yielding, less disease). The growth stage on a Group 4 (planted early) is a moving target. If you checked a plant today and it has eight nodes it might be at R3. Come back in three weeks and let it add 8 inches in height and it might still be at R3.
The proper place to gauge the crop is to count four nodes down from the terminal, starting with node 1 as the first fully expanded trifoliate.
When the fruit on the main stem is 1/4 to 3/16 inch in length on over 50 percent of the plants in the field, this is the R3 growth stage. Given continued growth, this is a moving target.
The range of susceptibly of the crop may be 25 to 85 days, averaging from about 45 to 48 days. As you plant earlier, the period of susceptibility lengthens. This creates the concern regarding spraying too soon.
If we can make it to R4 to R5 on Group 4s, we can control other diseases (possibly late-season cercospora) and protect seed quality. On our Group 5s, an R3 through R4 application has been a consistent moneymaker. When you need to spray will be based on what is present in the field.
We will spray a lot of this crop, but cropping history, yield potential, current needs and timing will determine what approach we utilize.
When you spray and how many times (in 2005) will be a reflection of how much risk you are willing to take. Given the lack of rust the risk is not much.
We have a crew looking for rust twice a week. One of the best field pathologists in the country is heading up the effort in Mississippi. Sampling involves checking ultra-early-planted soybeans (sentinel fields), volunteer soybeans, kudzu and other legumes. We just completed our eighth survey throughout the state and have not observed any positive findings.
Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected]