There are several schools of thought regarding Asian soybean rust I feel need clarifying. I hear that many farmers plan to spray at first bloom. Preventive control is the best method for controlling diseases, but paying attention to what is found will be better than trigger spraying this growing season.
Although rust has been found in Florida and Georgia, prevailing winds appear to be in our favor in Mississippi. Spraying prior to bloom or prior to pods beginning to form probably will be premature this year.
We had a pretty decent winter across the South and since rust entered the United States late in the fall, it very possibly did not overwinter at any significant level.
Another thing to consider is history. It took rust 102 years to reach the United States, and in every country it has inhabited since its discovery in 1902, the yield impact has been minimal the first year. Our experience should be no different.
Once rust is established, an at-bloom application followed by a second application approximately 21 days later may be the best approach. That would be preventive control and the best and most cost-effective way to control a disease.
Your decision to use a preventive spray this year or wait until rust is found within the state may be a result of how much risk you are willing to take. It has been recommended (in Brazil) that spray applications start at R1 (first bloom). If rust has not been found in Mississippi and you want zero risk, R1 is probably the time to start spraying. In the future we may find that our timings will differ due to numerous factors, such as overwintering potential, amount of inoculum, and future weather.
The R1 growth stage is when you see the first bloom on the main stem. The R3 growth stage is when developing pods are 1/4 to 3/16 inch long. R5 is the stage when pods are fully elongated and just beginning to fill seed. Count four nodes down from the terminal when making this determination. The R3 and R5 growth stages are when we applied Benlate fungicide twice years ago.
From R1 to R3 is about 21 days; from R3 to R5 is about the same. Research has shown no yield benefits from spraying fungicides prior to R1. The maturity group, planting date, and environmental conditions all determine the period of susceptibility. However, this may range from 25 to 85 days with an average of about 45 to 48 days.
If you plant early or plant a full-season variety, your period of susceptibility lengthens. In a worst-case scenario these two situations could increase spraying, and for this reason I recommend you wait until we see where and how widespread rust is. You could spray too soon. Timing is the key.
Do not get pushed or scared into jumping the gun. We are trying to make this easier, but you need to stay informed. Sort through the information to arrive at the right decision.
One thing that shocks me is the number of growers who are going to spray regardless. On one hand, I am proud that most consider rust a serious threat, but I am surprised many might spray whether rust is present or not — I have seen worms eat a crop up because many acres went unsprayed, and we can see the worms.
Do not misunderstand. We are going to use foliar fungicides this season. We hope we can make the initial application on early planted beans after R3 and on Group 5's at R3. If we get to this point without rust, our next decision will be what and how much to use. That decision should be based on other diseases present, availability of materials, efficacy and (most importantly) cost.
Keep rust in perspective. Regardless of its presence, continue to use cropping systems and inputs that produce the greatest return.
Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected]