Leaf Rust economic threshold
Leaf rust in wheat is expanding exponentially in our area (East of Denver, CO and No. of I-70). The wheat is beginning the head-out. How does one determine the economic threshold for spraying, specifically, once the flag leaf has had some infection? What fungicides are effective?
Adams County Colorado field crops
I think that you have already reached the threshold and it would have been wise to spray as soon as you saw rust pustules on the lower leaves. I would think that you have stripe rust as a primary agent and you may have some leaf rust as well. To know the economic threshold we need to make some assumptions. I assume that you have wheat that you expect to yield over 40 bushels per acre. The spray and application is going to cost about 4 bushels per acre so the spray needs to save that amount of yield to be economically feasible. You will need to spray a curative fungicide and not a preventative fungicide. Here are two links that should help you. The second one is about fungicides but I would rely heavily on your fungicide applicator for availability and choice of fungicides.