We have been evaluating foliar fungicides for the control of leaf rust (Puccinia triticina) since 1985, and believe we have accumulated enough data to be able to predict which varieties will provide a positive return on investment when sprayed with a foliar fungicide. We do not have as much data on stripe rust (Puccinia striiformis), but have concluded that it is a more devastating disease than leaf rust, and fungicide applications on infected wheat are likely to be even more profitable.
A common treatment in all of our foliar fungicide studies has been Tilt at 4 ounces per acre for the control of leaf rust. This treatment was compared to the untreated check in each experiment to lay the groundwork for predicting the economic benefit of a foliar fungicide application to control leaf rust on susceptible wheat varieties. The following parameters were used in these experiments:
All trials were replicated 4 to 6 times in a randomized complete block experimental design.
Final flag leaf infection levels were rated in early May, which roughly corresponds to the soft dough stage in kernel development.
Tilt was the only common treatment throughout these trials, but it should be noted that Tilt is a first generation fungicide, and the weakest fungicide in almost all of our trials. Headline, Quilt, and Stratego were more efficacious in most trials, and produced better leaf protection and greater yield increases.
How can this information be used to develop a way to predict the profitability of fungicide applications to control leaf rust?
We made the following comparisons:
We compared the percent yield increase in the treated wheat with the final percent flag leaf infection level in the untreated wheat, and it appears that, as anticipated, the relationship is linear: the lower the infection level, the lower the percent yield increase; the higher the final infection level, the higher the percent increase. In summary, the greater the infection, the more leaf damage and the lower the yield.
How can this information be used to predict a variety's response to a fungicide?
If we use the final flag leaf rust ratings from the variety trials from last year (Table 1), we should be able to predict closely which varieties are most likely to respond positively to a fungicide application this coming year, assuming no major rust race changes in one year.
For practical purposes, you can add the leaf and stripe rust values together to come up with the total leaf area infected. Both diseases do similar damage to the leaf tissue.
We are also assuming that the leaf rust pathogen will be present in the wheat crop. We can remember only two years out of the past twenty where rust was not present. Since wheat was planted all the way down to the Texas coast this year, we likely will have plenty of rust spores blowing up this way on the southwesterly winds.
Summarizing all of our research since 1985, any variety with a final flag leaf rust rating of 48 percent or greater produced a yield increase ranging from 12 percent to 40 percent (average 26 percent) when sprayed with a foliar fungicide. Assuming a minimum potential yield of 40 bushels or more per acre, the fungicide treated wheat would produce an average of 10.4 bushels per acre more than the untreated wheat.
Even at wheat valued at $2.00 per bushel, the fungicide treatment would pay for itself. Using this line of reasoning, Pioneer 25R57, Coker 9553, Terral TV8558, and Pioneer 25R37 will respond positively to a foliar fungicide if leaf rust infections are present by the time wheat begins to head this spring. At $6.00 wheat, it would also likely pay to spray Terral TV8331, Terral TV 8466, and Pioneer 25R49.
Wheat prices at $9 to $10 will justify a fungicide application to most commercial varieties planted in Northeast Texas. The only variety with complete resistance to both leaf and stripe rust last year was Terral LA 841. Unless there is a change in rust races this year, it will probably not pay to spray this variety.
What fungicide is the best choice for a specific farm?
Quilt, Headline, and Stratego are the best choices. Tilt is also effective, but not as good as those three.
Table 2 summarizes a three-year profitability study of these fungicides when sprayed on varieties that are susceptible to leaf rust.
Stripe rust studies
A limited amount of data was obtained on stripe rust (5 trials), but it appears the yield response to fungicides to control stripe rust infections is at least as strong as we have seen with leaf rust. Yield responses appeared to be a function of when the disease appeared. If stripe rust occurs late (Feekes 9 to Feekes 10.5.1), a single fungicide application is all that is needed. On the other hand, if stripe rust infections occur early (Feekes 7 to 8), a split application of Quilt (7 ounces fb 7 ounces) or Stratego (5 ounces fb 5 ounces) would provide the best return on investment. In 2005, where stripe rust appeared at Feekes 7, these treatments produced yield increases averaging 200 percent over the untreated plots. See table 3.