Early Fungicide Applications of Headline to Wheat

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

 Reprinted with additions for Delaware from EARLY FUNGICIDE APPLICATIONS OF HEADLINE TO WHEAT By Don Hershman in the March 24, 2008 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.

 This year, BASF is recommending the use of a reduced rate of Headline (3 fl oz/A), applied early (tillering through early stem elongation), for disease control/yield enhancement in wheat. The idea is that the fungicide would be added to the herbicide or herbicide/insecticide mixture that most wheat producers are already putting out for weed and aphid control, respectively. BASF has issued a 2ee recommendation for this use; the target diseases are tan spot, speckled leaf blotch (Septoria tritici) and Stagonospora (Septoria) leaf and glume blotch.

 I have received numerous queries about this recommendation. By way of background, early season fungicide sprays are being recommended in most states where wheat is produced. As a result, this has been a major topic of discussion amongst wheat pathologists this winter. The overall consensus is that this or any fungicide treatment is only profitable if a disease develops at high enough levels to reduce yields. Thus, if there is little to no disease in a crop, there will be little to no economic benefit from a fungicide application. Similarly, treatments can be applied, but if the timing is off or the rate too low, disease control may be compromised and yield and/or quality losses may still occur.

 Now, assuming that one or more diseases can be found in a crop, the next thing to do is to determine if what is being found justifies a fungicide spray now or at some later date. From data I have seen, the only time where an early fungicide application seems to pay off is where tan spot, leaf rust, stripe rust, or powdery mildew is found in a crop early. Tan spot is typically most serious where wheat following wheat is a common production practice. It is often evident in other cropping systems, but I have only seen that disease reduce yields a few times in 24 years in Kentucky. [It is more common in Kent and New Castle counties but usually comes in late.] This is probably because most of our wheat is planted behind corn (a non-host crop for tan spot) and our wheat residue deteriorates rather quickly. The few times I have seen tan spot reduce yields is when it came in late in the season. The argument can be made that making early fungicide applications will “nip it in the bud” and this may be true; Headline is excellent against tan spot. However, in my experience, the risk that tan spot will reduce yields in any given field in Kentucky is low. As a result, I would not base my fungicide use decisions on tan spot control unless you see compelling evidence that tan spot control is needed. [In DE, later fungicide applications at early to late head emergence would probably be better if disease pressure is low or not present.]

 Speckled leaf blotch is not a very aggressive disease in Kentucky [or DE], but it can be found in almost every field, every year, at very low levels. Frequently during early spring the disease will develop on the Flag -2 or 1 leaf, but in most years and situations, the disease will shut down due to high temperatures before it reaches the flag leaf. When it does make it to the flag leaf, it does not tend to be very aggressive, again, probably due to the high temperatures that we often see in mid-May through mid-June. Speckled leaf blotch moves rather slowly in the canopy, and it is not hard to control if it appears that the disease is increasing. There is limited resistance to speckled leaf blotch, but I don’t believe there is much evidence to support an early application of Headline for control of this disease.

 Year in and year out, Stagonospora (Septoria) leaf and glume blotch, is probably our most important disease. [This is also true in DE.] Yields are rarely devastated, but it is common to see 10-20% yield losses, as well as reduced grain quality. However, this disease is more prevalent in mid to late season due to its higher temperature requirements, compared to speckled leaf blotch. The main point is that Stagonospora leaf and glume blotch almost never starts building up steam until after the flag leaf has emerged, and usually later. I seriously doubt that applying any fungicide in the spring, through early stem elongation, will make much difference in the outcome for Stagonospora leaf and glume blotch [ditto for DE].

 In susceptible varieties, powdery mildew, leaf rust and stripe rust can be devastating, and are very difficult to control if they are allowed to get a foothold in a crop. In some cases, an early application of a fungicide is exactly what is needed to ward off damage due to these diseases. However, in each of these disease situations, the 3 fl oz/A rate of Headline is too low to provide adequate control of these diseases. My guess is that this reality is the reason why BASF has not included these diseases in this particular 2ee recommendation.

 With high wheat prices anticipated for early summer, it does not take much of a return to recover the cost of an invested input. I will not argue that applying Headline to a crop early, and at a reduced rate, may result in a slight yield increase that may make the treatment perfectly acceptable. I acknowledge that controlling low levels of several diseases early may, collectively, result in slight yield increases compared to non-treated wheat. All I am attempting to do in this article is to lay out some biological realities that, on the surface at least, are not highly supportive of this treatment. However, the 2ee recommendation being described here assumes that a later fungicide application will be made for late season disease protection. In fact, a follow-up application is encouraged on the 2ee recommendation. When the two fungicide treatments are applied, there may be more benefit than would be seen from either treatment alone. To be honest, there has been very little research done with tillering to jointing applications of Headline at reduced rates. BASF summarizes the results of numerous side by side studies, but it is difficult to know what to make of the summaries. No disease information is included and the tests come from a range of states and both winter and spring wheat. There is limited data from Kentucky [and DE], so we cannot support or reject the treatment based on our experiences here. Thus, we certainly do not have all the answers. If I were farming, I would give the treatment a try on a portion of my crop and make sure I could document the yield impact of the treatment. I would also send in some representative plant samples to a qualified plant disease diagnostic laboratory and ask them to make an overall assessment of the diseases present and their levels.