The current conversation regarding the “plant health” benefits following fungicide applications is a difficult one to address as a plant pathologist. Not only can plant pathology be a confusing topic, by trying to explain when it is best to apply a fungicide and what type of response you will likely observe is even more difficult. Two strategies can be employed:
1) apply the fungicide prior to disease incidence/observation (prophylactic, preventive), or
2) wait until disease is observed and make an application with a labeled fungicide product.
Regardless of who you query on this particular topic one thing is certain, the most consistent response from a fungicide in terms of yield can be obtained when disease occurs. Please note that I used the word consistent since targeting the potential “plant health” benefits following the application of a particular fungicide product are generally considered to not always be consistent across a large acreage, regardless of crop. In crops other than soybean this particular topic of “plant health” appears to be one that will continue to stress plant pathologists since most of us are not observing the touted benefits in our efficacy plots or in large plots either. We, as the group of plant pathologists, conduct efficacy trials to determine how fungicide products perform against specific fungal diseases. For example, I plant a susceptible wheat variety for efficacy trials to attract leaf and stripe rust and make fungicide applications to attempt to determine the product with the greatest efficacy against those two diseases. In those particular situations, what appear to be large yield increases occur when yield loss is prevented as a result of foliar disease.
Once again this year we have detected stripe rust earlier than what has historically been the “norm” (see: http://www.mississippi-crops.com/2013/02/28/wheat-stripe-rust-detected-in-mississippi/). How this will influence yield at the end of the season is difficult to predict at this time. But, with that said, keep in mind that with the wet weather we have experienced for an elongated period of time this winter we likely could not harvest such high yields as we did last season (purely speculation on my part due to the standing water we have experienced in some wheat fields throughout the state and the fact that wheat doesn’t like “wet feet”). As was stated in the previous post concerning stripe rust, do not expect a fungicide to alleviate such issues as stress and nutrient deficiencies. A fungicide should be expected to prevent yield loss as the result of a foliar fungal disease. In addition, be mindful that wheat varieties with adult plant resistance typically don’t express that characteristic until approximately boot stage (Feekes 10.0). Moreover, once wheat varieties reach a stage when adult plant resistance may protect the plant those varieties that are rated as resistant may not respond as well to a fungicide application as would a susceptible variety. Again, disease reduced yield, a fungicide protects the plant and a variety resistant to stripe rust will have an extremely limited potential for disease development.
Regarding fungicide selection for disease management keep in mind that fungicides respond differently depending on when they are applied in the overall disease cycle as well as what disease is occurring at that particular location. I’ve included two tables regarding fungicide trials that were conducted during 2012 in Stoneville, Miss. The specific variety planted was GA Gore, a rust-susceptible variety. Information presented in Table 1 relates to fungicide applications made at approximately Feekes 10. Table 2 is related to fungicide applications that were made later than what was intended (Feekes 10.5), immediately preceding flowering. Stripe rust was the predominant disease. However, low levels of leaf rust were also observed in plots. Plots were harvested at different times and this could account for some of the variation in test weight between Figure 1 and 2. More importantly, some of the greatest yield benefits were the result of a reduction in the overall observable disease between non-treated and treated plots. Again, the most consistent yield response is a result of a fungicide application in situations when disease is likely to occur.