Should You Make a Wheat Foliar Fungicide Application?

We have reached the time of year when automatic foliar fungicide applications are suggested for wheat. As most would suspect, I am a firm believer that fungicides provide the most economic response when used on varieties that are reported to be susceptible to the more damaging foliar diseases (e.g., stripe rust). At present, stripe rust has not been identified in a commercial wheat field. However, last night I was able to identify minimal stripe rust infection on some volunteer wheat growing under the MSPB-funded rainout shelter in Stoneville. I suspect the infection occurred in the fall. Interestingly enough, telia had started to form on some of the leaves, which suggests the environment under the plastic covering of the rainout shelter is no longer conducive for stripe rust development (too hot). Wheat in the vicinity of the rainout shelter is not infected. I suspect this is an isolated incident and certainly not cause for concern. Moreover, volunteer wheat is normally more susceptible to foliar disease. I’m not suggesting a fungicide application needs to be made to wheat simply because I found 10 leaves containing stripe rust.

Firstly, I do not remember ever seeing such a clean wheat crop in the years I have had wheat responsibilities (going on 10 years). But, with that in mind, there are quite a number of fields that have received some form of drift from a local herbicide application. Keep this in mind when scouting for foliar diseases present. Paraquat will produce a tear

Paraquat injury on wheat leaves will produce a teardrop type lesion.

drop shaped lesion with a bleached center.

When considering a fungicide application in wheat there are several key characteristics to consider. Growth stage is important since the majority of fungicides have a specific pre-harvest interval (PHI) that likely cuts off at heading complete (Feekes 10.5), which immediately precedes the observation of flowers (Feekes 10.5.1). For the specific fungicide products labeled for application in wheat and their respective PHI see the associated table (NCERA 184 Wheat fungicide table).  See: https://www.mississippi-crops.com/2014/03/20/identifying-wheat-growth-stages-using-the-feekes-scale/ for more information regarding growth stages in wheat.

Yield

In MS we have an extremely limited data set regarding the “plant health” fungicide applications, in the absence of disease, which are now quite commonplace. The majority of the time when I conduct wheat fungicide trials I am managing a disease present in the field (see: https://www.mississippi-crops.com/2013/03/11/fungicide-applications-in-wheat-targeting-disease-to-prevent-yield-loss/ ) on an extremely rust-susceptible wheat variety because I am interested in the specific response of the fungicide. However, over the past few years I have been privy to data sets from other states and have had numerous conversations with other plant pathologists that conduct fungicide trials in wheat regarding plant health applications as well as the different responses of susceptible versus resistant varieties to fungicide applications. Similar to the plant health situation in corn production systems, plant pathologists have not observed large yield increases following a timed fungicide application especially to foliar disease resistant wheat varieties.

In general, fungicide applications provide the most consistent benefit when applied in situations either before disease occurs or after disease has been observed in a field of a susceptible wheat variety. Stripe rust resistant wheat varieties have not responded near as well to fungicide applications in adjacent states. In the years when I have conducted fungicide applications in response to stripe rust, I have observed a substantial yield response (see the below link regarding fungicide trials conducted during the 2012 wheat season). In addition, test weight has also been greater following the fungicide application in the presence of disease with some fungicides, but not all.

In the attached pdf file, yield response was either minimal or non-existent during the 2013 wheat season since no observable foliar disease (either leaf or stripe rust) was present in the sprayed trials (2013 Foliar fungicide wheat trial data from Western MS).

Test weight

The resulting test weight following a fungicide application appears to be similar situation to the yield response. In years where I have made fungicide application trials in response to foliar disease such as stripe rust, the response of the fungicide with regards to test weight has varied by fungicide product (e.g., Headline and TwinLine versus Priaxor (Table 2)) (see: https://www.mississippi-crops.com/wp-content/uploads/2013/03/wheat-fungicide-tables1.pdf). In the attached tables, note that test weight was not influenced by a fungicide application in the absence of foliar disease during the 2013 season in either of the two wheat varieties treated (see the above link to the pdf).

The answer

Based on my observations this season, with little to no foliar disease present in the wheat crop (and a Fusarium head blight (scab) predictive model that continues to suggest the environment in 98+% of the state hasn’t been conducive for scab to develop), I would forego making a fungicide application to wheat. However, I leave that type of a decision up to each of you. In my opinion, the wet, cold winter has appeared to reduce the threat of foliar disease this season.