Be on the Lookout for Wheat Rust Diseases

Carl A. Bradley, Extension Plant Pathologist, University of Kentucky

Stripe rust (Fig. 1) was diagnosed by the Plant Disease Diagnostic Laboratory at the UKREC this week in a wheat sample that came from Lyon County.  In addition, leaf rust (Fig. 2) of wheat has been observed in states south of Kentucky (Arkansas and Mississippi), and appears to be moving northward.  Some wheat varieties have high levels of resistance to these diseases; therefore, it is important to know the susceptibility of the varieties planted. Resistant varieties likely will not require any additional management for rust disease control; however, a foliar fungicide application may need to be considered for susceptible varieties. The 2016 multi-state university foliar fungicide efficacy table for wheat diseases can be found here:

Results from 2013 On-Farm Fungicide Trials in Corn

Paul Vincelli, Extension Plant Pathologist, University of Kentucky

In 2011, we set up three commercial-scale, on-farm fungicide tests in fields of yellow and white corn in West Kentucky. It was challenging to balance scientific standards of experimental design with the need to keep it doable for the aerial applicator and the producer. Nevertheless, we did it and, to my surprise, a single fungicide application resulted in 20+ bushel yield increases in two of the three tests. We also observed improved stalk strength in one of the trials. Surprisingly, all of this occurred in the absence of significant disease pressure. (For more information, see the January 18, 2012 issue of this newsletter, at http://www2.ca.uky.edu/agcollege/plantpathology/extension/KPN%20Site%20Files/pdf/KPN1291.pdf.) Please note that, in one of those two trials, the fungicide application included an insecticide, so we can’t be sure how much of the yield increase was due to the fungicide. In any case, the results were impressive, and we wanted to repeat those experiments to see how robust our results were.

Corn Fungicide Efficacy Ratings

Paul Vincelli, Extension Plant Pathologist, University of Kentucky

The Corn Disease Working Group has prepared an updated table of efficacy ratings for corn fungicides. This table only provides efficacy ratings for diseases. It does not rank fungicides for physiological “plant health” effects.

Diseases of primary concern to us in Kentucky would include gray leaf spot and, sometimes .northern leaf blight. In selected fields in some years, southern rust may justify a fungicide application under Kentucky conditions, but in most fields, levels of southern rust are so low that routine fungicide application is difficult to justify. It would be very uncommon for the other diseases listed to pose yield-impacting threats under Kentucky conditions.

This table is a “work in progress”, so efficacy ratings are subject to change, should ongoing research indicate that changes are justifiable. In the meantime, these ratings may be useful to corn producers.

Corn Fungicide Efficacy Chart

Observations on Corn Diseases

Observations of Paul Vincelli, Extension Plant Pathologist, University of Kentucky as of August 21, 2009.

I (Paul Vincelli) just returned from a trip inspecting corn diseases through western Kentucky as far west as the Mississippi River, and here is a quick summary and comments.

1. Southern rust is prevalent in Kentucky, having been found in Fayette County and every western Kentucky field inspected. See most recent article in Kentucky Pest News, http://www.uky.edu/Ag/kpn/kpn_09/pn_090818.html. This disease can progress very rapidly on corn, since almost all corn hybrids vary from moderately susceptible to highly susceptible. However, incidence and severity vary widely from field to field. In some fields, it is hard to find; in others, many plants show leaf reddening and desiccation in the lower canopy and the upper canopy has a few pustules. It is a little difficult to decide when to pull the trigger on spraying fungicide, if for no other reason than to protect stalk health. Most fields I inspected were at early dent, which in my opinion is too late to treat with fungicides. The very few fields I saw that were in early dough might be candidates for treatment, but only if rust was present, easy to find, and producing abundant reddish sporulation in at least some spots in the field. Cool weather expected over the next few days will slow it down, which is good news. I think the main thing is that growers should scout all fields for stalk health as they mature, and schedule early harvests on those fields with weak stalks. In at least some cases, spending money on propane for grain drying probably makes more sense than a fungicide application.

2. Northern leaf blight is widespread but generally occurring at levels that will not hurt yields. There is also another look-alike out there: Diplodia leaf streak. This disease has lesions that look somewhat like Northern leaf blight, but the edges of the lesions are wavy like Stewart’s wilt. Also, the lesions might follow the secondary views like Stewart’s wilt. Lab diagnosis is the way to confirm this disease. The fungus that causes Diplodia leaf streak (Diplodia macrospora) is distinct from the common one that causes Diplodia ear rot and stalk rot (Diplodia maydis), but Diplodia macrospora will also cause ear rot and stalk rot. Diplodia macrospora may be increasing in occurrence in Kentucky; seed companies will want to keep an eye on this.

Worrying about Small Soybeans

Some farmers are concerned that the smaller growth of soybeans could result in reduced yields. Some are questioning the use of foliar fertilizers and/or fungicides to help make up the difference.

Soybeans were planted late across most of the state. According to the latest Crop and Weather Report from NASS, soybean flowering is at 35%, behind the five-year average of 47%. Most farmers' "internal clock" says that soybeans should be larger by now. Most years, that is correct. This is not most years.

Some soybeans are getting to flowering (growth stage R1) and may be a little smaller than in previous years. The cooler temperatures combined with later planting dates will cause smaller plants. The smaller plants could be a concern if rows are not closed in shortly after flowering. If the soybean rows are closed, then height is less of an issue. As long as the rows are closed, tall plants do not automatically equal high soybean yields.

If the rows are not closed and the soybeans begin to flower, then yield potential is likely lost. As the soybeans move into pod development and the rows are not closed, yield potential is likely lost. If the soybeans get to seed fill and the rows are not closed, yield potential is lost. This brings us to the main question: will a foliar fertilizer or a foliar fungicide help? The short answer...probably not.

Fungicides will not improve the speed at which soybeans grow and will not help with canopy closure, in the absence of a disease. Fungicides will help soybeans retain leaves, if a disease is present in the field. However, the cooler night temperatures and the smaller soybean plants both contribute to less of a threat from diseases this season.

Foliar fertilizers will not compensate for lower temperatures. They will not increase the speed of growth, assuming P2O5 and K2O levels are adequate in the field. They will make the plants greener and that might make someone feel better.

If your, or your neighbor, is absolutely set on spraying something, then consider the foliar fertilizer. It will likely make the plants greener and it should cost a little less than the fungicide. Or, take that money you would have spent on the foliar product(s) and take a trip someplace warm. Someplace where you don't have to see the soybeans for a couple weeks. It just might make everyone happier, including your friends! For others, keeping that money in the bank may be the best stress reliever right now.

Bottom Line:
Small soybeans or late-planted soybeans that do not reach full canopy by flowering probably have lost some yield potential. Cooler temperatures also reduce the chances of soybeans reaching full canopy by flowering. In hindsight, the best management practice would have been to plant in 7.5-inch rows. The narrow rows would have improved the chances of getting complete canopy closure by flowering. Foliar fertilizers and fungicides will not make up the difference in temperatures, planting date or row spacing.