Heat During Corn Pollination
Corn maximizes its growth rate at 86°F, but the point at which growth is hindered is not so specific. Overall, daytime temperatures in the upper 90’s will reduce the efficiency of the photosynthesis food factory in the plant, and organisms tend to prioritize energy into survival before reproduction. Hot temperatures can also directly reduce pollen production and viability while silks are very water dependent. Silks contain more water on a percentage basis than any other part of the corn plant and are sensitive to lack of precipitation, limited root growth from any cause and dry air conditions. Without adequate water, silks will desiccate and/or fail to elongate.
The good news for us in the Midwest is that the hot temperatures during the week of July 15 were just ahead of pollination for most of our late planted corn. We have since eased off to more favorable temperatures and the 2-week outlook is also promising. How can you be an active manager at this time? 1) be sure the crop has water if irrigation is an option. 2) leave the crop alone while the magic happens: wait with your fungicide. 3) conduct pollination assessments.
Root lodging prior to tassel usually means some acceptable level of recovery that will put on yield and hold an ear, but recovery by corn that is damaged post-tassel is less likely; the plants are in reproductive mode and have finished vegetative growth. Concerning hail damage, directly hit soybean flowers will likely fail to set pods just as silking corn ear shoots may no longer be receptive to pollen, but defoliation in and of itself also causes major issues as shown by Joe Lauer et al. (2004) at the University of Wisconsin (Figure 1). Note that 100% defoliation at early vegetative stage results in less than a 20% yield loss vs a near 100% loss if defoliated at R1.
It’s the time of year to focus on things we can control to preserve the current crops’ yield potential. The early reproductive growth stages for corn and soybeans give a signal that scouting should continue to determine if a foliar fungicide should be considered.
Rainfall, high humidity, dew and fog are some of the ingredients required for fungal disease development. Temperature is another ingredient that can vary the presence or absence of certain diseases. For example, cooler temperatures generally favor the development of white mold and northern corn leaf blight; whereas, warmer temperatures favor gray leaf spot and frogeye leaf spot.
Fields with susceptible hybrids or varieties should be scouted before applying a fungicide. If disease is present, accurately diagnose which disease exists. Fungicide selection should be based on the fungicide label and the disease you want to control. Most fungicide labels have a broad spectrum of diseases that they control.
Numerous factors should be considered before making a foliar fungicide application:
The agronomy team is available to assist you in making foliar fungicide and insecticide decisions.
On any leisurely drive through the country side, cupped soybeans can be seen virtually everywhere this year. We have been fielding many questions related to this issue recently.
There are several reasons which can cause this plant response. Of course, there is the obvious related to growth regulator herbicide drift and or volatility specifically dicamba products. Drift or tank contamination will usually show a gradient of heavy damage to less damage with the worst being nearest to the drift source. Volatility is less easily pinpointed and can be scattered across the whole field. It could be especially noticed in low lying areas of the field. Other instances occur through faulty equipment hoses, pumps or valves leaking product into the spray solution.
Damage from dicamba will have the typical cupping look. Low level dicamba damage will have a little cupping. It will also have some stippling and limited drawstring appearance. It will not have the 2,4-D rectangular leaf shape and will be more truncated at the leaf tip. The leaf tip will be very noticeable and be yellow. See figures below.
Another possibility is the plant exhibiting a physiological response to adverse environmental conditions. The heat and lack of moisture during post herbicide application certainly could have triggered this. The 2019 application was certainly set up for this scenario. Some have suggested that translocated herbicides could disrupt hormonal balances in the plant and cause cupping as well. Typically, once environmental conditions become more favorable soybeans grow out of this. If it is a dicamba, it may take a couple weeks to metabolize the herbicide and continue with normal growth.
We are also getting calls on weed escapes related to environmental stress and crop response. Weeds need to be “actively” growing for optimum weed control. If a plant shuts down during extreme heat and lack of water, it will not take up the herbicide as well. So, it will either take longer for control and there will be limited control. The heat stressed plants may have shriveled or rolled up leaves to reduce water loss. This creates a less than ideal target for herbicide deposition. Plants will also lay down more Epicuticular wax during hot dry periods to prevent water loss. This wax can interfere with the water-based herbicide solution entering the plant. Continue to monitor your fields for weed pressure and crop tolerance the next few weeks.