the Bulletin EXTENSION COLLEGE OF AGRICULTURAL, CONSUMER AND ENVIRONMENTAL SCIENCES Pest Management and Crop Development Information for Illinois

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1 the Bulletin Pest Management and Crop Development Information for Illinois No. 14 / July 7, 2011 Executive editor Aaron Hager, Extension Weed Science Specialist Copyright 2011, Board of Trustees, University of Illinois UNIVERSITY OF ILLINOIS EXTENSION COLLEGE OF AGRICULTURAL, CONSUMER AND ENVIRONMENTAL SCIENCES Also in This Issue Is Wheat Straw Worth Anything? 88 Applying Nitrogen Late in the Game 89 The Illinois Corn Crop in Midseason 90 Plant Diseases White Mold Q&A White mold, also known as Sclerotinia stem rot and caused by the fungus Sclerotinia sclerotiorum, is a disease of soybean and other broadleaf crops. Historically, white mold on soybean has been observed sporadically in central Illinois, more frequently in northern Illinois, and rarely in southern Illinois. In 2009, white mold was prevalent in central and northern Illinois, causing yield losses in many fields. Cool (temperatures below 85 F) and wet weather, especially when soybean plants are blooming, favor the development of white mold. These conditions occurred in central and northern Illinois in 2009, which was the primary reason for the high levels of the disease. White mold gets its name from the fuzzy white growth that develops on affected soybean plants. This growth is the mycelia of the fungus that causes the disease. White mold symptoms include wilting leaves, stems that appear to be bleached, and shredding of the stem tissue (photo on page 88). Small black structures termed sclerotia can be found on and inside plants that have been affected by white mold. I have received many questions about white mold in the past few weeks, probably because many of the fields seriously affected in 2009 have returned to soybean this year. These frequently asked questions and my answers follow. Q. White mold was a severe problem in my soybean fields in Does that mean there is a greater risk of white mold this season when these fields have returned to soybean? A. Yes, the risk has increased because more sclerotia went back into the field after the 2009 season. However, the risk of white mold is driven primarily by weather conditions. Wet, and especially cool (less than 85 F), weather is needed for spores of the white mold fungus to infect soybean plants and for disease to develop. So even though more sclerotia mean more spores present in a year favorable for the white mold fungus, disease will not develop unless favorable weather conditions occur. Q. I have observed small mushrooms in the soil of my corn and soybean fields. Are these apothecia of the white mold fungus? 87

2 A. The only evidence of the white mold fungus that can be observed in the soil is the small mushrooms known as apothecia (photo below). I received some reports last week that apothecia had been spotted in some Illinois fields. The apothecia are small and cup-shaped they look a bit like small golf tees. Q. Which foliar fungicides registered for use on soybean include white mold (Sclerotinia stem rot) on the label? A. Domark, Endura, Proline, and Topsin M are registered for use on soybean and include white mold on the label. (The active ingredient in Topsin M is thiophanate-methyl, which is off-patent other products containing thiophanate- methyl also may have white mold on the label.) Q. Do any herbicides besides Cobra suppress white mold? A. Cobra and Phoenix (both containing the active ingredient lactofen) are the only two herbicides that currently have white mold suppression on their label. Q. Has the University of Illinois done any research evaluating products for control of white mold? A. Yes, trials have been conducted at the U of I research farm near DeKalb. See Tables 1 and 2 for 2009 and 2010 results. Carl A. Bradley Crop Development Is Wheat Straw Worth Anything? As I traveled through the southern part of the state this last weekend, I saw a number of harvested wheat fields, some with bales of straw waiting to be picked up and others with baling yet to begin. In the last three or four years, baling wheat straw seems to have become a more frequent practice. While the uses for wheat straw are varied, a common denominator exists in terms of nutrient removal. Removing straw means exporting nutrients from the field. I want to explain here how to determine how much nutrient is removed when straw is baled. Knowing the amount of nutrient in straw is important to both determining the straw s value and knowing how much fertilizer is being removed and thus how much to apply back to the field to maintain fertility levels. The most precise way to determine the amount of fertilizer being removed is to weigh the bales and extract a representative sample of the straw. Using that sample, two values must be obtained from a testing lab: moisture content, to determine the dry weight of the bale; Soybean stem affected by white mold, also called Sclerotinia stem rot. Table 1. Effect of registered products on white mold incidence and soybean yield at the University of Illinois research farm near DeKalb in Treatment* Yield (bu/a) Nontreated Topsin 4.5 FL at 20 fl oz Proline at 3 fl oz Domark at 5 fl oz Cobra at 12.5 fl oz Endura at 8 oz (2x) LSD 0.05** *All products were sprayed at R1 (July 20, 2009). Endura was sprayed again 9 days later. **Least significant difference value. Mushroom-like structures known as apothecia growing from a sclerotium. (Photo courtesy J. Venette, North Dakota State University.) Table 2. Effect of registered products on white mold incidence and soybean yield at the University of Illinois research farm near DeKalb in Treatment* Yield (bu/a) Nontreated Topsin 4.5 FL at 20 fl oz Proline at 3 fl oz Domark at 5 fl oz Cobra at 6 fl oz Endura at 8 oz LSD 0.05** 11 NS 8 *All products were sprayed at R1 (July 10). **Least significant difference value. NS indicates that treatments did not significantly differ from each other for that column. 88

3 and nutrient content, which is reported on a dry basis. Of course, this approach might not be practical or possible for all producers, in which case one can estimate the value of straw by following a few simple steps. But keep in mind that there can be substantial variability in the actual value, depending on how the straw is processed. Step 1 of the calculation is to determine how much straw is being produced. The best estimates are obtained as a function of plant height and grain yield. These estimates vary somewhat, but a typical value is about 1 pound of straw per pound of grain. In this step, remember that although a bushel of wheat is 60 pounds, that includes 8.1 pounds of moisture (13.5%). So a bushel of wheat is 51.9 pounds of dry grain. The dry-grain yield in bushels per acre is multiplied by 51.9 and then divided by 2,000 to obtain the number of dry tons of straw produced. Step 2 is to determine how much straw is actually removed. Again, this is not an absolute value because it will depend on cutting height and how much of the cut straw is actually baled. In general, a good wheat crop yields around 2.5 to 2.8 tons of straw per acre on a dry matter basis. Step 3 is to know the nutrient content of the straw. A ton of wheat straw normally contains 9 to 12 pounds of nitrogen (N), 3 to 4 pounds of phosphorous (P) in the form of P 2 O 5, and 25 to 45 pounds of potassium (K) in the form of K 2 O. K content varies so much because K is not incorporated into organic compounds and so can be easily leached from the residue. The time elapsed, the amount and frequency of precipitation received after the crop reached maturity, and when the straw is removed from the field all impact K content. Typically, K leaching occurs rapidly; it is not unusual to see a decrease of as much as 80% after the straw has been rained on a few times. On the other hand, N and P content in the straw are more or less stable because they are present in organic forms and require microbial decomposition of the straw before they can be leached out. The fourth and final step in the calculation is to determine the fertilizer value. Use the information determined through the procedures described and the current price of fertilizer to arrive at a value of dollars per acre. Though I have focused on removal of major nutrients (N, P, K), I want to be clear that there are other costs associated with straw removal; they might not be easily observed in the short term, but they can have important impacts on a field s future productivity. In addition to the major nutrients, wheat straw contains calcium (Ca), magnesium (Mg), sulfur (S), and micronutrients. In Illinois, the soil s native supply of these nutrients is typically high, so there is no need to supply them through fertilization. But greater removal of these secondary nutrients by harvesting straw can accelerate depletion rates and result in future deficiencies. Removal of basic cations, like K, Ca, and Mg, can increase the need to lime soils to maintain adequate ph levels. Depletion of organic matter and N reserves can lead to less crop availability of N through the process of mineralization (conversion of organic N to inorganic N). Diminishing organic carbon contents can also result in negative impacts on soil conservation and on soil physical, chemical, and biological properties. For all these reasons, carefully consider all relevant factors when you estimate the true cost of straw removal. Fabián G. Fernández Applying Nitrogen Late in the Game The appearance of a corn crop is an excellent diagnostic tool for nitrogen (N). If corn is yellow-green or light green (especially in the older leaves), it is most likely N-deficient. While no one likes to wait until July to apply N, some fields or parts of fields do not yet have all the N they need; this is true for a variety of reasons, wet fields not the least of them this year. Now soils are drying and crops are growing very fast as they rapidly accumulate lots of growing-degree days. The rapid growth of the crop requires making it a priority to apply N as soon as possible to any areas that did not receive sufficient N or that lost some of the N that was applied. It is common to see light-green corn next to dark-green corn that is also further along in development. If N has been applied, the light-green corn is typically found in areas where water ponded. In areas where there is adequate N, often-waterlogged soils induce symptoms resembling N deficiency. For the most part, those symptoms should have disappeared. If they persist after about a week from the time soils dry, you can be pretty certain that the crop needs more N. Whether N-deficient areas represent many acres or just spotty zones, the question occupying farmers minds is how to fix the problem and how to do it most efficiently. One of the most important things to realize is that this late in the season, crops showing N deficiency have already lost some yield potential, and applying a full N rate cannot recover that lost potential. In other words, the corn crop will not be capable of using the full rate to make yield. Another important point is that the sooner you apply N, the better response you are likely to see. This year across Illinois, corn is all over in terms of development stages. While some fields are still at early vegetative stages, others are rapidly approaching reproductive stages. If your field fits the latter case, remember that you are very likely to obtain a yield response by applying N until tasseling. Studies have shown that even until silking, corn has a great capacity to use N and produce an increase in yield if the application is done in severely N- deficient fields. 89

4 Also keep in mind that areas needing N at this time are typically patchy, so targeted applications, rather than even applications across the field, are fundamental to reducing cost, increasing return on investment, and minimizing potential loss of N to the environment. One way to identify the location of trouble spots is with aerial photography or observation from a vantage point above the canopy. In most fields, the corn crop is reaching heights that would make it too difficult to find problem areas by walking the field. Aerial photographs can be converted into variable N-rate maps to guide the application. Canopy-sensing technology can also be used to guide application rates. Research has shown that canopy sensing is most useful when plants are bigger (around V10 stage) and obviously N- deficient. When plants are smaller, the sensor often captures too much soil area relative to leaf area and can overestimate how much N is needed. If you are using canopy-sensing technology, make sure the equipment is properly calibrated to an N-sufficient zone of the field. High-clearance equipment is likely the only way to make the rescue N application in some fields; fortunately, it is becoming more widely available. For rescue N, I suggest that the best alternative is between-row applications of dribbled or injected UAN solutions or urea plus a urease inhibitor such as NBPT (Agrotain). Another option is broadcast urea with a urease inhibitor. The urease inhibitor is important to reduce the potential for volatilization losses when the product sits on the soil surface until it is incorporated by water. I do not recommend broadcast application of UAN because of the high probability of canopy injury. Also, a slow-release product (polymer-coated urea) is not a good choice at this point because you want N to be available to the crop immediately. While the coating can protect urea from volatilization (just like a urease inhibitor does), it will take time for the coating to break down and release N, further delaying availability to the crop. Regardless of the N source, any product that is surfaceapplied will require water to move it into the root system so that the plant can use the applied N, so applying ahead of rain is a good approach. I would not apply a foliar product simply because the amount of N that can be applied is often very low and the cost per acre too high to make that profitable. Fabián G. Fernández The Illinois Corn Crop in Midseason While the large acreage reported at the end of June shocked the corn market, it goes almost without saying that good yields are still needed to end up with a large crop. While it is a little dangerous to guess at prospects before we see many tassels, we now have some idea about the physiological state of the crop as it enters the most critical yielddetermining part of the season. The latest report from the National Agricultural Statistics Service indicates that 8% of the Illinois corn crop was silking as of July 3 behind the 21% average of the past five years, but set to increase substantially over the next week as the crop planted in early May begins to silk. In our planting date study, the 109- day RM hybrid planted on March 31 silked on July 3, and the 113-day hybrid on July 5. That s after about 1,400 growing-degree days for the early one and 1,450 for the later one. The May 5 planting will probably silk later this week, after some 1,300 or so GDD. From travels through much of the northern part of Illinois over the past week, it s clear that most of the corn crop there is in good shape. Exceptions are places where heavy rainfall in the last half of June left standing water in low places and flooded bottomland; little crop will recover to produce much there. These areas are much less extensive than in 2010, but parts of central and southern Illinois have more of them. The amount of loss in affected fields will depend on how much stand remains and how quickly root function can be restored. The rest of the crop has benefited greatly from the dry weather and sunshine over the past two weeks. Root systems have clearly grown well and are tapping soil water beneath the top few inches. The state of the root system is signaled better through observing the crop than digging up roots. The primary indication of healthy roots is that the crop shows no signs of water or (in most cases) nutrient stress. Leaves in low-stress crops stay open (unrolled) through even the warmest part of warm days, and the rate of height increase is normal. Now that the surface soils are drying out, roots must be actively taking up water from deeper in the soil to keep the plant functioning well. Crop color is good to outstanding in most fields, though I ve seen a few fields with symptoms of N deficiency. These are likely fields where N was not yet applied or was applied so late that it was not yet being taken up or where a lot of N loss has occurred. In fact, with the dry weather of the past two weeks, some of the rescue applications that have been made, based on fear of N loss or on poor crop color when soils were very wet, may still be sitting in the dry surface soil, with no way to get to the roots. It s perhaps too early to say that such applications were a mistake, but some were likely premature, especially in fields where adequate N was applied earlier and there was no real indication that much of it had been lost. The fact that nonrescued fields are mostly looking well nourished now does remind us, though, that guesses about how much N has been lost (vs. how much root function has been compromised) are usually not very accurate. In any case, the dark green color means that photosynthetic rates are high, and as the canopy closes for real as leaf area and light interception approach their maximum the crop is producing sugars at maximum rates. This is exactly what it needs to do as it enters the pollination 90

5 stages. We don t have a good test to see how sugar levels are holding up in the plant, but with so much sunshine, moderate temperatures, and a healthy crop canopy, they should be as high as they can be at this point. Remember, though, that the effect of good conditions now can be overcome if it turns dry and hot later in July, in which case water will start to run out and kernel numbers will drop, along with yield potential. The main things to watch for now are signs that photosynthetic rates might be dropping and, by the time pollination ends and silks start to turn brown, how many kernels have been fertilized. Threats to photosynthetic rates are almost always visible as leaf symptoms, including loss of dark green color, curling up due to lack of adequate water, disease damage to leaf area, or loss of leaf area from hail or insects. After silks start to brown, it should be easy to see which kernels are fertilized and starting to increase in size. In round figures, we expect maximum yield potential when kernels number in the range of 15 to 20 million per acre. At 32,000 ears per acre, that would be 500 to 550 kernels per ear. If conditions remain good for 2 weeks after brown silk, there tends to be little abortion. But any of the leaf symptoms just mentioned occurring in the 2 to 3 weeks after brown silk may signal some loss of developing kernels. Coming through northeastern Indiana early this week, I saw corn planted in June that was undergoing significant drought stress. There is likely some corn in similar condition in Illinois, including some that was replanted. Unless it rains soon on such corn, the root system, which never had much chance to grow before soils dried out, will remain small, and the crop will be unable to grow well as a result. As we move through July, prospects for such corn are dropping quickly. Emerson Nafziger Contributing Authors Carl A. Bradley, carlbrad@illinois.edu, Fabián Fernández, fernande@illinois. edu, Emerson Nafziger, ednaf@illinois.edu,