Integrated Pest & Crop Management Newsletter University of Missouri-Columbia Vol. 16, No. 15 August 11, 2006 None Like It Hot By Bill Wiebold The recent period of hot temperatures may have affected corn and soybean yields more than assumed from observing stress symptoms of affected plants. Stress symptoms are almost always related to low water availability, but high temperatures can damage plants or reduce yields even if there is adequate moisture availability. It is important to remember that plants react to temperature differently than humans. Humans must evaporate water to dissipate heat. High humidity reduces evaporation and greatly affects the way a particular temperature feels. Thus, weather stations report heat indices that are an attempt to estimate how air temperature "feels" to humans. High humidity translates into heat indices that are often five or more degrees above air temperature. Heat indices have little relationship to the effects of temperature on plants. Sometimes leaf temperature is more important to plants then air temperature. Leaves function as solar collectors, that is, they are designed to absorb light energy. They do this in order to build sugars and produce other products necessary for life (and yield). However, very little of the light energy is actually used to do this work (photosynthesis). Light energy not used for photosynthesis causes leaf temperature to rise. Plants dissipate heat through water evaporation from cell surfaces, convection, and conduction. Changing liquid water to water vapor requires substantial energy and this energy loss causes the cooling effect. Conduction means that the warm leaf surface warms (gives energy to) the air touching the leaf. Convection means that cooler air is moved closer to the least surface and displaces warmer air. These three methods of heat dissipation are very much interrelated, and without them the leaf temperature would quickly rise to the point where plants could not survive. It is not uncommon for leaf temperature to be higher than air temperature, especially on bright sunny days with little wind. With good moisture supply, evaporation will be fast enough to keep leaf temperatures fairly close to air temperature. With limited moisture, stomates will close reducing water evaporation and increasing leaf temperature. Nearly all of the chemical reactions necessary for the life of plants are controlled by enzymes. The rates of these chemical reactions increase with temperature, so, for example, plant growth and weight gain are greater at 80 degrees than at 55 degrees. These enzymes have a three-dimensional shape and can warp (change shape) at high temperatures. An extreme example of temperature affecting protein is the frying of an egg. The heat causes the egg protein to change its shape and become solid. The effect of temperature on plant enzymes isn’t nearly that dramatic, but temperatures of 100 to 105 degrees can affect the shape of plant enzymes. When the shapes of the enzymes change, they no longer work as well. In other words, the reaction rate decreases. That is why 86 degrees is often given as the optimum temperature for corn and soybean growth. Although the optimum is fairly flat for about 15 degrees, temperatures above the optimum slow many of the important reactions including those involved in photosynthesis. So, high temperatures can harm crop plants and reduce yield. This direct affect from high temperature is probably small in most years, but when temperatures top 100 degrees as they did several times in July, yield was reduced, at least slightly. Unfortunately, high temperatures have other effects on plants. One almost hidden effect from increased temperature is the differential effects it has on photosynthesis and respiration. Photosynthesis is "income" for the plant world and respiration is an "expense." The difference between the two, net photosynthesis, is "net income." Within reason, high amounts of net photosynthesis often translate into high yield. Some respiration is essential, just as some expenses are essential. Respiration oxidizes ("burns") sugars to produce energy that is needed for many of the life processes. However, some respiration is wasteful because it burns away or oxidizes sugars that could have been stored in seeds as yield. Hot temperatures stimulate respiration more than photosynthesis and reduce the plant’s net income. This is particularly true during night when there is no photosynthesis. Warm night temperatures can decrease yield without showing any visible effects on the plants. Although high humidity can be beneficial to plants because water evaporation is reduced and this reduces water stress, high humidity also slows the rate by which temperatures cool at night. It is not uncommon for temperature to remain above 80° during summer nights if humidity is high (dew point above 65 degrees). So, although plants do not "feel" a high heat index, they are affected by the slow temperature decline during nights of high humidity through increased respiration. It is difficult to separate the effects of high temperature from the effects of water stress. Often these two stresses occur together and magnify the effects from each other. But, high temperatures can reduce yield even if plants exhibit no symptoms of water stress. Bill Wiebold 573-882-0621 ##################################################################### Stalk Nitrate-N Test - A Diagnostic Tool for Evaluating Nitrogen Management Practices in Corn By Manjula Nathan With the increase in N fertilizer prices and growing concern for environment growers are becoming more interested in fine tuning fertilizer N applications for corn production. There are many diagnostic tools that are available for improving N management in corn. Researchers at Iowa Sate University have come up with the stalk nitrate N test as an effective diagnostic tool in improving N management in corn. This test gives you information on how well you have managed your nitrogen and doesn’t provide information on how much fertilizer N to apply for the coming season. The stalk Nitrate N test is done in the lab where a 6" stalk (cut at 6 -8 inches above the soil surface at black layer stage, no leaves included) sample is dried, ground, and processed and analyzed for NO3- N. The numbers are compared to standards set by Iowa State University researchers based on field research. It is important to note for accurate results samples should be collected at black layer stage and not after harvest. After collecting a representative sample six-inch stalk samples cut 6 - 8 inches above the soil surface, make sure to split the sample into two vertically and let it dry before mailing it to the lab. This would quicken the process of drying. It is preferable to sample a least a minimum of 10 stalks from the area of interest to have good representation and reliable results. A stalk NO3-N test value of less than 250 ppm is interpreted as low, nitrogen was probably deficient during the growing season. Test values of 250-700 ppm is marginal, it is possible that nitrogen shortage limited yield in this range, and 700-2000 is optimum, yield was not limited by a shortage of nitrogen in this range. Values in excess of 2000 ppm means excessive, nitrogen rate was too high or some production factor caused a yield reduction. Factors other than excessive use of N can such as drought and hail damage can lead to excess N in the stalk. University of Missouri Soil and Plant Testing lab located at 23 Mumford Hall, UMC, Columbia, MO 65211 offers stalk NO3-N test in corn for $12 per sample. You can reach the lab at 573-882-0623 or get information from the lab’s Website at http://soilplantlab.missouri.edu/soil on submitting samples. If you have any other questions about the test you can contact Manjula Nathan at 573-882-3250. Manjula Nathan 573-882-3250 ##################################################################### Field Crop Disease Update- August 7, 2006 By Laura Sweets Corn Field crop diseases on corn have been rather quiet for the last few weeks. Earlier in the season, Stewart’s bacterial wilt, common rust, anthracnose and gray leaf spot were present in low levels in scattered fields across the state. However, the extended period of hot, dry weather as slowed the development of these foliage diseases and in most fields the ear leaves look quite clean. With the extended high temperatures and dry conditions some fields are starting to turn. Initial symptoms of stalk rots could be present in these fields. See accompanying article on corn stalk rots. Soybean Brown spot continues to be evident in the lower canopy of many fields and downy mildew has been quite common in the upper canopy of many fields. Downy mildew symptoms include light to bright yellow blotches on the upper leave surface with tufts of grayish-purple mold growth on the lower leaf surface beneath the yellow blotches. Sudden death syndrome has been showing up in some fields. The foliage symptoms, ie. yellow blotches between the veins with the veins staying green can be very striking. The entries in the SDS trials at Columbia and Waverly are showing a wide range of symptoms. Some entries have no visible symptoms, some are showing light yellow blotches between the veins on leaves in the upper canopy and other entries are showing severe foliage symptoms. These two trials were planted in mid-April and have been irrigated over the season. We are also still receiving reports of plants dying from Phytophthora root rot and plants showing severe stunting and yellowing from Rhizoctonia and/or Fusarium root rot. At this point in the season there are no managements options for these root rot diseases. Laura Sweets 573-884-7307 ##################################################################### Corn Stalk Rots By Laura Sweets Any factors which stress corn during the growing season may contribute to an increase in stalk rots that season. Although some of the leaf diseases (gray leaf spot, common rust and anthracnose) came in early in the season their development was slowed by the extended period of hot, dry weather over most of the state. So, although foliage diseases may not have a direct impact on yield, they certainly can contribute to an increase in stalk rot. With the environmental stress on corn and low levels of disease, this is certainly a year in which it would be wise to scout fields for corn stalk rots and to harvest fields with stalk rot problems as quickly as possible. A number of different fungi and bacteria cause stalk rots of corn. Although many of these pathogens cause distinctive symptoms, there are also general symptoms which are common to all stalk rot diseases. Early symptoms, which occur a few weeks after pollination, usually start with premature dying of bottom leaves. Eventually, the entire plant may die and appear light green to gray. Diseased stalks usually begin losing firmness during August. The cells in the interior of the stalk are dissolved, resulting in a loss of stalk firmness and strength. Stalks may then lodge, particularly if harvest is delayed or wind storms occur. Fusarium stalk rot and Gibberella stalk rot can be difficult to distinguish in the field. Both can cause a pink to reddish discoloration of diseased stalk tissue. Tufts of white mycelium may be evident at the nodes of diseased stalks. When stalks are split open the pith is usually shredded and discolored. Anthracnose stalk rot, caused by the fungus Colletotrichum graminicola, may be most evident at the nodes. Initially lesions are tan to reddish-brown but they become shiny black later in the season. These shiny black lesions may begin at a node and extend out from that node. The lesions may merge to discolor much of the lower stalk tissue. Internal pith tissues may also be discolored and may disintegrate as disease progresses. Diplodia stalk rot may begin as a brown to tan discoloration of the lower internodes. Stalks become spongy. The pith disintegrates leaving only the vascular bundles. Mats of white fungal growth of Diplodia maydis may be evident on affected tissues. Diplodia also produces fruiting bodies which may be seen as small black specks embedded in the white fungal mat. Charcoal rot may begin as a root rot and move into the lower internodes of the stalks. Pith tissues will be shredded and plants may break at the crown. The charcoal rot fungus, Macrophomina phaseolina, produces very small survival structures called microsclerotia which may be visible as very small, black flecks just beneath the stalk surface or on the vascular strands remaining in the interior of the shredded stalks. Stalk rots are caused by several different fungi and bacteria which are part of the complex of microorganisms that decompose dead plant material in the soil. They survive from one growing season to the next in soil, in infested corn residues or on seed. Stalk rot pathogens enter the corn plant in a variety of ways. The spores may be blown into the base of the leaf sheath where they may germinate and grow into the stalk. Spores may enter directly into a plant through wounds made by corn borers, hail or mechanical injury. When fungi are present in soil or infested residue as either spores or mycelium, they may infect the root system causing root rot early in the growing season and later grow up into the stalk causing stalk rot. Stalk rot becomes a problem when plants are stressed during the grain filling stage of development. Water shortage, extended periods of cloudy weather, hail damage, corn borer infestation, low potassium in relation to nitrogen, leaf diseases and other stresses that occur in August and September may be associated with an increase in stalk rot. Losses from stalk rots vary from season to season and from region to region. Yield losses of 10 to 20 percent may occur on susceptible hybrids. Tolls greater than 50 percent have been reported in localized areas. Losses may be direct losses due to poor filling of the ears or lightweight and poorly finished ears or indirect through harvest losses because of stalk breakage or lodging. Harvest losses may be reduced if fields are scouted 40- 60 days after pollination to check for symptoms of stalk rot. Stalk rot can be detected by either pinching stalks or pushing on stalks. If more than 10-15 percent of the stalks are rotted, the field should be harvested as soon as possible. Management of Stalk rots of Corn Should include the following: * Select hybrids with good stalk strength and lodging characteristics. * Plant at recommended plant populations for that hybrid. * Follow proper fertility practices. * Avoid or minimize stress to corn (especially during pollination and grain fill). * Harvest in a timely manner. Laura Sweets 573-884-7307 ##################################################################### Conservation/Efficiency Key to Profitable Harvest By Bill Casady The cost of energy continues to change the way we set priorities as we enter into an early harvest. August is traditionally hot, so it’s been no surprise that the days have been oppressive, but harvest is beginning in earnest in some parts of the state and will be in full swing statewide soon. Most estimates place the harvest about two weeks early on average across the state of Missouri. Harvest is an energy intensive season. Combines use about 1.5 to 2 gallons of fuel per acre, a value greater than any other field operation. Yet, regular tune-ups, oil and filter maintenance combined with good management of service vehicles to haul grain from the field can only improve efficiency just so much. A much larger expenditure of energy occurs at the drying and storage facilities. An early harvest can have significant value if we use the calendar to patiently wait for field dry down to occur. Patience then becomes the key to conserving energy. Weigh the value of a quick and early harvest carefully with the cost of high speed drying. Grain dried quickly from greater than 25 percent wet basis moisture content can add as much as $40 per acre to the cost of energy used in corn production. Natural air drying from about 20 percent moisture can be accomplished for as little as about $7 for current utility/propane prices. Field dried grain will cost essentially nothing to dry, but it is still very important to have a plan in place to aerate grain in a timely way. An early harvest has its disadvantages as well as its obvious potential energy saving advantages. Grain harvested very early must wait a long period of time for cool weather to arrive for a proper cooling aeration cycle that will protect and preserve grain. Weigh field drying against other factors such as the condition of the stalks, lodging potential and dropped ears before making your final decisions, but do not be in a hurry to store grain under the hot dry conditions of August. It’s not too late to complete some last minute sanitation procedures to protect new grain from pests that have taken up residence in and around bins and in all of the equipment that moves grain. Sanitation details are available in several places, but the series of articles describing the SLAM procedure for storing grain begin at (http://ppp.missouri.edu/newsletters/ipcm/archives/v13n18/ipmltr5.htm). Key Sanitation Principles Include: 1) clean up any left over grain from the bin site, 2) mow and control weeds and habitat for insect and rodent pests, 3) remove all accessible panels to remove any left over grain in grain handling systems and equipment, 4) use bin treatments to eradicate any insect populations. There are still many older bins and even a few new ones that have never been carefully checked to ensure maximum efficiency. Roof vents should be sufficient to allow at least one square foot of open space for every 1000 cfm of airflow. Check fans, burners and other equipment to be sure they are in proper operating conditions and keep fan blades clean and free of obstructions to improve airflow. Bill Casady 573 882 4370 ##################################################################### Soybean Rust Update- August 7, 2006 By Laura Sweets Development and spread of soybean rust in the southern United States continues to be slow. However, over the last few days soybean rust has been confirmed in additional sites in Florida and Georgia as well as confirmed for the first time this season in Mississippi. According to the USDA Public PIPE Website (www.sbrusa.net), soybean rust has been found on this year’s soybeans in nine different counties in five states (Alabama, Florida, Georgia, Louisiana and Mississippi). The rest of the positive reports for 2006 have been on kudzu. The Mississippi report from the first week in August stated that soybean rust had been found in Jefferson County in southwestern Mississippi on both soybean and kudzu. The rust on soybean plants was found in an area shaded by oak trees and the incidence was very low- only 4-5 leaves which were later removed. The rust infected kudzu was along a roadside and was a heavier level of infection. The Delta and northern soybean production areas of Mississippi have been quite hot and dry so the threat of rust remains low in those areas. Overall, dry to very dry conditions in many regions of the southern United States have slowed soybean rust development and spread. In Missouri we are continuing to monitor 25 sentinel plots. Most of the sentinel plots have reached the R3-R5 stage of growth. Thus far, soybean rust has not been detected in any samples submitted from sentinel plots. As original sentinel plots reach R5 growth stage, observers are trying to locate fields in earlier stages of growth to monitor. Much of the state of Missouri has also been unusually hot and dry so conditions have not been favorable for the development of soybean rust even is spores had been introduced from the southern United States. The southeastern part of the state has had slightly better precipitation so scouting in that part of the state remains a priority. In addition to the sentinel plot program, the University of Missouri is again participating in the Syntinel RustTracker Spore Trap Network. This is a soybean rust spore monitoring program using spore traps designed to trap spores and other particles present in air moving through the spore traps. Spores and other particles are trapped in petroleum jelly coated on a microscope slide mounted in the spore trap. The slides are removed from the spore traps on a regular basis and shipped to a central laboratory for microscopic examination. Since the spores of soybean rust aren’t unique enough to be distinguished from other rust spores and even some other fungal species, only tentative identification of spores resembling those of the soybean rust pathogen has been possible with this spore trapping system. There are two spore traps located in Missouri, one in Boone County and one in Ste. Genevieve County. The slide from Ste. Genevieve County that was microscopically examined on July 25 did have one spore which resembled a soybean rust spore. No other slides have had any spores resembling those of soybean rust. Both traps are located in sentinel plot fields and soybean rust has not been found on soybean plants in the sentinel plots surrounding the spore traps. Plants at both sites are at R4 stage of growth and weather conditions have not been favorable for rust infection nor disease development. Overall, because of continued low levels of soybean rust inoculum to the south, unfavorable weather conditions in many parts of Missouri and the advanced stage of growth of much of the soybean crop, the risk for soybean rust in Missouri remains low and further management actions are not necessary. Exceptions would be areas of the state which have or are receiving more normal rainfall and late planted or double cropped soybean fields. Those fields should be scouted on a regular basis. Laura Sweets 573-884-7307 ##################################################################### Weather data for the Week Ending August 9, 2006 By Pat Guinan -------------------------------------------------------------------------------- | Monthly | Growing Weekly Temperature (deg. F) |Precip (in.)|Degree Days^ -----------------------------|------------|------------ Ext- Ext- Depart| Depart|Accum Depart Avg.Avg. reme reme from |Aug 1 from |since from Station County Max.Min. High Low Mean avg. |Aug 9 avg. |Apr 1 avg. ------------------------------------------------------|------------|------------ Corning Atchison 90 69 97 61 79 +3 | 1.95 +1.17 | 2737 +543 St. Joseph Buchanan 92 70 100 64 80 +4 | 0.97 +0.36 | 2671 +437 Brunswick Chariton 94 69 102 61 80 +4 | 0.71 -0.42 | 2657 +381 Albany Gentry 90 68 98 58 78 +2 | 4.73 +3.79 | 2534 +294 Auxvasse Audrain 93 68 103 61 80 +4 | 0.00 -0.86 | 2613 +324 Columbia Boone 95 70 102 63 82 +5 | 0.34 -0.49 | 2692 +316 Sanborn Field Boone 96 72 104 65 83 +5 | 0.01 -0.81 | 2869 +445 Williamsburg Callaway 93 67 102 61 80 +4 | 0.00 -0.86 | * * Novelty Knox 89 68 99 63 78 +2 | 2.42 +1.41 | 2444 +204 Linneus Linn 91 68 98 60 78 +3 | 1.38 +0.48 | 2491 +313 Monroe City Monroe 92 67 103 59 79 +4 | 0.88 -0.03 | 2535 +246 Versailles Morgon 98 71 104 62 83 +6 | 0.06 -1.02 | 2880 +468 Green Ridge Pettis 98 71 107 61 83 +7 | 0.00 -1.09 | 2793 +548 Lamar Barton 98 72 104 69 84 +6 | 0.00 -0.85 | 2847 +339 Cook Station Crawford 97 66 100 56 81 +4 | 0.01 -0.78 | 2571 +120 Alley Spring Shannon 94 65 96 61 78 +2 | 0.00 -0.85 | 2480 +159 Round Spring Shannon 94 65 98 60 79 +3 | 0.14 -0.71 | 2513 +192 Delta Cape | | Girardeau 93 67 97 62 80 +2 | 0.28 -0.56 | 2735 +21 Cardwell Dunklin 90 71 96 68 81 +2 | 0.71 +0.20 | 3109 +178 Clarkton Dunklin 92 71 96 66 81 +2 | 0.88 +0.17 | 3028 +133 Glennonville Dunklin 91 71 95 65 81 +2 | 0.80 +0.12 | 3019 +134 Charleston Mississippi 93 72 98 67 82 +3 | 0.62 -0.17 | 2900 +224 Portageville- | | Delta Center Pemiscot 91 73 96 68 82 +3 | 0.41 -0.23 | 3132 +256 Portageville- | | Lee Farm Pemiscot 91 74 95 68 82 +3 | 0.64 -0.01 | 3144 +285 Steele Pemiscot 92 72 98 68 82 +3 | 1.19 +0.60 | 3233 +358 -------------------------------------------------------------------------------- * Complete data not available for report ^Growing degree days are calculated by subtracting a 50 degree (Fahrenheit) base temperature from the average daily temperature. Thus, if the average temperature for the day is 75 degrees, then 25 growing degree days will have been accumulated. -------------------------------------------------------------------------------- Pat Guinan, Commercial Agriculture Program (573) 882-5908