1. Symptoms of take-all in wheat 1 2. Assessing early-season hail damage on corn 4 3. Effect of stand loss on corn yields 6

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1 Number 194 May 29, Symptoms of take-all in wheat 1 2. Assessing early-season hail damage on corn 4 3. Effect of stand loss on corn yields 6 1. Symptoms of take-all in wheat In years with a wet spring, it is not unusual to see some fields in which there are large patches of wheat with white heads. In some cases, the wheat has lodged. In other cases, the patch of wheat with white heads remains standing. One possible cause of this is take-all root rot. Where excessive moisture has been a problem, another possible cause is drowning. Large areas of wheat can also lodge due to many other factors, including excessive nitrogen, weak straw, high winds or storms, poor root system, or several other causes. But most of these other factors won t kill the wheat and cause heads to turn white as will take-all and drowning. Take-all can occur almost anywhere in Kansas, but is most common in the central part of the state, especially under cool, wet conditions and in continuous wheat. This disease is relatively easy to diagnose in the field after heading, but may be more difficult to diagnose earlier in the spring. Wet soils, especially during the second half of the growing season, favor take-all development. Symptoms are less noticeable under dry soil conditions. Early planted wheat is most severely impacted. Symptoms are often first detectable around jointing time. Affected plants are stunted and yellow compared to healthy plants. Symptoms may be limited to blackened roots if soil moisture is low early in the development of the crop. If there is sufficient soil moisture throughout the growing season, the black-brown rot may extend into the crown and up the stem base, where a superficial, dark, shiny layer of the fungus surrounds the stem beneath the lowest leaf sheath. 1

2 After heading time, take-all symptoms are much easier to detect. Diseased stems weakened at their base may occasionally lodge, or lie flat on the ground. Although these symptoms are often the first obvious indications of take-all, they are the terminal stages of the disease. Whole plants begin to die and turn color prematurely. The white heads contain little or no grain. Plants may die individually or in large patches. Patches usually occur in wetter areas of the field. Occasionally whole fields are killed and this is how take-all gets its name. Since take-all is a root rot, affected plants pull out of the ground very easily. Diagnosis can be confirmed by the dark black discoloration on the roots and at the base of the stems. White heads occur in patches with take-all. Photo by Bill Willis, former K-State Extension Plant Pathologist. Black discoloration of roots and lower stem caused by take-all. Photo by Bill Bockus, K-State Plant Pathologist. 2

3 Take-all can be confused with Cephalosporium stripe, drowning, dryland foot rot, strawbreaker foot rot, winter injury, and crown rot symptoms. However, only take-all has the shiny, dark black discoloration on the lower stems. The fungus that causes take-all persists in infested wheat debris and on infected wheat plants. The take-all fungus can also survive between wheat crops on grassy weeds, and on volunteer wheat. Roots of the next crop become infected when they grow through soil near the infested debris. In the fall, the fungus moves from the old residue onto the roots of young wheat seedlings. This requires good moisture and close contact between the old residue and the new roots. There is evidence that the take-all fungus also spreads from plant to plant along root bridges, which may help explain why take- all occurs in patches. Take-all infestations often originate from weedy grasses in ditches or waterways. Pieces of infected roots and crowns are simply dragged into the field during tillage. Smooth brome, cheat, wheatgrass, and wild barleys are all possible sources. The take-all fungus also produces airborne ascospores which may play a role in initiating new infections. Take-all is not carried in the seed or by insects. Since the take-all fungus is short-lived, take-all is primarily a problem in continuous wheat. Take-all builds up slowly, so it's usually the third or fourth year of continuous wheat which is severely damaged. How can producers prevent take-all? * Rotate out of wheat for one to three years to a crop such as corn, soybeans or other legumes, grain sorghum, or oats. A rotation with barley or bromegrass will not be effective since those crops are also hosts of the take-all organism. A full year of fallow is an effective rotation. It is important to control weedy grasses in the rotational crop, since many grasses are hosts of takeall. * Maintain optimum soil fertility and ph levels to help reduce the severity of the disease. Lower ph levels actually help suppress take-all. When ph levels are 7.0 or higher in continuous wheat, severe take-all can result. * Control volunteer wheat and alternative grass hosts, such as annual bromes, smooth brome, wheatgrass, and wild barley. * Improve soil drainage on poorly drained fields. * Delay planting until after the fly-free date. Late planting can be helpful because it gives more time for the take-all fungus to die out. It also leaves less time for the new seedling roots to penetrate the old residue and become infected in the fall. * Burning followed by one or two tillage passes will hasten the breakdown of the wheat residue. Plowing may bury the residue deep enough that the take-all has trouble attacking new seedlings. 3

4 Clean-tilled fields also achieve higher soil temperatures, which are detrimental to survival of take-all. Of course, these methods are not appropriate for highly erodible land. Hard winter wheat varieties highly resistant to take-all are not available. Reports on the benefits of seed treatment with Baytan for take-all contol have been variable. Dividend seed treatment gives partial control of take-all. For more information, see Wheat Take-All Root Rot, K-State Extension Plant Pathology Fact Sheet, at: -- Erick DeWolf, Extension Plant Pathologist 2. Assessing early-season hail damage on corn Hail storms are common in Kansas this time of year, causing damage to both maturing wheat and young stands of corn. Wheat injury is easy to diagnose, but diagnosing injury to corn seedlings is more difficult. Hail damage always makes corn look bad, and can make for some sleepless nights. But while the physical damage is apparent, the actual effect on yield is not as obvious. Potential corn yield losses from hail gradually increase as the crop gets more mature, up to the silk stage, when peak yield loss occurs. After silking, yield losses from hail damage normally decline again. Why is that? There are several reasons, and all are based on the growth and development of the corn plant. * Emergence until stem elongation (VE to V5). Up through the 5-leaf stage of growth, the growing point of corn is below the soil surface. At the worst, hail damage would remove all five leaves, but typically not damage the growing point. A corn plant has 24 to 26 leaves at tasseling. If the plant loses five of those leaves early on, it will still have the potential to have 19 to 21 leaves at tasseling. Yield will be reduced, but by much less than one might expect from the appearance of the plant. * Stem elongation to tassel (V6 to VT). The growing point begins extending aboveground by the 6-leaf stage, although it is still protected by several layers of leaves and sheaths. The number of rows that will be in the ear is established by the 12-leaf stage. Stress during V8 to V11 can reduce row number. The number of kernels per row is not determined until about V17, just before tasseling. Hail damage and loss of leaf area during these stages of growth can cause increasing potential for yield loss. Hail can also cause stalk bruising during these stages of growth, but it is hard to determine the amount of damage from stalk bruising until later in the season. 4

5 * Tassel to maturity (VT to R6). At VT to R1 (tassel to silk), the corn plant is more vulnerable to hail damage than at any other stage. The tassel and all leaves are exposed at that time. No more leaves will be developed, and the corn cannot replace a damaged tassel. Furthermore, the stalk is exposed, with only one layer of leaf sheath protecting it. Unlike wheat, corn cannot fill from the stem if leaves are lost at this stage of growth. The six to eight leaves above the ear are the most important, and provide most of the grain fill. The four-week period centered around silking is critical to corn, and not only in regard to hail damage. Drought stress, excessive moisture, extreme heat, diseases, and even high winds can all stress the plant at this critical stage of growth and reduce yields. Early in this period, stress can reduce kernel number by limiting potential ear size. Stress right at silking can reduce the number of kernels fertilized. And stress just after silking can cause fertilized kernels to abort. Leaf Area - Yield Loss Relationships Percent Yield Loss V5 V7 V9 V11 V13 V15 V17-18 Silk Pre-blister Early Milk Late Milk Early Dent Late Dent Mature Percent Leaf Area Loss Growth Stage Graphic illustrating the relationship between estimated yield loss and percent leaf area loss at different growth stages of corn (based on data from Corn Loss Adjustment Standards Handbook, USDA-FCIC-280, pp. 7-98). -- Kraig Roozeboom, Crop Production and Cropping Systems Specialist kraig@ksu.edu 5

6 3. Effect of stand loss on corn yields Excessive moisture and other problems may have caused some stand loss in corn this year in parts of Kansas. Stand loss in corn usually causes relatively little yield loss, at least at populations greater than 24,000. The amount of yield reduction from stand loss depends on the stage of growth. Research by Barney Gordon at the North Central Experiment Field evaluated the effect of stand loss on yields, ear number, and ear size by removing plants at various stages of growth. Yield Loss From Three Levels of Stand Loss at Three Growth Stages 100 Percent Yield Loss Emergence V5 V Percent Stand Loss Growth Stage 6

7 Ear Number At Three Levels of Stand Loss at Three Growth Stages 2.0 Ears Per Plant Emergence V5 V Percent Stand Loss Growth Stage Ear Size At Three Levels of Stand Loss at Three Growth Stages 1200 Kernels Per Ear Emergence V Percent Stand Loss Growth Stage This study showed that: * Yield loss was much less than stand loss at every growth stage. 7

8 * Yield loss increased with stand losses that occurred at later growth stages. * Ear number per plant increased slightly at % stand loss, and more at 75% stand loss. * Seed weight did not change as a result of stand loss (data not shown). * Ear size increased with greater stand loss at early stages of growth. When considering replanting due to poor stands, keep in mind that planting corn in early June in much of Kansas can result in yield losses of up to % compared to a typical planting date. Based on the data above, it would probably be better to keep an existing stand even with as much as % stand loss than to replant in early June. Of course, much depends on the uniformity of the remaining stand and the weather for the rest of the growing season. -- Kraig Roozeboom, Crop Production and Cropping Systems Specialist kraig@ksu.edu These e-updates are a regular weekly item from K-State Extension Agronomy and Steve Watson, Agronomy e- Update Editor. All of the Research and Extension faculty in Agronomy will be involved as sources from time to time. If you have any questions or suggestions for topics you'd like to have us address in this weekly update, contact Steve Watson, swatson@ksu.edu, or Jim Shroyer, Research and Extension Crop Production Specialist and State Extension Agronomy Leader jshroyer@ksu.edu 8