Integrated Pest & Crop Management Newsletter University of Missouri-Columbia Vol. 16, No. 13 July 14, 2006 Common Waterhemp Population in Northwest Missouri Added to “Official” List of Glyphosate-Resistant Weeds By Kevin Bradley and Travis Legleiter As discussed in previous articles and at numerous field days and farmer meetings throughout the state, over the past year or more we have conducted extensive greenhouse research on a potentially glyphosate-resistant common waterhemp population discovered in northwest Missouri. This population was discovered in fields with a continuous soybean “rotation” where glyphosate has been used repeatedly as the sole active ingredient since 1996. The 2006 season was the first year that we were able to conduct research on this population at the actual field site and at this time we feel that we have met all the requirements that are necessary to declare a weed as “officially” resistant. Therefore, this common waterhemp population has now been confirmed as the ninth glyphosate-resistant weed in the world, and the sixth glyphosate-resistant weed that occurs in the U. S. The eight other confirmed glyphosate-resistant weeds throughout the world include buckhorn plantain, common ragweed, goosegrass, hairy fleabane, horseweed or marestail, Italian ryegrass, palmer amaranth, and rigid ryegrass (for the official lists and information pertaining to glyphosate- and other herbicide-resistant weeds see http://www.weedscience.org). Although results from our greenhouse research had already led us to believe this population was, in fact, resistant, we wanted to make sure of this in the actual field environment before this confirmation was made. In field trials, most of our glyphosate treatments were made in soybeans more than three weeks ago, and even extremely high rates of glyphosate have provided very poor control of this common waterhemp population (Figure 1). At this point in our research, we have observed that preemergence soybean treatments like Spartan, Valor, Boundary, Dual II Magnum, and Intrro have provided good control of this glyphosate-resistant waterhemp population. These treatments will continue to be evaluated throughout the summer to determine the length of residual activity on waterhemp that may emerge later in the season. Additionally, we have numerous experiments to evaluate glyphosate-resistant waterhemp control with conventional herbicides like Phoenix, Ultra Blazer, Aim, etc., when applied alone or as a postemergence glyphosate tank-mix partner. In our corn experiments, we have observed a similar trend in that many of our preemergence treatments like Lexar, Lumax, Degree Xtra, Harness Xtra, etc., have provided excellent control of the glyphosate-resistant common waterhemp population. We have also observed good control with postemergence treatments of Distinct and Callisto, but as expected postemergence applications of glyphosate and/or glyphosate-only corn programs have provided poor control of this common waterhemp population (Figure 2). All of these experiments are ongoing and first-year results will be discussed in more detail as we are able to gather and summarize the data, but at this time one thing seems clear from the field research we have conducted thus far: we can control a glyphosate-resistant waterhemp population in corn with a herbicide program that has alternative modes-of-action (other than glyphosate). While rotating to corn seems like a simple solution to a serious problem, it is much easier said than done in our state. Why? Because unlike all of the major corn and soybean producing states that surround us, Missouri is a state that plants a considerable amount of continuous soybeans. In 2006, for example, approximately 2.75 million acres of corn and 5.3 million acres of soybean were planted in Missouri. These figures alone show that somewhere, somehow, we have a lot of acreage being planted to continuous soybeans. This concerns us, since glyphosate-only programs in Roundup Ready soybeans are certainly the norm, and since the examples of glyphosate-resistant weeds found to date have been discovered in areas where a Roundup Ready crop, such as soybean or cotton, has been planted continuously without rotation and where repeated applications of glyphosate have been made year after year. As discussed in a previous article written this year (“Glyphosate-Resistant Weeds: Pay Now or Pay Later. Which is Better?”), we feel that the identification of this glyphosate-resistant waterhemp population should be viewed as a serious wake-up call to growers who are planting continuous soybeans and relying solely on glyphosate for weed control. Common waterhemp is our most troublesome weed in corn and soybeans and has shown its ability to adapt to other herbicide chemistries (ALS-inhibitors, Protox-inhibitors, triazine herbicides) in the past. Also, there is no reason to believe that independent selection events cannot occur throughout the state, meaning that other farmers who have been in a continuous soybean rotation and who have relied solely on glyphosate may also start to see instances of glyphosate-resistant common waterhemp in their fields as well. This appears to be what has happened in the southeastern United States with the glyphosate-resistant palmer amaranth that has surfaced in continuous Roundup Ready cropping systems during the past two years. We are already receiving calls this year from around that state about inconsistent control of common waterhemp with glyphosate. This is not that unusual as we receive these kinds of calls each year, but there seem to be more this year than ever before. Are there other glyphosate-resistant common waterhemp populations in Missouri or surrounding states? It’s difficult to say but given the number of acres that common waterhemp infests, the answer is probably yes. If you are concerned that you may have a glyphosate-resistant weed in your field, one of the first things to consider is the herbicide use history of the site where you are experiencing a problem. If you have been applying glyphosate as the sole herbicide for the past several years in this location, then the chances are much higher that you may have a resistant weed population that is either already established or becoming established in this location. If so, one of your first courses of action should be to rotate to a herbicide(s) with an alternative mode-of-action next year. Kevin Bradley 573-882-4039 ##################################################################### MU Entomologists Find Potato Leafhoppers in Alfalfa By Duane Dailey COLUMBIA, Mo. – Summer cold fronts and associated thunderstorms bring more than rain, they can drop clouds of potato leafhoppers onto Missouri alfalfa fields. “After the last two storm fronts, we started finding lots of adult leafhoppers,” said Wayne Bailey, University of Missouri Extension entomologist. Leafhoppers are carried on upper-level winds from Gulf Coast states where they overwinter in large numbers. The leafhoppers, both adults and nymphs, suck juice from the leaves and stems of alfalfa plants. Close examination of individual leaves shows a wedge-shaped injury on the leaf tip that dies and turns yellow. Eventually the whole leaf turns yellow, Bailey said. Fields heavily infested with the pests take on a yellow color. The damage not only reduces hay yields for the current year but also reduces stand vigor for the next year. Newly seeded alfalfa fields and fields recently cut for hay are most vulnerable, Bailey said. A stubble field starting to grow again after hay harvest can be heavily damaged by a small infestation of leafhoppers. The recommended economic threshold for treatment is only 10 hoppers per 50 sweeps when a stand is less than three inches tall. For alfalfa 12 inches or taller, the threshold is 100 hoppers per 50 sweeps. Scouting sweeps are made with a 15-inch insect net. “Alfalfa producers should be scouting their fields at least twice a week, especially after thunderstorms,” Bailey said. “If high numbers of insects are found, an insecticide treatment should be considered,” he said. “If the alfalfa field is ready for a third cutting of hay, an insecticide treatment can be avoided." University of Missouri research shows that harvesting with a disk- mower conditioner can reduce hopper nymph counts by 90 percent. Fewer adult leafhoppers are killed by mechanical harvesters, as the hoppers jump out of the way. Newer alfalfa varieties, with glandular hairs on their stems and leaves, are resistant to leafhoppers. “The hairs form a physical barrier which holds the hoppers away from the plant surface.” Information on leafhopper controls can be obtained from MU Extension regional agronomists. Insect information can be found on the MU plant protection program’s integrated pest management Web site at http://ipm.missouri.edu. Source: Wayne Bailey 573-864-9905 Duane Dailey Senior WriterExtension & Ag Information University of Missouri 573-882-9181 daileyd@missouri.edu Kevin Bradley 573-882-4039 ##################################################################### Rice Sheath Blight Advisory By Brian Ottis As we approach the later reproductive stages in rice development, it is important to be on the lookout for sheath blight disease. Sheath blight is caused by Rhizoctonia solani, the same pathogen responsible for aerial blight in soybean. Although we don’t have the disease pressure in Missouri similar to states further south, there are always problem fields that need to be scouted frequently. Several factors affect sheath blight development in the field. Some of these factors we can control, but Mother Nature most often determines where disease will show up. When considering sheath blight, some of the factors we can control are variety selection, seeding rate, and fertility. If you discover fields on your farm with a history of sheath blight problems, consider switching to a more resistant variety. For example, most of the semi-dwarf varieties grown today are more susceptible to sheath blight than the conventional-height and hybrid types. A few examples of susceptible varieties are CL131, CL161, Cybonnet, and Trenasse. If you have a red rice problem and utilize the CLEARFIELD* production system, a new variety, ‘CL171’ should be available in limited quantities next year with better sheath blight resistance. CLEARFIELD* hybrids should also be considered in fields with a history of sheath blight as they are rarely affected by sheath blight. Seeding rate is another factor we can control in the fight against sheath blight. Rice has an uncanny ability to tiller to compensate for voids in the canopy. If you have had a history of sheath blight in your fields, you may have noticed that it tended to be worse in areas where rice was double-planted on the ends of the field. When the canopy is thick, air movement through the canopy is impeded, which allows moisture to persist longer providing perfect conditions for disease development. Lower seeding rates produce a more ‘open’ canopy, which allows more efficient air movement through the canopy. Sheath blight development is also related to nitrogen fertility. In cases where too much nitrogen has been applied, or overlapped on the edges, sheath blight development can be severe. Excessive nitrogen leads to lush vegetative growth, which creates a similar situation to high seeding rates. In the event that sheath blight develops on your farm, there are a couple of management options that must be considered. First, determine if a fungicide application will be warranted by considering the variety, yield history, and stage of growth. If the rice is heading, a fungicide for sheath blight is usually not warranted at this stage as the panicle has ‘escaped’ the disease, or is too late. If the variety is CL161 or another susceptible variety, scout these fields twice a week if possible. Once the disease is found at a third of the stops in the field, an application of a strobilurin fungicide is usually warranted between ½” internode and late boot. On less susceptible varieties, such as Wells and Francis, scouting does not need to be as frequent, but a fungicide application may still be warranted if disease begins moving up the culms toward the flag leaf. With these varieties, wait until late boot before applying fungicide as sheath blight may not become severe enough to warrant an application. Research has shown that yield loss associated with sheath blight is most likely due to infection of the flag leaf, which supplies most of the energy to the panicle during grain fill. Therefore, even if disease thresholds have been reached, it may be better to wait until the disease is two to three leaves below the flag leaf until the application is made in order to get more ‘bang for your buck.’ In the case of hybrid rice, research has shown that more often than not, a fungicide application did not pay for itself in protected yield. Similar results were found for medium-grain varieties such as Bengal, Medark, and Jupiter. Brian Ottis Rice Agronomist Delta Research Center 573-379-5431 ##################################################################### 2006 Crop Injury Diagnostic Clinic July 25-26 & July 27-28 Bradford Research & Extension Center For registration information, contact Thresa Chism, 573-884-7945 or by e-mail at ChismT@missouri.edu For information on the content of the clinic sessions, contact Tim Reinbott, Superintendent of the Bradford Research & Extension Center, 573-884-7945 or by e-mail at ReinbottT@missouri.edu. ##################################################################### Field Crop Disease Update- July 10, 2006 By Laura Sweets Corn Corn foliage diseases are evident in many corn fields although both incidence and severity of these diseases is relatively low. Common rust, gray leaf spot and anthracnose appear to be the most common foliage diseases this season. Stewart’s wilt of corn has not been particularly prevalent this season. The last issue of the Integrated Pest and Crop Management Newsletter contained descriptions of corn foliage diseases. Common rust is evident as small pustules on the upper leaf surfaces. These pustules quickly rupture to reveal masses of rusty, red-brown spores. Common rust may develop on lower leaf surfaces and even on stalks and ears but right now seems to be most prevalent on the upper leaf surfaces on leaves in the lower half of the canopy. Initially, gray leaf spot appears as somewhat oval to elongate reddish-brown lesions. The causal fungus tends to be restricted by the leaf veins so as lesions increase in size they take on the parallel edges characteristic of mature gray leaf spot lesions. Lesions may range from less than an inch to several inches in length. Gray leaf spot is showing up on lower leaves in the canopy and lesions are just beginning to expand and take on the parallel edge appearance. Anthracnose is most common early in the season on the very first leaves that develop on corn plants. This year anthracnose has moved up in the canopy and can be found on the lower quarter to third of the leaves on a plant. Anthracnose lesions are oval to elongate in shape and tend to be reddish-brown to brown in color. Frequently anthracnose lesions will be surrounded by yellow to pinkish-yellow haloes. Foliage diseases tend to have the greatest impact on yield when they are present in high levels on the ear leaf. The earlier in the season that ear leaves become infected the greater the potential for yield loss. So far most of the corn foliage diseases that I have seen have been on the lower leaves in the canopy and the ear leaves are quite clean. The corn foliage diseases are favored by extended periods of free moisture on the leaf surfaces. Frequent light rains, heavy dews that stay late in the day and overhead irrigation can lead to an increase in corn foliage diseases. Most of the management recommendations for minimizing losses due to corn foliage diseases are preventative measures such as planting resistant hybrids, rotating crops so corn doesn’t follow corn in the same field or using tillage to reduce the amount of infected residue left on the soil surface. Several fungicides are labeled for use on corn to control foliage diseases. These fungicides tend to be most effective if applied at the first sign of disease when weather conditions are favorable for disease development. Check fungicide labels for application restrictions such as do not apply after silking, do not apply within 30 days of harvest, , do not apply within 40 days of harvest, etc. Soybean A number of different disease problems are beginning to show up in soybean fields throughout the state. Stunting and yellowing of plants may be the result of Fusarium root rot, Rhizoctonia root rot or Phytophthora root rot. With Fusarium and Rhizoctonia root rot plants may showing yellowing of the lower leaves, general stunting and poor root development but the entire plant may not yet be dead. Fusarium root rot tends to cause a rotting of the main taproot. The taproot may be discolored (ranging from light brown to a dark blackish-brown) and deteriorated. Rhizoctonia tends to cause red to reddish-brown lesions on the stem at or near the soil surface. With Phytophthora root rot the entire plant may have an off-color, wilted appearance. Entire plants may be dead. One key symptom of Phytophthora is the dark-brown discoloration of the main stem from the soil line up the main stem and even out side branches. At this point in the season there are no control measures that can be taken for these soybean root rot diseases. If weather conditions are stressful, i.e. hot and dry, affected plants may die. If weather conditions are mild with adequate moisture, affected plants may survive but not recover. Stunted, yellowed or poorly growing plants should also be checked for soybean cyst nematode. If plants are carefully dug up and soil gently removed from the root system, it may be possible to see the white to yellow bodies of the females on the roots. If SCN is suspected, it would be wise to submit a soil sample for SCN analysis. Some soybean foliage diseases are also showing up in low levels in many parts of the state. Septoria brown spot is occurring but is not as widespread or severe as it has been the last few years. Bacterial blight has been evident on some samples coming in from soybean rust sentinel plots. Bacterial blight lesions tend to be small, black to blackish-brown lesions with a slight yellow to light green halo. One or two samples have shown early symptoms of downy mildew. The upper leaf surfaces have bright yellow, irregular blotches and the downy mildew fungus may be obvious on the lower leaf surface as a purple to gray mold growth. Cercospora blight is also showing up on some samples. In some cases it has been developing as the reddish-purple discoloration across much of the upper leaf surface. In other fields it is showing up as the distinct brown lesions in areas of the leaf margin that are showing yellowing similar to that caused by potassium deficiency. None of the samples submitted thus far have had high levels of any of these foliage diseases. Soybean rust has not been detected on any samples submitted from soybean rust sentinel plots. See article containing an update on soybean rust in this issue of the newsletter. Finally, A few scattered plants are showing what may be very early symptoms of SDS or sudden death syndrome of soybean. Leaves in the upper to mid canopy are showing yellow, irregular blotches in the interveinal leaf tissue. The few fields in which I have seen this symptoms were planted prior to the wet period the end of April the first of May. Laura Sweets Ag Ext.-Plant Sciences 573-884-7307 ##################################################################### Soybean Rust Update- July 10, 2006 By Laura Sweets Soybean rust has been moving quite slowly in the southern United States. As of July 10, the USDA Public Soybean Rust Web site reported that soybean rust has been confirmed from 25 U.S. counties in five states. Most of these finds are on kudzu and many occurred early in 2006. Over the last week there have been several new finds on kudzu as well as three reports from soybean samples collected from sentinel plots. See the USDA Web site at www.sbrusa.net for exact locations and further information. The breakdown by state is as follows: Florida Soybean rust was confirmed in early to mid-January on kudzu in Gadsen, Alachua, Duval, Hernando, Hillsborough, Leon, Marion, Pacso, Lee and Polk Counties. In February, soybean rust was confirmed on kudzu in Miami-Dade. On May 16 soybean rust was confirmed on soybeans in a sentinel plot in Martin County. Georggia Soybean rust has been confirmed on kudzu from three counties, Thomas, Brooks and Miller. Soybean rust has just been confirmed on a soybean sample submitted July 3 from a sentinel plot in Decatur County. One of one hundred leaflets in the sample was infected with 7-8 pustules on the infected leaflet. Alabama Soybean rust has been confirmed on kudzu in 5 counties (Montgomery, Baldwin, Mobile, Henry and Houston Counties) and on soybeans in a sentinel plot in Baldwin County (June 29, 2006). Mississippi Soybean rust has not been confirmed in Mississippi yet this season. Louisiana Soybean rust has been confirmed on kudzu in two counties- the Lafayette County positive was found on June 29 and the Iberia County positive was found on July 7. Texas Soybean rust was confirmed in a soybean planting in Hidalgo County, Texas on February 12 but that site was harvested shortly thereafter. In Missouri we have been monitoring samples from 23 sentinel plots located throughout the state. Samples of 100 leaflets/plot were collected every other week through the vegetative stages of growth. As plants moved into reproductive stages of growth sampling has been on a weekly basis. Thus far, all samples from Missouri have been negative for soybean rust. The risk of soybean rust in Missouri is very low at this time. Although scouting of fields, especially as they move into reproductive stages of growth is recommended other control measures are not warranted at this time. A reminder on foliar fungicide use on soybeans would be that several chlorothalonil formulations (various trade names), Headline and Quadris do have full federal registrations and are labeled for use on soybeans for a number of diseases in addition to soybean rust. The section 18 fungicides have quarantine exemption labels for use on soybeans only if there is a threat of soybean rust and only for management of soybean rust. The section 18 fungicides are not labeled for general plant health or for control of other soybean foliage diseases. Laura Sweets Ag Ext.-Plant Sciences 573-884-7307 ##################################################################### Weather data for the Week Ending July 10, 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 |Jul 1 from |since from Station County Max.Min. High Low Mean avg. |Jul 10 avg. |Apr 1 avg. ------------------------------------------------------|------------|------------ Corning Atchison 84 64 87 54 74 -3 | 0.77 -1.15 | 1801 +400 St. Joseph Buchanan 82 64 85 59 73 -4 | 1.57 -0.04 | 1729 +293 Brunswick Chariton 83 61 86 55 72 -6 | 1.38 -0.02 | 1730 +254 Albany Gentry 83 61 86 53 73 -4 | 0.08 -1.88 | 1616 +184 Auxvasse Audrain 83 62 87 57 72 -6 | 1.50 +0.07 | 1696 +220 Columbia Boone 83 63 87 57 73 -5 | 0.46 -1.13 | 1730 +190 Sanborn Field Boone 83 65 87 58 74 -5 | 0.67 -0.94 | 1865 +289 Novelty Knox 84 61 91 55 73 -4 | 0.13 -1.31 | 1558 +115 Linneus Linn 83 60 86 55 72 -5 | 0.22 -1.37 | 1594 +199 Monroe City Monroe 85 61 89 56 73 -4 | 0.34 -0.88 | 1620 +131 Versailles Morgon 85 63 91 57 74 -4 | 1.05 -0.32 | 1860 +278 Green Ridge Pettis 83 63 88 59 73 -5 | 1.12 -0.26 | 1796 +358 Lamar Barton 83 63 87 56 73 -6 | 0.62 -1.47 | 1878 +234 Cook Station Crawford 83 58 87 48 71 -7 | 0.59 -0.45 | 1653 +33 Alley Spring Shannon 85 56 89 47 70 -7 | 1.67 +0.37 | 1646 +123 Round Spring Shannon 85 58 89 49 71 -6 | 1.35 +0.05 | 1669 +146 Delta Cape | | Girardeau 85 61 91 54 73 -7 | 0.26 -0.91 | 1856 +25 Cardwell Dunklin 86 65 90 58 75 -7 | 0.48 -0.51 | 2206 +188 Clarkton Dunklin 86 64 93 57 75 -7 | 2.84 +2.16 | 2117 +131 Glennonville Dunklin 85 64 89 58 75 -7 | 2.41 +1.69 | 2099 +121 Charleston Mississippi 84 65 91 60 75 -5 | 1.05 -0.37 | 1987 +187 Portageville- | | Delta Center Pemiscot 86 67 92 60 77 -4 | 1.19 +0.25 | 2198 +233 Portageville- | | Lee Farm Pemiscot 86 66 92 60 77 -4 | 1.66 +0.68 | 2203 +245 Steele Pemiscot 88 66 92 59 77 -4 | 2.01 +0.95 | 2297 +324 -------------------------------------------------------------------------------- ^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