Vol. 17, No. 12 June 15, 2007 Stewart’s Bacterial Wilt of Corn By Laura Sweets The variations in weather conditions this spring have put stress on young corn plants. In some fields seedlings have been showing yellowing and/or stunting from cool, wet soils immediately after planting and some leaves are showing injury from the unusually cool night temperatures. However, with the more recent warm weather, corn in many parts of the state has really taken off and is now 12 to 18 inches tall. So symptoms of Stewart’s bacterial wilt are beginning to develop on these rapidly growing young corn plants. On young corn plants the symptoms of Stewart’s bacterial wilt include linear, pale green to yellow streaks that tend to follow the veins of leaves and originate from feeding marks of the corn flea beetle. Lesions may extend the length of the leaf. Plants may appear stunted or somewhat distorted. If the bacteria become systemic within the plant, the entire plant wilts and may die prematurely. Cavities of a brown, soft rot can develop in the stalk pith. On field corn the disease tends to be limited to the leaf blight phase of the disease in which foliage symptoms develop but the pathogen does not become systemic within the plant. With the leaf blight phase of Stewart’s bacterial wilt, the linear, pale green to yellow lesions develop on the leaves. These lesions tend to parallel the leaf veins and to have wavy, irregular margins. These streaks soon become dry and brown. The bacterium which causes Stewart’s bacterial wilt overwinters in the guts of some species of adult corn flea beetles. Adult beetles feeding on corn seedlings in late spring and early summer can contaminate the feeding wounds with the causal bacterium. Flea beetles can continue to spread the bacterium throughout the season by feeding on infected plants and then healthy plants. The potential for Stewart’s bacterial wilt to develop on young corn plants is greater after mild winters when higher levels of the corn flea beetle may be present. Most field corn hybrids have enough resistance to Stewart’s bacterial wilt that additional management is not necessary. Laura Sweets SweetsL@missouri.edu ********************************************************************** Head Diseases of Wheat By Laura Sweets This has been a difficult year for wheat in many ways. And, unfortunately, the problems continue. Although some wheat fields look remarkably good considering the weather stresses they have encountered this year, in some fields various head problems are occurring. Loose smut is much more evident than it should be. Scab is developing in scattered fields throughout the state. Septoria tritici causes Septoria leaf blotch but may also cause dark blotches on heads and Stagnospora nodorum is actually more common on heads than leaves. Bacterial stripe (leaf disease) and bacterial black chaff are also evident in some fields. Finally in some fields the root disease take-all is evident as white heads in the field. Loose smut is obvious as heads emerge from the boot and for several weeks after that. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very obvious, black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads are wind carried to adjacent plants in the field and infect through the flowers. The fungus that causes loose smut survives within the embryo of wheat seeds. If infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season. If seed from a field that has a “small” amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut. Loose smut is best controlled by planting either disease-free seed or using a systemic fungicide seed treatment. Scab or Fusarium head blight was discussed in last week’s issue of the Integrated Pest and Crop Newsletter. Symptoms of scab are evident in fields now. The characteristic symptom of scab on wheat is a premature bleaching of a portion of the head or the entire head. Superficial mold growth, usually pink or orange in color, may be evident at the base of the diseased spikelets. Bleached spikelets are usually sterile or contain shriveled and or discolored seed. Septoria leaf blotch had not been particularly widespread or severe this season but with scattered pop-up rains and high humidity over the last few weeks this disease has come on more strongly. Foliage symptoms may be evident on the flag leaves and the dark brown to black blotches on the heads are also quite evident in some fields. Stagnospora nodorum may also cause leaf lesions but is usually more common on heads- again causing dark blotches on glumes of part or all of the head. Bacterial stripe or black chaff is a bacterial disease that produces symptoms on both leaves and heads. Water-soaked lesions may develop on young leaves. These develop into reddish brown to brownish-black streaks on the leaves. Glumes and awns show brown-black blotches or streaks. Fungicides are not effective against bacterial stripe or black chaff so the use of resistant or tolerant varieties and crop rotation are the main management options. Take-all is one of the more common root and crown rot diseases in Missouri. The fungus that causes this disease may infect seedlings in the fall. Symptoms are usually most evident after heading as white heads on wheat plants. Entire heads on infected plants may be bleached (white heads) and sterile. Infected plants are also stunted and slightly yellow, have few tillers and ripen prematurely. Plants with take-all typically have poorly developed root systems and roots are sparse, blackened and brittle. With sufficient soil moisture, a black-brown dry rot may extend into the crown and up the lower stem. This shiny, black discoloration of the lower stem and crown may be evident if the lowest leaf sheath is scraped off with a knife or fingernail. A management program for take-all should include planting good quality seed of adapted, disease resistant varieties, planting in well-drained sites under good seed bed conditions, rotating with nonhost crops for one to three years, controlling weed-grass hosts and volunteer wheat, using seed treatment fungicides and maintaining good plant vigor with adequate fertility. Laura Sweets SweetsL@missouri.edu ********************************************************************** Sheath Blight Damaging Missouri Rice Fields? By Allen Wrather Sheath blight is present in a few Missouri rice fields, and it is causing damage. It is not spreading rapidly in these fields due to the dry weather. However, farmers should scout their rice fields weekly for this disease from internode elongation to beginning bloom to determine if the field should be sprayed with a fungicide. Yield loss may be severe if this disease persists and eventually attacks the flag leaf and head. Varieties such as Clearfield 161, Cocodrie, and Cybonnet are very susceptible to this disease; Wells is susceptible; Francis, XP723, and XP730 are moderately susceptible; and XP710 is moderately resistant. Fungicides should be applied when the disease threshold is reached for the variety growing in the field. Stratego, Gem, Quadris, Quilt, and Moncut are labeled fungicides for control of this disease. University of Missouri Extension Regional Agronomist can supply information about sheath blight thresholds, or you can call me at the Delta Center. More information about sheath blight identification, control, and thresholds for treatment of fields with fungicides is available at aes.missouri.edu/delta/muguide/mp646.stm Here’s the situation. Sheath blight is caused by a fungus that most often infects rice plants soon after the flood is applied. Sheath blight lesions develop first on the stem near the water line. These lesions are circular to oblong (1 inch long and 0.5 inch wide), with grayish-white centers and purple-brown borders. Infection and spread of the disease is more severe when humidity is high, rain is frequent, and average daily temperatures are around 82 degrees. As the disease progresses, entire leaves may be killed and the disease may spread to rice heads. Damage can range from partial infection of the lower leaves to premature plant death. When the disease is severe, yield and milling quality of kernels can be reduced. Following these suggested procedures will give rice farmers a better chance to produce high yields and profit in 2007. Allen Wrather, Professor Division of Plant Sciences ********************************************************************** Plan Now to Attend the MU Weed and Pest Management Field Day on July 11th By Kevin Bradley The annual Weed and Pest Management Field Day will be held this July 11th at the Bradford Research and Extension Center near Columbia, Missouri. As in recent years, we have expanded the focus of this field day to include a variety of pest management topics that are of interest to agricultural industry representatives, agrichemical dealers, Extension specialists, and producers. Some of the research topics and trials that will be on display at this year’s field day include foliar fungicides for corn and soybean, corn and soybean fungicide seed treatments, corn rootworm transgenics, glyphosate-resistant weed research in Missouri, invasive weeds and their management, pasture weed management considerations, new herbicide and cropping systems, and much more. As usual, you will also have the opportunity to view plots that showcase a wide variety of herbicide treatments and weed management systems for use in either corn or soybean. The program will begin at 8:30 a.m. and will include guided wagon tours with stops that feature presentation of results and talks by university weed scientists, entomologists, plant pathologists, and agronomists. There will be a $10 registration fee collected at the time of check-in. This will cover costs associated with lunch and will provide each attendee with a tour booklet that describes the layout and location of each experiment. After lunch, you are welcome to spend the afternoon browsing the plots on your own. For certified crop advisors, 2 CEU credits for the field day are pending. If you plan on attending the field day, you must pre-register before July 9 by calling 573-884-7945 or by sending an e-mail to chismt@missouri.edu. The Bradford Research and Extension Center is located 7 miles east of Columbia, off of highway WW. For more complete directions call 573-884-7945 or visit http://aes.missouri.edu/bradford/index.stm. Kevin Bradley BradleyKe@missouri.edu ********************************************************************** Weather Data for the Week Ending June 10, 2007 By Pat Guinan -------------------------------------------------------------------------------- | Monthly | Growing Weekly Temperature (deg. F) |Precip (in.)|Degree Days^ -----------------------------|------------|------------ Ext- Ext- Depart| Depart|Accum Depart Avg.Avg. reme reme from |Jun 1 from |since from Station County Max.Min. High Low Mean avg. |Jun 10 avg |Apr 1 avg. ------------------------------------------------------|------------|------------ Corning Atchison 83 58 90 52 72 3 | 0.2 -1.39 | 977 301 St. Joseph Buchanan 80 59 88 52 70 1 | 1.28 -0.55 | 928 213 Brunswick Chariton 80 60 88 53 71 1 | 3.32 1.43 | 955 217 Albany Gentry 81 57 91 47 70 0 | 0.5 -1.28 | 896 209 Auxvasse Audrain 80 60 87 51 70 0 | 1.12 -0.39 | 973 236 Columbia Boone 81 60 89 51 71 1 | 0.65 -0.94 | 982 193 Sanborn Field Boone 82 62 89 53 72 1 | 0.66 -0.97 | 1055 239 Williamsburg Callaway 81 60 90 53 71 2 | 1.1 -0.56 | 996 282 Novelty Knox 79 56 86 49 68 -2 | 0.89 -0.65 | 858 150 Linneus Linn 80 56 87 48 69 0 | 3.53 1.83 | 900 220 Monroe City Monroe 79 57 87 52 69 -1 | 1.29 -0.34 | 920 176 Versailles Morgan 82 61 89 52 72 2 | 1.87 0.38 | 1045 207 Green Ridge Pettis 82 61 87 50 71 2 | 0.73 -1.36 | 973 265 Lamar Barton 82 62 85 54 72 1 | 8.61 7.02 | 984 112 Cook Station Crawford 81 57 88 49 69 -1 | 2.05 0.45 | 955 84 Alley Spring Shannon 83 56 89 51 70 0 | 1 -0.57 | 917 108 Round Spring Shannon 83 56 89 52 70 0 | 1.51 -0.06 | 937 127 Delta Cape 85 59 91 56 73 0 | 0.15 -1.00 | 1111 98 Girardeau | | Cardwell Dunklin 87 65 91 61 76 0 | 0.98 -0.24 | 1256 103 Clarkton Dunklin 87 64 94 61 76 1 | 1.04 -0.20 | 1212 84 Glennonville Dunklin 87 63 94 58 75 0 | 0.66 -0.38 | 1202 79 Charleston Mississipp 85 62 92 59 75 2 | 0.25 -0.97 | 1174 184 Portageville- 87 65 94 61 77 2 | 0.6 -0.83 | 1273 152 Delta Center Pemiscot | | Portageville- 87 65 92 62 77 2 | 0.43 -0.93 | 1279 171 Lee Farm Pemiscot | | Steele Pemiscot 89 66 95 62 78 3 | 0.44 -1.02 | 1336 207 ‡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 GuinanP@missouri.edu