We have attached the most recent edition of Sangamon-Menard Extension’s “Crop Update” for your use.

In This Issue:

• COMMON CORN LEAF DISEASES –
• BLISTER BEETLES ARE BACK –
• SOYBEAN REPRODUCTIVE STAGES –
• SOYBEAN APHID SPEED SAMPLING –
  NOTE: SANGAMON-MENARD EXTENSION WOULD APPRECIATE YOUR HELP IN DOCUMENTING A COUPLE UNUSUAL DISEASES THIS YEAR. THE FIRST IS A RHIZOCTONIA-LIKE ROOT ROT THAT APPEARS ESPECIALLY WIDESPREAD IN OUR AREA. PLANTS APPEAR STUNTED AND EXHIBIT A STRAW-BROWN ROT THAT STARTS RIGHT AT THE SOIL LINE. THIS IS ACCOMPANIED BY EXCEPTIONAL RED/RED-BLACK DISCOLORATION. THE SECOND IS A DISEASE CAUSING SDS-LIKE SYMPTOMS OCCASIONALLY ACCOMPANIED BY A ROT OF ROOT TISSUE. PLEASE CONTACT US WITH THE LOCATION (COUNTY, TOWNSHIP, SECTION), PLANTING DATE, CONTACT NAME, CONTACT INFORMATION, VARIETY, AND BRIEF HISTORY OF THE FIELD IN QUESTION.

  COMMON CORN LEAF DISEASES

  *COMMON RUST *- Caused by the fungus Puccinia sorghi, Common Rust does not overwinter in the state of Illinois. Instead, the fungus responsible for the disease actually overwinters in tropical (gulf coastal) regions of North America and is blown back into the state each year. In the south, P. sorghi may infect corn or its alternate host, wood sorrel. Should favorable conditions prevail (60-75 degree temperatures and wet weather) as the red colored uredospores enter the state, those spores will germinate on a leaf resulting in oblong, rough edged, pustules. The pustules are typically red to brown in color (a result of the red uredospores housed within) and they “crack” open forming a long “jagged” opening that runs the length of the pustule. Such pustules may be found on either the upper or lower surfaces of the corn plant (Note: Southern Rust appears on only the leaf surface. That disease also tends to have a more circular shaped pustule). Common sources of polygenic (multi-gene related) resistance and the general (post-tassel) timing of extensive Common Rust symptoms in the state typically cause this disease to be “minor importance” in field corn. Teliospores (resting spores) are produced toward the fall, changing the pustule color from a red-brown to a dark brown/ black color. However, an inability of the teliospore to overwinter in the state makes its appearance an interesting diagnostic feature but little else.

  NORTHERN CORN LEAF BLIGHT – Caused by the overwintering fungus Exserohlium turcicum (formerly Helminthosporium turcicum), Northern Corn Leaf Blight (NCLB) attacks corn, sorghum, sudangrass, foxtail, Johnsongrass, etc. The fungus is initially disseminated via windblown/rain splashed conidia (football shaped spores) that arise from infected debris. Conidia are asexual spores that arise from strands of fungal hair. NCLB conidia best infect a susceptible host when temperatures are moderate and water is present (i.e. heavy rains/dews). Inch to couple inch long, tan cigar shaped lesions develop on the bottom leaves first and move upward. The center of the lesion usually appears “dirty” because it is covered with spores. The most severe symptoms of this disease appear between July and August and the exact impact of this disease can vary significantly depending upon the hybrid. Resistance has typically stifled NCLB development until a few week post tassel (Note: The 2-3 weeks before and after tasseling mark a critical period for the corn plant). Resistance typically is more akin to tolerance thus allowing the disease to establish yet inhibiting exceptional development. Resistant plants may therefore display numerous small brown flecks on the leaf surface. Susceptible plants may suffer 50 percent yield losses given the right combination of condidtions. A few other management options for NCLB include burying residue where NCLB has been a significant problem (follow conservation agreements), maintaining balanced fertility, application of just enough yet not too much “N,” and practicing crop rotation.

  SOUTHERN CORN LEAF BLIGHT – The fungus Bipolaris maydis (previously Helminthosporium maydis) is responsible for symptoms that plant pathologists term Southern Corn Leaf Blight (SCLB). Attacking corn and sorghum, this disease is spread by windblown/rain splashed spores. The fungus overwinters within the state on crop debris or occasionally within kernels as either spores or masses of fungal hair. The spores, termed “conidia,” germinate at both “ends” or “poles” rather than on one end or “pole” of the spore (Note: many fungal spores commonly germinate from only one of their two poles). Conidia only infect when free water is found on the leaf surface and when temperatures are between 60 to 80 degrees. Observations of SCLB tend to be most frequent when high humidity and wet weather are punctuated by dry, sunny conditions. Symptoms can be difficult to isolate from those associated with other diseases in the field. However, textbooks typically state that SCLB lesions are “oblong, up to ½ inch wide, up to 1.5 inches long, with parallel sides.” Those lesions are typically tan or gray in color and may have a brown to purple colored border depending upon the hybrid in question. Symptoms typically appear most severe between July and August. SCLB is of course most famous for a “version” of the disease termed “Race T” which proved exceptionally devastating thirty-five years ago. That race spread throughout the corn belt due to wide spread use of Texas male sterile (TMS) cytoplasm in hybrid seed production. Normal cytoplasm was utilized in seed production the following year along with tillage practices that reduced residue. Where as “Race T” is known to infect leaf and stalk tissue, “Race O” is known for leaf infections only.

  GRAY LEAF SPOT – Perhaps one of the most famous corn leaf diseases, Gray Leaf Spot (GLS) is caused by the fungus Cercospora zea-maydis. The fungus attacks corn and pasture grasses following a winter spent on infected residue. The fairly hardy spores of this organism are spread via wind and rain. They prefer 70 to mid 80 degree temperatures, wet weather, and high humidity to best germ and infect their host. Moisture prone areas, such as bottom ground and irrigated fields, are more prone to this disease and in need of management. Actual symptoms do not develop until about 2 to 4 weeks following penetration of the leaf. They appear as long (up to two inch long), skinny rectangular lesions that are restricted between the veins of the plant. In exceptional cases, these lesions may coalesce forming larger regions of necrotic leaf tissue. Symptoms on the plant usually progress from the lowest leaves up and become most evident as our area races into the heat of July. Resistance does exist to this disease and represents one of the most effective and most used management tools for GLS. Most resistance available in today’s hybrids is more akin to tolerance (polygenic rate reducing resistance) and will thus allow limited/insignificant lesion development to occur. Crop rotation reduces the likelihood of encountering this disease as well since the fungus can not survive long minus a suitable host.

  BLISTER BEETLES ARE BACK

  Blisters beetles have been observed in area beans over the last few weeks. Such observations should not lead to apprehension but instead should lead to vigilance as producers prepare to cut hay.

  Half inch to inch long beetles that appear to have a very long neck (what actually is the thorax/mid-region of the insect), blister beetles vary in color. Some are black/dark gray and others are striped red, etc., but they are all usually noticed feeding in mass upon leaves in three-foot wide areas. As they feed, they remove all of the leaf tissue except for the most prominent veins. Females deposit their eggs in waste areas or pasture. The resulting larvae are (in some ways) beneficial, because they feed on grasshopper eggs.

  Cantharidin, a chemical agent contained within their body results in a blister once brought into contact with tissue. Should livestock, especially horses, accidentally consume a blister beetle while feeding on hay, the chemical also can damage the stomach. Every several years, a horse is killed in the state following the consumption of blister beetle contaminated hay.

  Producers should scout hay before they cut it for signs of this insect. If several exist in an area, that area should not be clipped. Insecticides are not an option since the carcasses of dead beetles also contain cantharidin. While not always preferred by equestrians, the first to early second cutting are safe from blister beetle contamination. Blister beetle adults can be found from July to August, and hay produced after that date runs the risk of contamination unless it has been scouted by the producer.

  SOYBEAN REPRODUCTIVE STAGES

• R1 One open flower at any node on the main stem.
• R2 Open flower at one of the two uppermost nodes on the main stem with a fully “unfolded” trifoliate.
• R3 1/4 inch long pod at one of the three uppermost nodes on the main stem with a fully “unfolded” trifoliate.
• R4 3/4 inch long pod at one of the four uppermost nodes on the main stem with a fully “unfolded” trifoliate.
• R5 1/8 inch wide seed in a pod on one of the four uppermost nodes. Noted when seed can first be felt in one of those pods.
• R5.5 Green seed fills half of the pod chamber of a pod on one of the four uppermost nodes.
• R6 Green seed fills the pod chamber of a pod on one of the four uppermost nodes.
• R7 One pod, anywhere on the main stem, now matured/browned.
• R8 95% of the pods have now matured/browned.

SOYBEAN APHID SPEED SAMPLING

“Speed scouting,” a scouting technique developed by the University of Minnesota, has been evaluated over the last couple years as a possible Illini method for soybean aphid evaluation. While the U of I has not fully “embraced or endorsed” this method, it could prove an efficient scouting tool and is at least looked upon favorably by University specialists. Sangamon-Menard Extension here provides a review of this technique “still under review.”

Eleven plants are initially scouted with the producer marking a “minus sign” on a worksheet for those plants displaying less than 40 aphids/plant (i.e. non-infested) or with the producer marking a “plus sign” for those plants displaying 40 or more aphids/plant. If 6 or fewer “plus signs” are recorded when 11 plants are examined, Minnesota’s speed scouting worksheet notes that the field should “not be treated.” If 11 “plus signs” are recorded, the worksheet states that the field should “be treated.” If 7 to 10 “plus signs” are recorded, the worksheet states that the current situation is not clear and 5 more plants should be evaluated in an attempt to “clear up” the current soybean aphid situation. Should 5 additional plants need to be evaluated, the producer then uses a new set of “plus sign” thresholds. Ten plus signs (the total from the previous 11 sampled plants and the additional 5 sampled plants) indicate that the field should not be treated, 15 plus signs indicate that the field should be treated, and any “plus sign tally” in between indicates that the situation is still unclear. If the situation is unclear, another 5 more plants must be sampled. The “plus sign” threshold for no treatment, treatment, or further sampling recommendations increases by 4 with each additional 5 sampled plants(i.e. 14 to not treat, 19 to treat, and 15-18 for further sampling when 21 plants are surveyed: 11 plants, 5 additional plants, and 5 additional plants). The process continues until the situation is either clarified or a total of 31 plants have been examined. If such counts still dodge absolute “treat” or “don’t treat” recommendations, the recommendation is made to reexamine the field 3 to 4 days later.

Sampled plants should be 30 paces (30 rows) apart and should be selected from various, representative, locations throughout the field. Regardless of the recommendation, every field should be reexamined in 7 to 10 days. The University of Minnesota notes that the speed sampling sheet works best when utilized “pre pod fill.” A “speed scouting” worksheet is available from the University of Minnesota.

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