Welcome to Sangamon-Menard Extension’s June 23rd edition of “Crop Update.” This publication is updated weekly during the growing season.

In this Issue:

• IN BRIEF – Fungicides, rootworms, Japanese beetles, and soy aphids.
• BROWN-SPECKLED, YELLOW SOYBEANS – Check out the roots.
• EXCEPTIONALLY UNUSUAL PROBLEMS IN CORN – No answers for this.

IN BRIEF

• FUNGICIDES – We have received a few questions regarding prophylactic foliar fungicide applications in both corn and beans. U of I Extension has noted that a yield benefit can occur due to prophylactic applications of fungicide in beans but notes that most applications (nearly 2/3) either barely break even or result in a net loss (i.e. try some plots/experiment). Contrary to the understanding of some, foliar applications will also not deter Sudden Death Syndrome (a spring infecting root disease whose symptoms appear in August). Likewise, using forecasted weather as a trigger for fungicide applications often proves a poor use of ones resources. In a world of tight margins, we recommend one think twice and scout for evidence of economically significant disease pressure before a fungicide is used.

• ROOTWORMS AND THEIR DEVELOPMENT – Visits with Denny Kopp, Lincoln Land FS; and plot tours with LLFS in Greenview and Mr. Harry Schirding of Petersburg detected significant to exceptional rootworm injury. Those tours also detected “well progressed” rootworm development consisting of 2nd and 3rd instar larvae, pupae, and adult beetles. Root examinations probably need to begin as soon as possible lest root regeneration confuse ones insecticide evaluation. In addition, sticky traps for adult beetles need to go into bean fields by mid-July.

• IOWA STATE NODE INJURY SCALE – Those evaluating roots will probably want to use ISU’s node injury scale. A “0.00 rating” equals no feeding. A “1.00 rating” is the equivalent of 1 node of roots eaten to within 2 inches of the plant. A “2.00 rating” stands for the removal of 2 nodes. A “3.00 rating” stands for the removal of 3 or more root nodes. The decimal portion of that rating provides the percentage of nodes missing. A rating of 0.25 is typically considered significant.

• JAPANESE BEETLE DEFOLIATION – Defoliation in corn would have to be exceptional to warrant an insecticide application due to this pest. Hail injury studies indicate 30 percent defoliation (30 percent removal of all leaf tissue at about a dozen leaves) would be required in much of the field to pay for an insecticide. Sangamon-Menard Extension reminds producers to keep leaf injury from this silk-clipping pest in perspective. Remember that they often congregate in limited areas of the field (i.e. end rows).

• SURE THEY ARE APHIDS? – “Small, lime green, and noted on the underside of the leaflet” sounds like the description for Aphis glycines (the soybean aphid), but many are easily confused when asked to differentiate between soybean aphids and potato leafhopper nymphs. Potato leafhopper nymphs are yellow-lime green colored, wedge-shaped, and 1/16 to 1/8 inch long when full grown. Tear drop shaped soybean aphids (similar size) can be found throughout the northern half of our state, but only an occasionally lone aphid has been noted downstate.

BROWN-SPECKLED, YELLOW SOYBEANS

A few producers noticed yellowed beans over the last week. While herbicide-induced yellowed beans have become a norm, the symptoms observed recently did not seem consistent with the “yellow flash” typically experienced following glyphosate applications. The leaves in these fields appeared yellow while the veins remained green as if plants were suffering Manganese or Iron deficiency. In addition, leaves in these areas were often speckled with small brown spots. In one particular field, Sangamon-Menard Extension noted “green” tire tracks in otherwise yellowed beans.

The fields in question likely were suffering from a nutrient deficiency (Manganese appeared most likely), and the “speckling” likely resulted from a nutrient deficiency as well. However, finding the underlying “true” cause means that one must dig deeper…literally.

An examination of root material in our previously noted piqued field showed roots that were “leopard spotted” with orange, orange yellow, slightly brown, or occasionally brick red lesions. Roots displaying less descript lesions often shriveled up, from the soil line down, when left in the open air for just a few minutes. Root symptoms similar to those described above are consistent with Rhizoctonia root rot. While manganese availability may have been marginal in this yellow bean field, the real issue “working these plants over” appeared to be a lack of root material caused when Rhizoctonia decimated roots and compromised the plant’s ability to procure micronutrients.

Rhizoctonia root rot is caused by the fungus Rhizoctonia solani. A very hardy fungus that survives as either fungal fruiting bodies (sclerotia) or as fungal hairs (mycelia), R. solani has a very wide host range. An excellent saprophyte, the fungus is known to reside and feed upon field residue. However cool, wet weather followed by warm-hot weather encourages it to infect seedlings. All major Midwest field crops can be infected by this fungus which can slash stands by half and reduce yields a couple score in the most severe cases. Infected roots sometimes develop the red to red-brown colored lesion we noted near the soil surface. These lesions and other less descript lesions may eventually girdle the root bringing the plant “to its knees.” Plant stress and heavy soils also encourage the onset of this disease.

Several options, when used in combination, can reduce problems with Rhizoctonia. First, maintaining adequate drainage/improving less than adequate drainage may decrease the wet conditions initially needed to jump start Rhizotonia solani. Second, any practice that decreases plant stress should narrow the window of opportunity available for this pathogen to infect seedlings. Such decreased stress allows plants to outpace this rotting fungus. Third, seed treatments may be a useful management option where historic Rhizoctonia problems occur. Finally, producers should use those soybean varieties displaying high vigor and rapid germination. While residue may encourage the presence of this saprophyte, the benefits of minimum, strip, and no-till systems outshine occasional problems with Rhizoctonia. Infrequent problems with this disease should not precipitate a shift in tillage.

EXCEPTIONALLY UNUSUAL PROBLEMS IN CORN

Extension offices, dealerships, and agronomists have been pummeled with questions about “unusual symptoms” in cornfields over the last week and a half. While hybrids, tillage systems, and herbicide programs have varied where such symptoms appear, the description of the problem observed remains unbelievably consistent. Corn plants exhibit a brown discoloration often trending toward a brown rot around the collar region of the plant that somewhat extends up the midrib. Many of these plants display a puckering of “collar region” leaf tissue near such damage, and several plants are observed “bending over” just above this injury which is found a few to several nodes up the stalk. The damaged area around the collar reminds one of a cigarette burn on upholstery, except the “burn” in this case is very small and several dozen such burns appear all clustered together where “leaf meets stalk” as of a crop version of flesh eating strep were at work. In some cases, Sangamon-Menard Extension has observed the formation of “smut galls” in this damaged area or on those collars found above this damaged area. What caused such symptoms? We (the collective “we” in this case) really are not sure why such symptoms appeared or what caused them. Several theories abound and include the following:

• A “bad” interaction between the plant, environment, and “something” (a herbicide possibly) caused the observed damage.
  - Note: Many plants do display damage at a very specific, consistent leaf collar throughout the field in question as if something happened throughout the field at specific stage of development. However, herbicide injury appears unlikely at this time because the symptoms just do not resemble known injury symptoms and random plants appeared affected. Additionally, different products were used on the fields in question, yet symptoms again remained unbelievably consistent.
• Soil in the whorl allowed bacteria to enter that most vulnerable portion of the plant resulting in a bacterial infection that rotted away leaf tissue ending with the unusual symptoms noted by scouts.
  - Note: A bacterial infection may have caused the described rot. However, some “symptom intense” areas often did not receive weather that would have resulted in soil being violently splashed into the whorl. Minus whorl introduction, bacteria likely would need an “entry point.” In other words, they would need a wound to enter the plant.
• Physoderma brown caused the rot noted in such unusual fields.
  - Note: Caused by the fungus Physoderma maydis, this disease forms circular to oval-shaped lesions that are most evident on leaf sheaths and leaf tissue near/wrapped about the stalk. Lesions eventually turn red-brown, brown, or even black and coalesce into much larger irregular shaped regions of similar discoloration. Should one use their imagination, the lesions observed in our “unusual corn fields” do appear similar (all-be-it atypically severe) to those caused by Physoderma. Given the right conditions, the disease may even produce a watery, node-restricting stalk rot that can result in lodging. However, Physoderma is typically a “cosmetic disease” that makes the plant look less attractive yet results in little to no yield loss. It would initially need some type of wound to cause such severe symptoms.
• An insect injured the plant, allowed bacteria/fungi to enter the resulting wounds, and caused the unusual symptoms. Stink bugs, known to cause growth abnormalities, would be the most likely culprit.
  - Note: A little use of the imagination could convince a scout that clustered brown lesions actually resemble clustered insect feeding/probing. Such injury would provide sites for either fungi or bacteria to enter the plant. With time, this could result in the unusual symptoms noted in our area. Unfortunately, unusually high insect populations appear devoid in many (not all) of these fields. Stink bugs, evident in some fields, appear totally absent in others. As Mike Roegge, Adams-Brown Extension, notes – “they would have to be smart to all feed on the same collar on all plants.”

Where does that leave the producer who wants to know what caused such symptoms and what can be done to avoid it? Frankly, the producer is left in good company. We are all left with a mystery that probably will not get solved this year, that may appear again next year, and that really leaves one devoid of management suggestions.