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One word describes the summer of '09 … cool. A quick review of the Illinois State Water Survey's "Crop Degree-Day" Calculator shows our area accumulation of Growing Degree Days to be behind normal for this time of year. With this comes concern for the corn crop and the question, "Will corn achieve adequate maturity before the first frost?"

Growing degree days (GDDs) are an accumulation of temperature readings that agronomists have found very useful in estimating corn maturity. A growing degree day is determined by adding the highest Fahrenheit temperature, not surpassing 86^o, and the lowest Fahrenheit temperature, not below 50 ^o, reached during the day. The resulting number is divided by two, and fifty degrees are subtracted. Corn hybrids need about 1,400 GDDs to tassel, and about 2,700 GDDs to reach full maturity. Not getting close to the "2,700 mark" by frost means plants are not fully mature at that time; and this, in turn, equates to some amount of yield loss. In other words, kernels are not allowed to fill as much as they could because a frost prematurely shuts the plant down.

So how does the situation look this year? Each year, Illinois receives an average of 3,000 GDDs, but the amount of GDDs accumulated for a particular crop in a particular field depends upon when that field was planted. In Illinois, during 2009, only 20-25% of corn was planted by mid-May. An additional 65-70% was planted by June 1, with about 5-10% of the crop planted after that date. From May 15 to August 1, about 1,500 GDDs had been accumulated in our area, so about 20-25% of Illinois corn had achieved that many, or more, GDDs as July turned to August. June 1-planted corn achieved about 1,300 GDDs and mid-June corn had achieved about 1,000 GDDs.

A mid-October frost would mean the accumulation of about 1,400-1,500 GDDs which would mean that more than 90% of the corn crop in Illinois would reach maturity (2,700 GDDs) by that frost date. A late frost would mean that most of the Illinois corn crop would reach maturity.

Producers have been reporting some evidence of gray leaf spot in our area, and this is of little surprise since conditions have been ripe for its return.

Gray leaf spot is a fungal disease of corn that prefers warm, humid weather similar to the muggy conditions we have experienced as of late. The fungus overwinters on corn debris and is moved to the plant either through wind or rain, which means that the disease appears on the older or lower leaves first.

Once infected, the plant takes a little while to show symptoms of infection. Symptoms appear as one-half inch to two inch long pale rectangles that are about 1/8 to 1/2 inch wide on the leaves. With time, these regions can grow and form large necrotic areas on the leaf tissue. Yield reductions are minimal if the leaf spotting does not reach above the ear leaf. However, yield losses can surpass a quarter on susceptible hybrids when spotting reaches above the ear leaf by milk stage, and can be double digit at early dough. Losses taper as the crop passes hard dough.

Scouting should begin now, if it has not already, and should continue until two weeks after tasseling. For future reference, U of I Extension recommends that producers keep careful track of weather conditions and start scouting the first week of July. Earlier planted fields may need to be scouted starting the last week of June.

Gray leaf spot is not an easy problem to solve. Resistant hybrids are the first step. Rotation also helps to eliminate some of the overwintering spore problems, and plowing, where soil conservation issues permit, will also aid the situation. If the problem begins too near the ear leaf, fungicide applications can help.

University of Illinois Extension recommends that producers pay particular attention to the lower three leaves of the corn plant from the period just before tasseling to two weeks after tasseling.

Soybean aphids were detected by Mason County Extension in a field outside of Havana during late July. A review of soybean aphid thresholds is therefore important.

Two different approaches can be taken to estimating a need to treat or not treat for soybean aphids. The first method is a total plant count of aphid numbers and the second is termed the speed scouting method.

The first method involves estimating average soybean aphid numbers per plant. Up to the point where a ¾ inch pod can be found on the top part of the plant (R4), the U of I recommends that management take place when populations reach 250/plant. Mason County Extension has often recommended two observations spaced three days apart at that time, and has recommended management if the population appears to be increasing during the second visit. Once seed in the uppermost pods has reached "BB" size (R5), the U of I has deemed a return on insecticide investment questionable. Mason County Extension has often recommended management if populations close in on 500/plant between that stage and the stage where seed fills up half of the pod chamber (R5.5). Once the plant reaches R6 (seed fills a pod found at one of the uppermost nodes), return on investment is so questionable that management is not recommended.

The second method for gauging soy-aphid intensity is termed "speed scouting" or "speed sampling." This technique was developed by the University of Minnesota.

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-infected) 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 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. The process continues until the situation is either clarified or a total of 31 plants have been examined. If such counts still do not yield enough "plus signs" to refute or encourage treatment, the recommendation is made to reexamine the field 3 to 4 days later.

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 at: http://www.soybeans.umn.edu/pdfs/2005aphid/speedsampling_blank.pdf.

Soybeans are one tough crop when it comes to pests. Soybean cyst nematode, soybean rust (which has yet to enter our state at a troubling time), soybean aphids (which can be problematic from time to time), and a few minor diseases are the only noticeable blemishes on the soybean's "crop resume."

However, from mid-July to mid-August, soybeans are occasionally attacked by outbreaks of insect pests that can feed on leaf tissue. Those pests can occasionally, all-be-it rarely, reach alarming levels. Accompanying Japanese beetles (the now most infamous defoliator) are bean leaf beetles, blister beetles, grasshoppers, green cloverworms, thistle caterpillars, and woolybear caterpillars.

Bean leaf beetles are not new to most producers. As a matter of fact, July is the second time that these leaf feeders get "press attention." They overwinter under residue in Illinois as adults, and are originally a concern in fields that are planted early. During July and August, they make a second appearance presenting a possible defoliation problem for mature soybeans as they feed over large areas of a field. Bean leaf beetles will vary in color and spotting, but these ¼" long beetles can always be identified by a black triangle located just behind the head.

Blister beetles are another potential soybean pest. They overwinter as adults emerging in swarms that land in isolated areas of the field. They can be disastrous in these isolated regions as they strip away foliage between the veins of the plant, but they almost never become a "field-wide" problem.

Sod and fence row areas of the field are "most loved" by grasshoppers as they lay their eggs in these areas. Eggs are the overwintering stage. Scouting these sodded areas in June can detect and eliminate intense grasshopper pressure before it enters the field. However, this "crystal ball" is past and scouting the bean field is the only means by which one can determine the necessity of grasshopper control.

Diseases are particularly hard on green cloverworm, a pale green larvae with two white stripes and three false legs in the mid section of its body. A three by three foot section of white cloth called a "beat cloth" may be utilized to determine the extent of disease. The producer simply "beats" the soybean foliage on either side of the cloth and examines the cloth for insects. Diseased insects will have one of three characteristics. They will either be dried out and shriveled, covered with a white cocoon-like cottony mass, or riddled with small, white eggs laid just behind the head. If disease is present green cloverworms will probably be decimated. They normally must piggy-back with other insects to cause problems.

The painted lady butterfly is a beautiful red, black, brown, and white mottled insect whose larva, known as the thistle caterpillar, can be another potential soybean pest. The thistle caterpillar, which commonly feeds upon thistles growing along the roadside or within pastures, does occasionally defoliate soybeans. The pest weaves a nest of leaves around itself, where it lives in isolation from other thistle caterpillars and the outside world. It can defoliate but typically harms only a few isolated plants.

Woolybears, called "woolyworms" by children who collect them as pets during the summer, are white hairy larvae when young. As they age, the worms turn dark brown or red. It is not uncommon to see quite a few of these worms crawling across roads in the fall. Two generations generally occur each year, and the pests are usually more abundant and damaging during dry years. The recent rains that have dowsed our area may have brought diseases into the woolybear population, thus limiting their potential as a pest. However, when scouting, a beat cloth should be used to determine the accuracy of this statement in the field in question.

Finally, the Japanese beetle, that 1/2" long brownish bronze and metallic green beetle identified by several tufts of white hair located just behind the wings, can and does defoliate area beans. However, yield-reducing defoliation is still limited (though not impossible).

The University of Illinois recommends control measures when 30% defoliation has occurred before bloom, and when 20% defoliation occurs between bloom and pod-fill. While that level of defoliation can be possible, it tends to be rather rare making soybean defoliation horror stories few and far between.

Ragged, tattered, brown-speckled soybean leaflets are showing up in some fields in the central part of Illinois. These symptoms are characteristic of the bacterial leaf disease "Bacterial Blight." We don't see this disease all that often, so keep an eye open for these telltale symptoms.

Bacterial blight is caused by the bacterium Pseudomonas syringae. This bacterium over winters on residue and can also, sometimes, spread via infected seed. The bacterium is transported to the plant when it is blown and splashed onto the surface of soybean trifoliates. P. syringae remains on leaflets until proper moisture conditions prevail when it then will infect the plant. During wet weather, the bacterium enters the leaflet either through the water regulating stomata or, more typically, via wounds. The bacterium moves into the spaces between the cells and multiplies. At the same time, the space between the cells is filled with a slime-like bacterial excretion while a toxic bacterial by-product inhibits the formation of chlorophyll. The latter two substances result in symptoms easily observed via the naked eye within 5 to 7 days of infection. The bacterium spreads best when storms are particularly windy.

Bacterial blight initially speckles the soybean leaflet with angular, yellow spots that eventually turn light brown. Those lesions initially appear slightly water soaked. Leaflets appear slightly puckered, similar to the puckering observed with some soybean viruses. As the tissue encompassed by the lesion dies, the lesion turns a dark red-brown to black color. These necrotic lesions appear all-the-more stark because they are surrounded by a yellow halo. The lesions usually are seen on soybean leaflets but the can appear on other portions of the plant as well. As bacterial blight progresses, the disease looks even more unsettling. Given enough moisture, and optimum temperatures for development of the bacterium (70 degrees with a bacterial envelope of 39 – 95 degrees), the black lesions coalesce forming large regions of necrotic tissue. The interveinal appearance of these necrotic areas may resemble well-aged Sudden Death Syndrome. Given a little more time, the necrotic regions begin to drop from the leaflet forming large, gaping, ragged holes. When combined with a little wind, the ragged appearance of leaf tissue intensifies.

In season management of bacterial blight is non-existent. However, hot, dry weather stalls development of P. syringae. Some resistance does exist in soybean varieties, which restricts yield losses to a less than double-digit range. Those varieties without resistance may see yield losses near 15 percent when the disease is prevalent and ideal conditions prevail. Since farm implements provide both a means of transportation for the bacteria and the slight mechanical injury needed for infection, cultivation and other operations should proceed with caution in those fields exhibiting more intense bacterial blight symptoms. Where conservation agreements allow, deep plowing may bury residue thus decreasing symptoms during the years following a bacterial blight outbreak. As usual, the use of high quality (pathogen free) seed may also aid the producer hoping to make the appearance of bacterial blight a once in a lifetime event.

Disclaimer – Asian soybean rust has not yet progressed into the Arkansas, Louisiana, Mississippi tri-state area, has become more prevalent in Louisiana, and displays limited distribution in the southwestern part of the United States (areas prone to distribute spores into the Midwest). This article is intended to remind producers about this disease just in case that distribution pattern should change. That said – the window of opportunity for this disease in narrowing.

• HISTORY – Soybean Rust was first reported in Japan in the early 1900's. Within the sixty years the disease was reported in India and Australia, and within another ten, soybean rust problems in Australia had reached "fairly intense" levels. The United States did not see the disease in native soybeans until it appeared in Hawaiian seed lots during the mid-nineties. With the turn of the century came the spread of the aggressive version of this disease to South Africa and South America. The more devastating species responsible for this disease made its first appearance in the soybean producing regions of Brazil and Argentina about seven years ago and appeared in our area soon after.

• CASUAL AGENT – There are actually two species of fungi that cause the disease pathologists term "soybean rust." The first is termed Phakopsora pachyrhizi, which is sometimes termed the Asian or African species of rust. The second is termed Phakopsora meimbomiae, which is sometimes termed the Latin America species of rust. Of the two species, P. pachyrhizi is deemed the most aggressive causing ten to eighty percent losses or more.

• EPIDEMIOLOGY – Soybean rust originates as a small hardy spore termed an oospore, which germinates on the leaf following prolonged wet periods. As the germ tube progresses from the spore, it develops a "swelling." From this swelling, fungal hairs (termed hyphae) penetrate the leaf tissue and start growing between plant cells. Initially the disease forms water soaked, gray areas that change into brown, four to five sided lesions. Pustules develop in these regions and do so on either surface of the leaf. The entire cycle from spore to pustule to spore may take only about a week. Lesions start out on the mid to lower portion of the plant, which is fairly unusual for a airborne disease like rust, and those lesions will develop on all parts of the plant at all growth stages. Yield losses partially result from reduced photosynthetic area, which reduces pod number/seed size. Some argue that increased evapotranspiration injures the plant as well.

• SCOUTING & MANAGEMENT REMINDER – Soybean rust will first appear on the lower part of the plant and will then progress upward. Strobilurin fungicide treatments are applied preventively and will thus not eliminate established rust. Triazole fungicide applications, while also preventative, should also provide curative action should the disease become evident.