Fungicide Applications to Soybean: Stop, Look and Listen before Spraying
By Don Hershman

Each year since 2003, Kentucky soybean producers have had to decide if applying a fungicide to their soybean crop was in their best interest or not.  Quite naturally, soybean producers want to maximize the yield potential of their soybean crops. Application of a fungicide, when needed, has been shown to be one means to achieve this end. However, producers also realize that maximizing yield does not necessarily mean maximizing profits.  Treatments must be economical in order to be viable.  It’s that simple.

There are some who would tell producers to apply a fungicide no matter what, because the past yield performance of treating is just too favorable NOT to apply.  I do not subscribe to this philosophy.  Instead, I tell producers to make fungicide use decisions based on need, as determined by a disease risk assessment, for each field where a fungicide appliction is being considered.  It is not acceptable, and in the long term may not be sustainable, to treat fields without first establishing the need to spray. 

The data figures, below, are a case in point. The figures come from a summary of multi-state, multi-cultivar, small plot trials conducted in various Midwestern states by Pioneer Hi-Bred International, Inc, Agronomy Sciences, during 2004-2008.  Although Kentucky is considered to be a southern state, its soybean crop is more like that in lower Midwest than states like Arkansas, Louisiana, or Mississippi. Thus, the Pioneer studies done in the Midwest are directly applicable to us.  I have decided to focus on the results of the Pioneer studies since neither Pioneer, nor their parent company DuPont, sell foliar fungicides that are being commonly being used in soybean.  Pioneer also does not carry the label “too conservative” that has recently been applied to many university studies by some soybean producers.  The Pioneer studies and this article focus on Headline and Quadris since these two fungicides represent the majority of soybean acres sprayed in the Midwest and Kentucky

Figure 5 summarizes the results for Headline applied at 6 fl oz/A at the R3 (early pod) growth stage.  Figure 6 summarizes the results for Quadris applied at 6.2 fl oz/A at R3.  Fungicides were applied, without reference to disease risk, in 15-20 gal/A, and included adjuvants recommended by their respective manufacturers (BASF [Headline] and Syngenta [Quadris]).  Little foliar disease developed in any test, any year.

The results of both treatments are fairly similar, and the associated “piano graphs” are not unlike what you may have seen from a variety of sources, including many Land Grant universities.  Almost without exception, there is a trend towards an overall positive yield response when treating.  Our studies and many others have shown a similar trend.  Headline produced a higher yield in treated plots 78 out of a 100 times (78%), while Quadris produced the same statistic, but treatments differed in average yield response over all trials (3.7 for Headline vs. 2.9 bu/A for Quadris).  However, when economics were applied (Pioneer placed this at 3 bu/A and I think few would argue with this figure), Headline produced an economical result 51% of the time and Quadris 47% of the time.  University trials typicially show a less favorable response when data are subjected to appropriate statistical analyses, but lets assume for the sake of argument that 50% is about right.

Folks, 50% represents the same response I would get if I were to flip a coin 100 times.  The bottom line in the above examples is that while the overall positive yield response looks very favorable, the true measure of the treatments – economics- is about a 50-50 proposition at best.  I hope you count this as compelling evidence that you should avoid spaying without determining the need to spray.  There is no statistic available to support my next statement, but my guess is that the economic response rate would be >80% if spray decisions were based on a disease risk assessment.  It is true that benefits associated with stess tolerance and/or improved growth efficiency, as indicated on the Headline label and in marketing literature for both Headline and Quadris, may also occur when the fungicides are applied. However, there is presently no way to predict when, and to what extent, these benefits will occur.

Figure 5. Yield reponse to Headline @ 6.0 fl oz/A (2004-08); the dashed red line indicates the 3 bu/A breakeven point. Source: J. Trybom and M. Jeschke. 2009. Foliar Fungicide Effects on Soybean Yield. Crop Insights, Vol 19, No. 1.

Figure 6. Yield response to Quadris @ 6.2 fl oz/A (2004-08); the dashed red line indicates the 3 bu/A breakeven point. Source: J. Trybom and M. Jeschke. 2009. Foliar Fungicide Effects on Soybean Yield. Crop Insights, Vol 19, No.1.

Now that I have established that basing fungicide spray decisions on a disease risk assessment is essential, just what does this assessment involve?  Figure 7 lists the various factors that need to be considered as part of your risk assessment in soybean. Most of the factors determine the risk that one or more foliar fungal diseases will develop and reduce yield.  The factors do not account for the many other diseases that can hurt soybean yield, but are not controlled adequately (or at all) by foliar fungicides applied at the R3 growth stage.  This underscores that there is, and always will be, a risk associated with applying a fungicide. You might make all the right decisions and do all the right things, and a disease, such as charcoal rot, might negate the value of treating.  Similarly, if a crop experiences serious drought or some other event or condition that compromises yield, all benfits associated with spraying a fungicide might evaporate.  Farming, as you all know, is just risky business.

Figure 7. Factors that favor disease and, therefore, a response to foliar-applied strobilurin fungicides, such as Headline and Quadris.

At the moment, the Northeast is experiencing a major outbreak of late blight, and store-bought transplants have been identified as the original source of inoculum.  Apparently, plants produced by an unnamed company were distributed to a number of “big box” stores and were infected with the late blight pathogen, Phytophthora infestans.  The end result is that they are seeing an unprecedented distribution of late blight across many states in the Northeast.

We recently found two cases of late blight on tomato in Kentucky – from Laurel and Larue counties.  It is too early to tell if our late blight case originated from the same source(s) responsible for the cases in the Northeast, or if natural movement of inoculum occurred from these areas.  We do know, though, that we have experienced multiple spells of weather recently that would have been ideal for late blight and we’ve got more rain in the forecast for next week. 

The following comes from a recent article published by Dr. Meg McGrath of Cornell University (http://www.growingproduce.com/news/avg/?storyid=2111), and provides an excellent description of the disease:  “Classic symptoms are large (at least nickel sized) olive green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the lesion border is yellow or has a water-soaked appearance. Leaf lesions begin as tiny, irregularly-shaped brown spots. Brown to blackish lesions also develop on upper stems. Firm, brown spots develop on tomato fruit. Photographs are posted on the Web at:
www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/lateblight_tomato.htm and vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm. Home gardeners can learn more about the situation by going to Http://blogs.cornell.edu/hort/?s=late+blight+disease.”

At this point, we need to make sure that tomatoes and potatoes are protected by regular fungicide sprays (products for late blight can be found in ID-36).  For most, this will mean weekly sprays; spray intervals should be shortened to 4-5 days during rainy periods.  Not only will we get protection from late blight, but also other diseases; tomatoes in particular are being hammered by bacterial spot in many parts of the state.  Make sure that copper fungicides are a part of every spray that goes out on tomatoes – you’ll get protection against bacterial spot and speck, and some suppression of late blight.

As you read Dr. McGrath’s article, you will see a number of fungicides that are labeled for control of late blight, and most (if not all) are listed in ID-36.  One of the biggest issues that we will run across is finding these products.  Because we don’t have a lot of trouble historically with late blight, and because our overall acreage of tomatoes and potatoes is small, our ag dealers don’t stock many of these fungicides.  What I can tell you is that, of the products commonly available to commercial growers in KY, we will have the best success with preventive sprays of either chlorothalonil (Bravo and related generics) or mancozeb (Dithane, Penncozeb, or Manzate).  These protectants will function well unless we hit long periods of late blight-favorable weather, or if the disease is active in a field.  In these cases, we’ll need something stronger.  Commonly available fungicides like Quadris, Pristine, or Cabrio are mediocre performers against late blight.  Ridomil Gold products may or may not work, depending on the strain of the late blight fungus that is active – many are resistant to Ridomil.  Some of the better products, in my experience, include Previcur Flex, Gavel, Presidio, Ranman, Revus, and Tanos.  Of these, Previcur Flex, Gavel, and Tanos may be the easiest to find in KY.  With any of these late blight-specific fungicides, resistance management is critical; always tank-mix with a protectant product and follow label guidance (number of sprays per season, rates).

Homeowners have fewer choices available to them.  Essentially, they can use chlorothalonil, mancozeb, or copper products to protect against late blight.  Once disease is found, or if conditions are very favorable for disease, these products do not function as well as the late blight-specific materials available to commercial growers.  The best course of action for homeowners is to maintain a regular fungicide schedule, and to be prepared to destroy any symptomatic plants immediately.

For the present, our fungicide programs on tomatoes and potatoes can rely mainly on chlorothalonil and mancozeb (be mindful of the 5 day PHI on tomatoes / 14 day PHI on potatoes).  Only go with stronger materials if late blight is found, or if conditions favor disease and active late blight is known to be present in your area.  I can’t over-emphasize the need for a regular fungicide program.  Since late blight is such a fast-moving and devastating disease under favorable conditions, our growers shouldn’t wait until they see symptoms before applying fungicides.  If you have any questions, or if you find late blight in your area, please let me know.  I will do my best to keep you up to speed on this situation.

Grape root borer is potentially the most destructive insect attacking grapes in Kentucky. Larvae of this insect tunnel into the larger roots and crown of vines below the soil surface. Symptoms of Grape root borer attack include poor vine growth and fruit set, even loss of some vines. Because damage is restricted to below ground, problems often go unnoticed until vine decline is observed. Damage caused by larval feeding can range from just a few feeding sites to complete root system destruction. Multiple Grape root borer in a single vine can result in complete root girdling. Grape root borer is not common with new grape plantings, but problems often begin to develop after several years of grape production. Grape root borer is one pest of grapes that is often ignored until it becomes a serious problem affecting the vineyard. Moths active usually begins in early July.

Figure 9. Mating grape root borer moths.

Injury by Grape root borer is often most severe in low, poorly drained areas of the vineyard. In mid-summer, growers should examine around the bases of vines out to a distance of 18 inches for empty pupal skins of grape root borer.   In Kentucky, a control action for GRB is recommended if more than 5 percent of the vines are found to have GRB pupal cases emerging from the soil.

During recent weeks, Verticillium wilt has been diagnosed in the plant disease diagnostic laboratory on smoke tree and catalpa.  Verticillium wilt of woody plants is caused by the fungus Verticillium dahliae, or in some cases by V. albo-atrum.  The fungus is capable of causing a serious vascular wilt of a wide range of woody plants.  Several of our common landscape trees such as ash, katsura tree, magnolia, maple, redbud, and tuliptree are susceptible to Verticillium wilt.

Symptoms.  By invading the xylem tissues of the tree, Verticillium disrupts the movement of water from the roots to the leaves.  As a consequence, leaves wilt and branches die back.  This often occurs one branch at a time or on one side of the tree (Figure 10) over a period of several years, but sometimes in only a matter of months or a year.  Sometimes, branches simply fail to leaf out in the spring - the result of infection the previous year.  Verticillium wilt may also cause marginal browning and leaf scorch, abnormally large seed crops, small leaves, stunting, poor annual growth, and sparse foliage.  However, some or all of these symptoms may also be caused by girdling roots, construction injury, bacterial leaf scorch and drought.  The catalpa specimen observed last week was at first thought to be bacterial leaf scorch but scorch could not be confirmed with a specialized laboratory test (polymerase chain reaction) and a site visit revealed patterns more suggestive of Verticillium wilt (Figures 11 & 12).

In the landscape and nursery, one should try to observe additional diagnostic symptoms.  Usually, there is staining of xylem and cambial tissue, visible as streaks if you cut into the wood (Figure 13).  The color of this staining will vary for different trees often being greenish black in maple (Figure 14), yellowish green in smoke tree, dark brown in redbud, and brown in ash and catalpa.  Be aware that often young twigs and branches and some tree species simply don’t show the streaks of stained xylem tissue under the bark and that other fungi and other factors can cause staining.  For a positive laboratory diagnosis of Verticillium wilt, stained vascular tissue is essential.


Figure 13. Smoke tree leaf scorch and xylem staining evident in cut branch at right.


Figure 14. Small maple tree showing xylem staining symptom in the trunk. Note that staining does not completely encircle the xylem and could lead to one-sided wilt symptoms (P. Bachi photo).

Disease biology.  The Verticillium fungus survives as resistant, dormant microsclerotia for many years in soil, making effective crop rotation in the nursery or landscape difficult.  The fungus infects plant roots through wounds, or in some cases, direct penetration of susceptible root tissue. In the nursery, the Verticillium fungus could also be transmitted from plant to plant by grafting and budding.  From the root infections, the fungus spreads into the plant through the xylem.  Xylem tissues become blocked so that stems and leaves no longer are supplied with adequate water and mineral elements.  After the tree dies, the fungus is returned to the soil as tiny resistant fungal microsclerotia.  Microsclerotia can also be spread by wind, in soil, and on equipment.  Many herbaceous and weed hosts are also susceptible so it is hard to avoid contaminated soil.  Verticillium wilt is favored by landscape stresses such as wounding and drought.  It is possible that much of the Verticillium observed in Kentucky now relates back to stresses imposed by the drought last summer.

Management.

• Where Verticillium wilt has been diagnosed, only replant with disease resistant plants.  Conifers such as hemlock, pine, taxus and spruce are not affected.  Other trees that are typically free of this disease include: beech, birch, crabapple, mountain ash, dogwood, hackberry, hawthorn, hickory, holly, honeylocust, mountain ash, oak, pear, planetree, sweetgum, sycamore, willow, and zelkova.  The red maple cultivars Armstrong, Autumn Flame, Bowhall, October Glory, Red Sunset, Scarlet and Schlessinger have also been reported as resistant.
• Keep plants as healthy as possible.  Good plant health care includes good site selection, proper transplanting, good water management, a prudent fertility program, and pruning out dead branches.  Be aware that while pruning out infected branches is a useful general horticultural practice for maintaining plant vigor and aesthetics, it does not eliminate Verticillium from the plant since infections originate and spread from the roots.
• Fungicides are not effective for control of this disease.

Millipedes are long, many segmented creatures that use their two pairs of legs per body segment to move along with deliberate speed. There are several species in Kentucky with a variety of shapes and colors.

Millipedes can be very abundant in forest litter, grass, thatch, and in mulched areas. These places provide needed food, shelter, and dampness. Usually, millipedes stay out of sight unless abundant rainfall or some other event, such as the mating season, puts them on the move.

While harmless and in fact, helpful recyclers, millipedes generally are not welcomed with enthusiasm. They often invade crawl spaces, damp basements and first floors of houses at ground level. Common points of entry include door thresholds (especially at the base of sliding glass doors), expansion joints, and through the voids of concrete block walls. Frequent sightings of these pests indoors usually mean that there are large numbers breeding on the outside in the lawn, or beneath mulch, leaf litter or debris close to the foundation. Because of their moisture requirement, they usually do not survive indoors for more than a few days.

Wood cockroaches are typical cockroaches that are about 1 inch long. They live beneath loose bark, in wood piles, stumps, and hollow trees where they feed on decaying organic matter. These males can fly and are attracted to lights at night so they can be accidental invades of home around wooded areas. They also may be brought indoors on firewood. While they resemble the common household pest species, wood cockroaches rarely become established indoors.

Managment

• Minimize moisture & remove hiding places - The most effective, long-term measure for reducing entry of millipedes and wood cockroaches is to minimize moisture and hiding places, especially near the foundation. Leaves, grass clippings, heavy accumulations of mulch, boards, stones, boxes, stacked firewood or similar items laying on the ground beside the foundation should be removed, since these often attract and harbor pests. Items that cannot be removed should be elevated off the ground.
• Seal entry points - Seal cracks and openings in the outside foundation wall, and around the bottoms of doors and basement windows. Install tight-fitting door sweeps or thresholds at the base of all exterior entry doors, and apply caulk along the bottom outside edge and sides of door thresholds. Seal expansion joints where outdoor patios, sunrooms and sidewalks abut the foundation. Expansion joints and gaps should also be scaled along the bottom of basement walls on the interior to reduce entry of pests and moisture from outdoors.
• Insecticides - Exterior applications, in the form of barrier sprays, may help to reduce inward invasion when applied outdoors, along the bottom of exterior doors, around crawl space entrances, foundation vents and utility openings, and up underneath siding. It also may be useful to treat along the ground beside the foundation in mulch and ornamental plant beds, and a few feet up the base of the foundation wall. Heavy accumulations of mulch and leaf litter should first be raked back to expose pest hiding areas. Insecticide treatment may also be warranted along the interior foundation walls of damp crawl spaces and unfinished basements. There is no benefit from treating indoors.
  
  

Some incredible insects appear during the summer; here are a few that you might see in the next few weeks. The dobsonfly has an ominous appearance but does not pose any threat other than a weak pinch if handled.

Figure 15. Male dobsonfly

Dobsonflies are large (3+ inch), prehistoric looking insects that appear near flowing water in mid- to late summer. A pair of very long antennae comes off the front of the boxy flat head. Males have a pair of long pointed mandibles that cross like sabers; females do not have these. Two pairs of large smoky, pliable wings interlaced with many veins cover the abdomen to complete the major features of this insect.

Dobsonflies follow an erratic fluttery path thru the air. They will pinch if handled but are not dangerous. The adults live for only a few days, long enough to mate, lay eggs, and die. Eggs are placed on overhanging branches or undersides of bridges over streams, or on stones. The eggs hatch at night after 5-6 days and drop into the water. The larvae called Ahellgrammites@ live for several years under stones in streams where they feed on insects that live in the water. They are used as bait by fishermen.
 

Figure 16. Eyed Elater

The false eyes on the pronotum must cause potential predators to think again before grabbing hold of the eyed elater, a 2-inch long black beetle that is speckled with white spots. Their real eyes are much smaller and are located near the base of the antennae.

Eyed elaters appear around damp areas near woods about this time of year. They belong to the click beetle family – beetles that can flip over if placed on their backs. The larvae have a wireworm form and live as predators under bark.