Agronomy Update: Summer Time Disease and Insect Watch-Outs

Western Bean Cutworm

It’s that time of year for many crop insects to start causing concern.  This is especially true for Western Bean Cutworm (WBCW).  WBCW overwinters in pre-pupal stage then develops into a pupae in spring.  Typically, around the fourth week of June or early July they have completed development and emerge as moths. The week of July 15 was an intense week for moth flight this year.  Moth captures increased by 10 to 20-fold that week.  Begin scouting now for egg masses and larval hatch.  WBCW can be especially damaging because several larvae can feed on one ear.  This feeding also encourages or facilitates mold to develop in the damaged region of the ear.

• Evaluate 20 plants in 5 locations
• Threshold is 5% of plants having egg masses and or newly hatch larvae at or near 90% tassel
• It is critical to catch larvae before they are protected by entering the ear.

For further information see:  http://corn.agronomy.wisc.edu/Management/pdfs/A3856.pdf

More on Insects

Our ever-ubiquitous iridescent insect foe, the Japanese beetle, is back again this year. What should you know about protecting your corn and soybean acres?

• Japanese beetles like to congregate and colonize, so get a handle on the big picture.
• In soybean, treatment thresholds are 30 percent defoliation before flowering and 20 percent from R1-R5.
• Corn fields should be treated when 3 beetles are present per ear shoot and they have clipped silks down to ½” prior to peak pollination. They can move in fast, especially if a field is the only one flowering in a particular area (a concerning point in a year with such a wide planting window).

Thistle caterpillars (Figure 1) are making an appearance after favorable over-wintering conditions in the south. This spiny caterpillar forms tent-like leaf webs and usually causes only minor damage in soybean. Treatment is the same as Japanese beetle; when defoliation hits 30 percent before flowering and 20 percent during flowering.

Corn rootworm beetles are emerging across the Midwest and recent storms have exposed fields with weakened root structures from larval feeding. Now is the time to assess the potential for damage in future years while also watching for silk clipping. Sticky traps or beetle counts can be effective means of determining risks. Talk to your local Dairyland Seed Agronomist for more details.

Septoria Brown Spot

Septoria brown spot is an ever-present fungal pathogen in almost every Midwestern soybean field.  The onset of the disease can develop soon after planting and is often evident throughout the entire growing season.

Fungal spores are dispersed from infected plant tissue by wind and rain events to young plant tissue forming lesions on both upper and lower leaf surfaces in the lower canopy.   The leaf lesions are commonly irregular and purplish in color moving upward especially as warmer and wetter environments begin to be more common.  As canopy density increases and less light is made available to these infected leaves within the lower canopy, lesions begin to overlap one another forming much larger and darker brown sections.  Infected leaves typically begin to turn yellow and will likely completely senesce from the plant.  During hotter and dryer periods, the disease my subside for a time, but the pathogens disease cycle will repeat again and again as the seasonal changes begin to take place.  There are certainly varietal differences in susceptibility to this pathogen; however, there are no known sources of resistance at this time.

Management practices such as planting soybeans after soybeans, not providing adequate weed control, especially velvetleaf, and planting soybeans following several other legume species can greatly increase the incidence of this disease showing up in your soybean fields.  Rotation to non-host crops such as alfalfa, corn, and small grains is recommended to reduce risk.  Incorporation of infested crop residue is also quite beneficial to assist in keeping this pathogen in check.  In years when infection prevalence is early and/or exacerbated by unfavorable wetter and cooler growing conditions, products containing  Group 3, 7, and/or 11 fungicidal classes (or mixes thereof) applied at the R3-R5 growth stage can greatly reduce progression of this disease upward into the mid and upper portions of the canopy and protect yield potential to a great degree.

Tar Spot:  2019

Many growers remember all too well the “new” corn disease tar spot that showed up in 2018. As we try to learn more about the disease, what can we expect in 2019?

Tar spot is a fungal disease.  As with most fungal diseases, higher moisture conditions favor disease development.  That would include frequent rainfall, longer hours of leaf surface wetness, and dense foliage.  Tar spot also seems to prefer cooler weather as opposed to hotter weather.

The June weather in most of the Dairyland Seed footprint seemed to be more conducive to fungal disease development.  However, July was generally drier and warmer; thus, less conducive to disease development. 

To date, tar spot has been found in only a few fields and at extremely low levels.  A few fields in two counties in northwest Indiana, three counties in northwest Illinois, one county in eastern Iowa, and one county in central-western Michigan have had positive identification of the disease to date.

We discuss this disease now to encourage you to begin scouting for it (and other fungal diseases). Target fields that had tar spot last year, thus corn on corn fields.  Look first at corn that is nearer to or already tasseling.  Look in the lower and mid sections of the corn plants for lesions on the leaves. Tar spot lesions will be black and more or less circular, and may be found on the upper or lower surface of the leaf. Be careful not to confuse tar spot with insect frass (poop). Frass often is black and about the same size (maybe smaller) than tar spot lesions. Frass will usually wipe off a leaf with a little moisture (“spit test”) and gentle rubbing; tar spot lesions will not wipe off.

If you find or suspect tar spot, call one of us for positive identification.  Many of the universities are actively tracking this disease this year, so watch university newsletters as well as our site for additional information. If disease pressure develops quickly over the next couple of weeks, fungicide application may be beneficial. We can assist you in analyzing your situation and helping you to make management decisions.

Brian Weller Western Region 507.456.3034	Dan Ritter Central Region 219.863.0583	Branden Furseth Northern Region 608.513.4265	Rod King Eastern Region 574.596.6721	Terry Jones Eastern Region 419.630.3115