INSIDE THIS ISSUE:

* Watch out for silk clipping 
   by Japanese Beetles

* Corn roots and stress

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Watch out for silk clipping by Japanese Beetles

As most of you are probably aware, Japanese beetles are present in huge numbers this year.  I have walked corn fields in which the beetles are stripping 60-70% of the foliage off corn leaves.  Although we traditionally think of Japanese beetles as soybean defoliators, they also love to feed on corn silks and this is my concern.  Due to the later planting season and what seems to be an earlier emergence of the adults, they will be present in full force when the corn begins to silk.  With this in mind, and with the added stress of hot dry weather on pollination, I think it's important to keep a very watchful eye on your fields.  The adults can hang around for 6 to 8 weeks so they won't be gone any time soon.  The general rule of thumb for Japanese beetles is that if the beetles are clipping silks shorter than 1/2" from the tip and less than 50% pollination has occurred, then treatment may be warranted.  Several products provide adequate control such as Sevin and Warrior.  The Pyrethroids will generally have a couple weeks of residual while Sevin lasts about a week.  Click these links to learn more.

Japanese beetles and insecticides for their control U o Ill

Japanese beetle fact sheet OSU

Corn roots and stress

As we walk fields and dig up plants, it is becoming obvious to many that root development has been severely retarded in some fields.  Several reasons exist as to why this might be.  Due to the late spring and wet conditions, many growers had no choice but to plant in less than favorable conditions.  The result was a smeared or packed seed slot that limited root development.  Jim Wolf dug up the plant pictured here and found this as a great example.  Brace root formation has also been restricted due to the very high temperatures and dry conditions at the soil surface.  As the brace roots try to grow into this forbidding environment, the root buds at the tip are literally burned back.  The good news is that these roots will continue to grow along the soil surface until they find a place to move into the soil or until cooler temperatures and/or rainfall occurs.  If your corn goosenecks over, brace roots can also form where a node touches the soil surface.  One other factor that has limited root growth is that the warm temperatures following the later planting date have really encouraged rapid foliar growth.  I have seen corn grow faster this year than I think I have ever seen before.  This rapid top growth has reduced the amount of time the plant spent developing root systems and also diverted nutrients normally used for roots and moved them into leaf tissue.  Once tasseling begins, root development basically stops.  Root depth can really impact yield.  The chart below lists some "average" water holding capacity of a few soil types (not all of this moisture is available to the plant). 

Consider that one inch of moisture generally produces about 8-10 bushels of corn.  An extra foot of root depth could expose the plant to 2 more inches of moisture in a clay soil for example.  That could mean an extra 15 -20 bushels. Now imagine the difference in yield if you recharge that foot of soil two or three times during the growing season. Take all these figures with a grain of salt, but remember that things that limit rooting in a dry year such as compaction or a tight seed slot can really impact yield.  Of course with the right rain, it could also have no impact.  I have added a couple of links discussing water use in corn and rooting issues below.

Stress on the corn crop U of Ill July 12th

Water stress effects on corn growth and development OSU 

Corn resilience and repeated stress NDSU