The story you are about to read is true. The name of the agronomist involved has been changed to protect the dumb. The moral of this story is to not wear blinders into a field while making crop problem diagnoses.
Late last week, an experienced corn specialist (let's call him 'Bob'), was making quite a few field visits, diagnosing this problem and that problem. The usual mix of causes were found from field to field: twisted whorls and yellow tops, ALS herbicide injury, collapsed grape larvae (or were they grape colapsis larvae?), white grub injury, seed rots and seedling blight, compacted soils, cultivator blight, etc.
In one field, 'Bob' discovered that 5 to 10 percent of the plants were essentially dead 'from the neck up', meaning that the first two or three leaves were okay but the remainder of the whorl was dead. A smaller percentage of the plants in the field were technically alive but deformed in their growth. An occasional plant showed one or more series of 'shotholes' in the leaves. Leaves of some of the affected plants displayed necrotic streaks along their length.
Being a wise and experienced agronomist, 'Bob' knew that the whorl damage and death was often caused by insect damage to or near the growing point area of a young plant. Sure enough, upon splitting the stalks of the affected plants 'Bob' discovered that the growing points were not in good condition. "Ahah! I knew it!", he exclaimed proudly to the farmer standing at his side looking on with amused interest.
In fact, his first thought was of stalk borer because there appeared to be a feeding cavity where the growing point should have been. However, further inspection of the plants did not reveal any entry or exit wound that such an insect critter would leave behind. "Dang it!", he said.
Well, the next guess was some sort of piercing insect like the stinkbug. This experienced agronomist had participated in the Great Stinkbug Hunts of the early 1980's in notill fields of southern Indiana and was well versed in diagnosing that type of injury. In fact, he had often recreated stinkbug injury symptoms by drilling through the stems of young corn with tiny drill bits. However, the discolored cavity did not quite look like that caused by the proboscis of a stinkbug penetrating the stem of a corn plant. "Dang it!", he said.
Just then 'Bob' remembered that a several other Midwestern states had reported the occurrence of billbug injury in recent weeks. Now, 'Bob' had never seen billbug injury before but he knew that those pests somehow penetrated corn stems and injured meristematic areas with their snouts. Furthermore, the owner of the field was by now looking sternly over his shoulder wondering when he was going to make a concrete diagnosis. "Eureka!", he exclaimed. "I betcha this injury was caused by billbug feeding!", he confidently proclaimed and thereupon left the field.
On the way home, 'Bob' called one of his insect buddies (John Obermeyer) at Purdue and proudly described his nice piece of detective work. John was skeptical of the diagnosis, partly because 'Bob' reported so few plants exhibited the characteristic 'shot' holes in the leaves that would result from either stinkbug or billbug injury to corn. He also thought the description of the cavity at the growing point area did not fit the diagnosis. Well, the chastened agronomist figured he would visit the field again and look for more evidence that would support either stinkbug or billbug injury and prove to John that the diagnosis was correct.
So, back to the field on Monday for more investigative work. 'Bob' looked for evidence of nutsedge infestation because several references noted that the weed is a major host for billbug and is usually present when billbug injury occurs. None was found. He looked for billbug larvae in the stem or roots of the injured plants because several references noted that such things should be present. None were found. 'Bob' looked again for the characteristic 'shot' holes in the leaves of the affected plants, but only a small percent of the plants showed such holes. He looked for evidence of entry and exit wounds in the stems that could implicate either insect pest. Only a few plants exhibited such evidence. "Dang it!", he said.
The now thoroughly frustrated agronomist decided that it was time to 'call in the calvary' and so took a handful of injured plants back to the university's Plant and Pest Diagnostic Laboratory. His friend John looked over the plants as did another colleague in plant pathology (Gail Ruhl).
Lo and behold, the correct diagnosis of the injured plants turned out to be the wilt phase of Stewart's Disease, that nasty bacterial disease transmitted by flea beetles. Gail identified the bacteria streaming out of the damaged tissue.
I had never, I mean, uh..., 'Bob' had never seen the wilt phase of Stewart's Disease before and, consequently, never considered that possibility when diagnosing wilted or dead whorls. Indeed, the wilt phase of this disease is fairly rare, particularly with commercial corn hybrids.
The occurrence of wilted or dead whorls in young corn or deformed corn plant development almost always suggests a problem with the growing point area. The problem can be the result of physical damage to the growing point area by piercing or penetrating insects like the stinkbug or billbug. Other insects like black cutworm or stalk borer injure the growing point area if they tunnel and/or feed in that region of the young corn stem. Equipment tire traffic (e.g., turning on end rows) can easily injure the growing points of young corn older than about leaf stage V4. More common seedling blight organisms can also injure the growing point by virtue of their development into the crown area of young seedlings.
And now, 'Bob' realizes, the wilt phase of Stewart's Disease can also be the culprit of wilted or dead whorls in young corn or deformed growth.
Life-long learning: it's not just a catchy phrase!
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For other information about corn, take a
look at the Corn Growers Guidebook on the World Wide Web at
http://www.kingcorn.org
