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By Staff | Jul 24, 2015

After several months of what seemed like some of the coolest and wettest May and June weather we have seen, getting thrown into the blast furnace of 90- to 95-degree weather is quite a shock.

To me, the temperature range of 74 to 84 is just perfect while 93 or 94 is too warm. This is what the inhabitants of the West Coast states and many of the European inhabitants have been enduring in recent weeks.

While most of the Midwest states have had an excess of moisture parts of the area in Northwest Iowa and points remain dry with the corn plants showing a gray color with rolled leaves during this second full day of heat.

They need rain in the near future to avoid yield loss and additional stress on the corn.

The corn crop

The warmer temps pushed the average corn fields into tasseling a few days ahead of schedule. As of July 20, about 80 percent of the fields are at that stage.

This is typically a plus in maximizing yields since more moisture is available at this date versus mid-August, and moisture stress is less of an issue.

The kernels will also be filling during longer days when more GDUs are available.

Ideally, the days cool down with highs in the mid-80s and nights around 50 degrees with plenty of sunshine.

The question that is often asked is if corn plants grow after the tassel has emerged. The answer is that the tassel emerges when two or three leaves still need to fully extend.

The overall plant height with tassel will typically gain 15 to 18 inches after the tassel first pokes out.

Those last inches can be the ones that prevent making that last pass through the fields with a high clearance sprayer for a fungicide or final fertilizer application.

That final growth spurt or a wind that tips the plants at an angle can be the obstacle to a preplanned last pass meant to push the yields above any previous level.

Another question is if these late N applications can still have an beneficial effect. The new rule of thumb, as formulated based on actual field trials, is that corn can benefit on N applications clear through the tasseling stage.

The form of N also makes a difference. The individual responsible for late season trials on N-serve found that N in the ammonium form was more valuable than in the nitrate form as less plant energy was spent on its uptake versus the nitrate.

Disease issues

The appearance of plant diseases always hinge on having the inoculum, weather that keeps the leaves wet for many hours each day and susceptible plants. The susceptible plants were those streaked due to mineral deficiencies. The inoculum overwinters in the residue or blows in. The corn leaves that were staying wet up until 10:30 each morning last week created a very favorable environment for spore germination. Thus, alert growers were scouting their fields and finding diseases such as NCLB, anthracnose, GLS and eyespot. If you have not scouted be sure to do so in the near future. Be sure to study a scouting and ID guide to know what the treatment thresholds are given the value of the crop and cost of any treatment.

Now is when better plant health through variety selection pays dividends as does monitoring and meeting required mineral levels of the plants. Last year a very virulent form of NCLB showed up and really nailed varieties with B37 backgrounds. Were the varieties more susceptible or has the fungus mutated to a more virulent form? This weather change to hot and dry should slow the fungal diseases. The first sprayed varieties may require a second treatment.

The soybean crop

The majority of the soybean plants responded favorably to the warmer weather by growing taller. Having the soils dry out helped them and lessened the chance of serious root diseases. Having many hours of leaf wetness can lead to many leaf diseases.

The incidence of Septoria leaf spot is continuing to increase as the plants get closer to closing the rows. Again, scouting to see if the lower leaves on soybean plants are beginning to yellow is your clue to either applying a treatment product or using another management strategy.

The early stages of downey mildew are also appearing as small patches of fuzz on the undersides of the leaves. These patches will increase in number and size as the season progresses. This disease can influence final yields.

Get ready for more instances of white mold as it could become serious later this summer. Sclerotinia likes to infect during early flowering periods that are wet when soil temps are less than74 degrees and when they are splashed onto the lowering abscising flowers.

The fungus invades the plants eventually causing them to decline in health and productivity.

Bob Streit is an independent crop consultant and columnist for Farm News. He can be reached at (515) 709-0143.

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