CORN
Crop Observation and Recommendation Network

July 21-28, 2003
C.O.R.N. 2003-24

In This Issue:

A) Wet Weather Brings on Phytophthora - Is Sclerotinia Next?
B) Kernel Development Stages in Corn
C) Soybean Insect Update
D) Corn Insect Update
E) Factors That May Affect Tissue Test Results

A) Wet Weather Brings on Phytophthora - Is Sclerotinia Next? - Dorrance CORN Questions

Warmer temperatures last week brought on the symptoms of Phytophthora stem rot in my plots at OARDC NW branch at Hoytville and the first samples from farmer fields came into the office. Phytophthora loves warm weather - 70 and higher - so up to now these cool nights have slowed down the development of the stem rot lesions. The study at NW Branch looks at soybean varieties Rps genes combined with little to no partial resistance. Close to 50% of the plants were dying in the soybean variety with Rps1a with no partial resistance. For the variety with low levels of partial resistance, only 1 to 2 plants per plot were beginning to show symptoms. For fields with flooding injury - if Phytophthora is present in those areas, additional plants will begin to die this week. It is a good time to check to see if the genes in your varieties are working.

Flowering is delayed in many areas of the state and the cool night temperatures are perfect for infections of older blossoms by the fungus, Sclerotinia sclerotiorum. This is better known as white mold or Sclerotinia stem rot - but in order for this to occur several factors must come into place first. One, sclerotia have to be present in the field, if you have never had Sclerotinia in your field - you can scratch this one off your list - (note the last widespread occurrences were in 94, 96, 97, a little in 98). Second, the canopy must be closed prior to flowering - this provides the optimum environment for the small mushrooms or apothecia to form. Next what are the control options for this year? If you've planted a variety with some levels of partial resistance to Sclerotinia stem rot, relax, don't do anything and pat yourself on the back for making a good choice. If conditions are extremely favorable - some disease will develop but not as much if a highly susceptible line was planted like Williams 82. If you know your variety is a typhoid Mary when it comes to Sclerotinia, your best option is fungicides applied between R1 and R2, Topsin M is the only material labeled. However, you MUST get this material into the lower canopy on the bottom third of the plant. Plan on driving slow-3 to 6 MPH and apply at least 20 gal/A of water. Other remedies have been touted for control of Sclerotinia, namely the herbicide Cobra. I don't think that this is good year to even experiment with this product. These beans have been under stress since the day they were planted. Applying Cobra at these late growth stages (R1 to R2) will set them back some more and in these fields that have had repeated flooding and poor root systems, they many not recover from injury to the plants. The rest of the season will have to be perfect soybean weather in order for beans to recover and avoid a yield hit at the end of the season. For more than you will ever want to know about Sclerotinia stem rot - check the North Central Soybean Program's Plant Healthy Initiative WebSite - http://www.ncsrp.com/whitemold/managemt.htm. Particularly the article on the variability in control across the region for Cobra applications: http://www.ncsrp.com/whitemold/cobrapub.htm

B) Kernel Development Stages in Corn - Thomison CORN Questions

Following pollination, kernel development (or grain fill) is the most critical period in the development of the corn plant for the determination of grain yield. Kernel development proceeds through a number of stages which have been characterized by such terms as blister, milk, roasting ear, soft dough, dent, etc. Since these descriptive terms can sometimes be difficult to interpret, alternative systems have been proposed. A staging system widely used by agronomists and crop consultants divides kernel development into 6 stages, designated numerically as R1, R2, through R6. The table below lists kernel developmental stages in sequence and provides a brief description of each phase.

Kernel Development in Corn

*R-stages 1 through 6; specific number of days associated with each stage may vary from season to season, from location to location, and from hybrid to hybrid.

Keep in mind that the values for average number of days per stage and approximate days from silking in the table above are based on timely corn planting (e.g. early May). When corn is planted late, as was the case in many fields this year, it generally requires fewer heat units to achieve R6, physiological maturity or "black layer", and this may affect the number of days per stage and days from silking.

Stress conditions such as drought, high temperatures, nutrient deficiency, disease or insect injury, excessive shade, hail damage, and overpopulation during grain fill may cause complete abortion of kernels toward the ear tip ("tip dieback"). Ear tip kernel abortion occurs when the youngest kernels resulting from the most recent pollination are cut off from nutrient flow because the supply is insufficient to fill all the kernels that have been set. Such kernel abortion is most likely to occur during the first two weeks after pollination (during R2, the blister stage). These same stress factors may also reduce kernel size and weight. Premature plant death resulting from diseases (such as stalk rots) or frost cuts off starch accumulation and results in small, light-weight (low test weight) kernels.

C) Soybean Insect Update - Hammond and Eisley CORN Questions

Soybean aphids are continuing to increase in fields throughout Ohio, with some of the fields nearing or reaching threshold. Treatment thresholds are when a majority of the plants throughout the field have over 250-300 aphids.

Bean leaf beetle first generation levels are also reaching high levels in some fields. A field sampled north of the OARDC Northwest Branch had over 30 beetles per 10 sweeps. Although probably not reaching defoliation thresholds unless other defoliating insects are present, these larger bean leaf beetle populations give an indication of potential problems with pod feeding from this insect in late summer. Fields with high first generation bean leaf beetle populations should be watched closely for pod feeding in late August. Growers are advised to check their soybean fields for these and other insects.

D) Corn Insect Update - Eisley and Hammond CORN Questions

It is still not too late to inspect lodged cornfields to see what might have caused the lodging problem. Inspection of first year corn root systems in Allen and Van Wert Counties last week found none to slight corn rootworm feeding injury even though the fields had some lodging. The lodging appeared to have been caused by excessive moisture and high winds.

Early results from western corn rootworm trapping in soybeans indicate the populations are lower this year than last. This might be a good year to place some yellow sticky traps in your soybean fields to monitor the populations for next year.

E) Factors That May Affect Tissue Test Results - Watson CORN Questions

CORN 2003-23 contained information on tissue testing in Corn. This article follows up with some information on interpreting these results. Nutrient concentrations within the plant can vary because of many factors. Consequently, knowledge of fertilizer/manure management and the weather can aid in the interpretation of the test results. Be sure to maintain records of the plant analysis from each field and compare them over time to get a good idea how the nutrient levels may be changing.

The following are some considerations that may be helpful in evaluating your corn crop:

Nitrogen:
Below sufficient range:
-May be due to insufficient amount of N fertilizer or manure applied
-May be due to fertilizer or manure not applied at the right time
-If manure was applied, an extremely cool season may have reduced N release from the manure during the early plant growth stage
-May be due extremely wet weather, caused leaching of the nitrate-N
Above sufficient range:
-May be due to excessive amount of N applied
-May be due to poor plant growth because of other factors

Phosphorus:
Below sufficient range:
-Low soil P fertility
-May be due to insufficient amount of P fertilizer or manure applied
-May be due to P becoming unavailable to the plant because of extremely acid soil; soil should be limed to correct pH range
Above sufficient range:
-May be due to excessive amount of manure or P fertilizer applied
-May be due to poor plant growth due to other factors

Potassium:
Below sufficient range:
-Low soil K fertility
-May be due to insufficient amount of K fertilizer or manure applied
-May be due to very dry soil conditions, especially if the K concentration in the soil is borderline
Above sufficient range:
-May be due to excessive amount of manure or K fertilizer applied

Calcium:
Below sufficient range:
-May be due to a very acid soil
-May be due to excessive amount of K applied to the soil
Above sufficient range:
-Usually not a problem

Magnesium:
Below sufficient range:
-May be due to low soil Mg level
-May be due to excessive amounts of manure or K fertilizer applied
-May be due to acid soil
Above sufficient range:
-Usually not a problem

Sulfur:
Below sufficient range:
-May be due to low soil organic matter
-May be due to manure or S fertilizer not applied
Above sufficient range:
-Usually not a problem

Micronutrients (Mn, Fe, B, Cu, Zn):
Below sufficient range:
-Hardly ever a problem for corn
-May be due to over-liming a soil which may reduce the availability of these micronutrients
-Very sandy soils can be low in one or several of these micronutrients
Above sufficient range:
-May be due to application of some waste material high in these nutrients
-May be due to over application of fertilizer sources
-May be due to soils becoming very acid, making these nutrients highly available for plant uptake; especially for Mn, B, and Cu which can become toxic to the plant.

Remember that extra monitoring of the corn crop with plant analysis may prove very important if there are unseen nutritional problems beginning to develop.

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C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Harold Watters        Web Editor: Nathan Watermeier

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