C.O.R.N. Newsletter 2005-23

Soybean Aphid Update

Authors: Ron Hammond, Bruce Eisley

We are receiving more reports of aphid thresholds being reached across the state, along with the observations of large numbers of winged aphids being found on the plants. Reports from other states, especially those to our north, indicate the development and flight of winged aphids in heavier numbers. These reports and presence of more winged aphids in Ohio suggest that the mid-to-late July flight of large densities of winged aphids into our state is occurring. This is very similar 2003, the last year of heavy aphid populations. And in that year, our greatest problems came in the last week of July and the first half of August. Thus, the next 2-4 weeks will be critical in our battle against the soybean aphid. Diligence is recommended. If soybean fields are not being monitored for soybean aphids, it is essential you start. Remember that the treatment threshold is 250 soybean aphids per plant. Because of this new and heavy occurrence of winged aphids, fields that were sprayed a few weeks ago should be rechecked over the next few weeks for possible re-infestation.

Corn Silk Feeding Insects

Authors: Ron Hammond, Bruce Eisley

In last week’s CORN newsletter, we discussed adult western corn rootworm silk feeding on corn. We inadvertently left off Baythroid, Decis, and Sevin XLR from the list of chemicals that we know to be labeled for this use. We apologize for this. Thus, the chemicals that are labeled for silk-feeding beetles include:

Ambush*
Asana XL*
Baythroid*
Capture 2 EC*
Decis
Lannate*
Dimethoate
Mustang MAX*
Penncap-M*
Sevin XLR
Warrior *
* Use is restricted to certified applicators only.

Soybean Rust Update July 25

Authors: Anne Dorrance

No soybean rust was found in Ohio again last week nor were there any positive finds in Kentucky. “Rust-like” spores were found again in Kentucky but again, no active infections were found. Growth stages of full season beans are reaching R3 this week in many parts of the state and we will be in R5-R6 by mid-August. Based on the amount of US inoculum, it is highly unlikely that we will be in a situation where massive spraying will be warranted.

Phytophthora Root and Stem Rot

Authors: Anne Dorrance

Phytophthora sojae which causes root and stem on of soybeans, has been found in a number of fields this year in Ohio, including southern Ohio. Phytophthora is an oomycete that loves wet saturated soils. If you find the stem rot phase of this disease this means 2 things. 1) that Rps gene no longer effective to 100% of the isolates in that field and 2) levels of partial resistance (field resistance, tolerance) in that variety were too low. From over 6 years of field studies, with partial resistance scores of 5 or lower on OSU scale or 5 and higher for other seed companies (yes, you do need to read the fine print), rarely do plants with failed Rps genes develop stem rot. Partial resistance can provide that extra bit of protection needed for this changing population. We’ve been asked about race typing fields. In Ohio, from studies supported by Ohio Soybean Council, we have identified that Ohio’s fields have Phytophthora populations that can take out most Rps genes. At least 60% of all fields surveyed have isolates that can kill plants with Rps1c and Rps1k while over 90% can kill plants with Rps1a. The best protection – for fields where you are finding stem rot this year choose a variety with an Rps gene, but with high levels of partial resistance. Also, improve drainage to limit the amount of time that field remains wet.

Staging Kernel Development in Corn

Authors: Peter Thomison

Kernel development has started in many corn fields that were planted in April and early May. 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 six 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 Stages in Corn

Stage*	Description	Avg. No. of    Days/Stage	Approx. Days from Silking
Silking (R1)	Fresh green silks, no visible blisters	4	---
Pre-blister	Silks brown, not necessarily dry visible kernel pimples,contain little clear fluid	4	8
Blister (R2)	Visible blisters w/abundant fluid	4	12
Early Milk	Mostly white kernels w/milky-white fluid, some yellowkernels	4	16
Milk (R3)	Mostly yellow kernels w/milky-white fluid, no solidsyet (“Roasting Ear” stage)	4	20
Late Milk- Early Dough	Solids beginning to form, kernel pasty texture (barelyedible)	4	24
Soft Dough (R4)	Pasty or semi-solid (not edible), no visible denting	5	28
Late Dough- Early Dent	Few kernels beginning to dent, especially near buttof ear	5	33
Dent (R5)	Majority of kernels dented or denting	8	38
Late Dent	Essentially all kernels dented, milk line may justbe visible	17	52
Black Layer (R6)	Maximum kernel dry weight, kernel moisture 27-32%	10	62

*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 later, as it was this year due to widespread replanting, 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.

The high temperatures and water deficits we’re currently experiencing in many areas of Ohio may cause varying degrees of kernel abortion 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 (i.e. sugars produced by the leaf canopy) 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). Four days of severe drought at the blister stage has the potential of reducing yields 30-40%, and at dough stage, 20 to 30%. Drought stress may also reduce kernel size and weight. Premature plant death resulting from drought, diseases (such as stalk rots), or frost cuts off starch accumulation and results in small, light-weight (low test weight) kernels.

Plant Tissue Analysis and the Weather

Authors: Robert Mullen

Before long the corn in Ohio will be at the correct stage for doing plant tissue analysis. The plant tissue test is helpful in assessing the nutritional status of the corn crop and in the evaluation of how well your fertilizer program worked. However, this year there are some considerations you should be especially aware of in making the best use of this diagnostic tool. It seems that Ohio weather is different for each year’s growing season. This year is no exception, with the very cool and wet early spring and then quite dry in June and July in many areas. The plant tissue diagnostic test is most useful when the soil has supplied adequate moisture to the crop during the growing season. Even though the nutrients were at optimum levels at planting time, with the advent of droughty conditions for much of the growing season, the nutrient concentrations within the plant tissue may not be indicative of the soils nutrient supplying ability. Reduced moisture supply will reduce the growth of the plant and tend to elevate the concentrations of some nutrients within the plant as compared to sufficient moisture conditions. This will make the interpretation of the test results less useful than would otherwise be the case.

Under poor moisture supply, the nitrogen of the leaf tissue (the ear leaf taken at initial silk time) may be above the sufficiency range especially if plenty of nitrogen fertilizer was applied to the crop. Near the end of a droughty growing season the corn plant can concentrate nitrogen in the form of nitrate-nitrogen to the level that it could be detrimental to cattle if the corn is fed as green chop. The nitrate-nitrogen will concentrate in the lower 12 inches of the corn stalk. This part of the plant should be tested for nitrate-nitrogen before it is used as feed.

In addition, the interpretation of the potassium concentrations in the ear leaf tissue at initial silk can be a challenge under drought conditions. The level of potassium may be lower than the sufficiency level if there was inadequate soil moisture that restricted root growth. This may show up especially on soils that are boderline in their potassium supplying ability for plant growth. Soil testing should be used in conjunction with plant analysis to provide more information about the level of potassium in the soil.

Nutrient Sufficiency Ranges for the Corn Ear Leaf at Initial Silk

Nitrogen (N)	2.90 – 3.50 %
Phosphorus (P)	0.30 – 0.50 %
Potassium (K)	1.91 – 2.50 %
Calcium (Ca)	0.21 – 1.00 %
Magnesium (Mg)	0.16 – 0.60 %
Sulfur (S)	0.16 – 0.50 %
Manganese (Mn)	20 – 150 ppm
Iron (Fe)	21 – 250 ppm
Boron (B)	4 – 25 ppm
Copper (Cu)	6 – 20 ppm
Zinc (Zn)	20 – 70 ppm

Nutrient concentrations within the plant can vary because of many factors. Consequently, knowledge of fertilizer/manure/soil 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. In addition, be sure to note whether or not the crop had adequate moisture for most of the growing season. This information will be valuable in making more accurate interpretations of the results of future plant tissue analysis.

Controlling Weeds in Wheat Stubble

Authors: Mark Loux

The summer through fall after wheat harvest is an excellent time to control weeds that are problematic throughout the entire crop rotation. A first step for control of weeds in wheat stubble is to determine the primary weed targets and their life cycle, since this affects the timing of mowing and herbicide application. Control practices can be altered to address one or more of the following weeds: summer annuals; winter annuals; warm-season perennials; or cool-season perennials. (Note – lb ae for glyphosate refers to lbs of acid equivalent per acre).

Summer annual weeds – a primary goal is to prevent seed production. This can be accomplished through mowing, tillage, or an herbicide treatment. The application of an herbicide is more likely to eliminate summer annual weed seed production than mowing or tilling, because weeds can regrow following mowing and tillage often stimulates additional weed emergence. Control should be implemented before mid-August to prevent most of the seed production. A combination of glyphosate plus 2,4-D is effective for most weed populations. Rates can be varied somewhat depending upon weed size, but the following should be effective in most fields on even large weeds: glyphosate (1.12 lbs ae/A) plus 2,4-D (0.5 to 0.75 lbs ai/A).

Winter annual weeds – usually emerge in late summer into fall. Mow wheat stubble now (before August in dry areas or before mid-August in areas with ample rainfall). Herbicide application between mid-October and mid-November is the most effective strategy. Glyphosate plus 2,4-D can be used where any crop will be planted next year. Where soybeans will be planted next year, additional options are: Canopy EX + 2,4-D; Sencor + 2,4-D. Additional options where corn will be planted next year: Basis plus 2,4-D; simazine + 2,4-D; Basis + simazine + 2,4-D.

Warm-season perennials (e.g. johnsongrass, wirestem muhly, hemp dogbane, common pokeweed, common milkweed) – these weeds must be controlled before a frost. Mow the field now (before August in dry areas or before mid-August in areas with ample rainfall), and allow the perennial weeds to regrow into early fall. Glyphosate plus 2,4-D or glyphosate plus dicamba are the most effective treatments, and they should generally be applied by mid-September. Where warm-season perennials are small yet and few summer annual weeds are present, mowing may not be needed at this time.

Cool-season perennials (e.g quackgrass, Canada thistle, dandelion) – herbicides are most effective in mid to late fall. Mow the fields by early August, or apply glyphosate (0.38 to 0.56 lbs ae/A) now to control summer annual weeds or reduce seed production. Apply glyphosate or glyphosate plus 2,4-D between mid-October and early November to control the perennial weeds. The application of glyphosate now is more effective for reducing annual weed seed production than mowing, but it is possible that it can injure perennials and prevent them from vigorous regrowth into the fall. Where dandelions are the primary target, the herbicide options listed for winter annual weed control (except Sencor) can also be effective.