CORN
Crop Observation and Recommendation Network

July 24 - July 30, 2000
C.O.R.N. 2000-23

In This Issue:

A. Still unsure it was Phytophthora?
B. Potato Leafhopper Activity Marginal But Economic
C. Control of Weeds in Wheat Stubble
D. Kernel Development in Corn

A) Still unsure it was Phytophthora? - Anne Dorrance CORN Questions

Now is the time to begin scouting for the stem rot phase of Phytophthora. Phytophthora sojae attacks soybean plants throughout the production season. As the temperature begins to warm, P. sojae will begin to move up the stem. This is the only pathogen that colonizes the soybean from the ground up and causes a chocolate brown lesion. In addition to Phytophthora, in many of these fields, Rhizoctonia is also present. Plants affected by Rhizoctonia will turn yellow and have a brick-red lesion at the base of the stem.

If Phytophthora is identified as the problem from symptoms on plant, there are several things you you should observe in the field now. Ask the following questions.

1) How much damage and how widespread is it in the field?

2) Can you see the tile lines (the only areas in the field with healthy beans)? If the answers are greater than 10% throughout the field or the tile lines are evident proceed to the next question

3) Which Rps gene was in the soybean variety that was planted?

4) What was the partial resistance (field resistance) score ? (Which scale is used, 10=good or does 10=dead)

5)Was a seed treatment used? What was the rate? (We recommend the highest labeled rates for control of P. sojae)

6) How many years in the last 10 has this field been in soybeans? (don't forget double crop)

If you have Phytophthora this year, then the Resistance gene (Rps) in the soybean variety planted was not effective against the populations of P. sojae in your fields. This is normal and we have documented these changes in P. sojae in many fields across Ohio. We have identified several fields in which the P. sojae population has become very diverse. The next question will be - is there an Rps gene that will provide protection in your field. It is impossible to race type all of the fields that show damage this year, but farm records and knowing which varieties were planted before will give us the best information to make a good decision. The next question becomes.

7) What Rps gene was planted in the three previous soybean plantings?

If you planted a variety with Rps1a or Rps1b and you had Phytophthora, there is a very high probability that varieties with Rps3, Rps1c or Rps1k will provide protection.

If you have already used varieties in the past with Rps1a, Rps1c, Rps3 and Rps1k, then you should select varieties with partial resistance or with 2 or more Rps genes. Look for varieties with the highest levels of partial resistance, treat the seed with the highest labeled rates of Allegiance or Apron XL, and improve the drainage by splitting the space between the tiles or tillage.

	Pythophthora stem rot - adult plant showing stem discoloration
	Phytophthora root rot - midseason wilting due to extensive root rot

B) Potato Leafhopper Activity Marginal But Economic - Hal Willson CORN Questions

Potato leafhopper (PLH) activity on alfalfa at the present time may or may not warrant rescue treatment. Unlike the alfalfa weevil outbreak on 1st cutting alfalfa, PLH activity on 2nd and 3rd cuttings of alfalfa has been relatively low this year. To date, no reports of severe PLH damage to alfalfa have been received and PLH activity in our research plots has been fairly marginal.

On July 12th, PLH activity in our research plots at the Western Branch station averaged 15 PLH adults per 10 sweeps on 9 inch alfalfa. Such a level of PLH activity exceeds the threshold of one PLH per 10 sweeps per inch of alfalfa regrowth, but may be regarded as marginal when alfalfa growing conditions are ideal. Treatments were applied on July 12th and 8 days later alfalfa growth in both treated and untreated plots appeared vigorous with minimal signs of hopper burn. Sampling of the alfalfa on July 20th found minimal PLH in all treated plots and 28 PLH per 10 sweeps in the untreated plots. Most of the PLH in the untreated plots were adults and PLH nymphs represented less than 10% of the PLH collected. However, stand height of the untreated alfalfa plots was about 3 to 4 inches less than the treated alfalfa plots. In summary, alfalfa growth had been stunted, but visible signs of hopper burn were not clearly evident unless one conducted a close inspection. Final assessment of the impact of such marginal PLH activity will be determined when the trial is harvested and yields are analyzed.

The scenario described may lead growers to assume that an alfalfa field is not being impacted by PLH, since visible signs of hopper burn are not obvious during a drive-by inspection. However, periodic sweep net sampling of an alfalfa field may detect the presence of PLH activity capable of causing an economic level of injury.

C) Control of Weeds in Wheat Stubble - Jeff Stachler and Mark Loux CORN Questions

The following facts are important for successful weed control in wheat stubble:

1. Eliminating the summer annual weeds before they go to seed will decrease next years summer annual weed problem.
2. The best time to control most perennial weeds with glyphosate is in late September to mid-October.
3. The most effective winter annual weed control is achieved by spraying herbicides in mid-October to mid-November.
4. Controlling weeds and volunteer wheat can reduce insect and disease problems next year.

For elimination of summer annual weed seeds, an application of glyphosate (1- 1.5 qt/A) and/or 2,4-D ester (1 pt/A) will be necessary. All glyphosate rates mentioned are based upon the 3.0 pound acid equivalent per gallon formulation. If a different glyphosate formulation is used, consult the label for the appropriate rate. The exact rate of glyphosate and need for 2,4-D is dependant upon weed species and size of weeds. For small weeds the lower rate of glyphosate will be adequate. If the weeds are large or tough to control species such as velvetleaf, smartweed, horseweed/marestail and morningglory are present use the higher rate and/or 2,4-D. Mowing the wheat stubble at a height of 4-6 inches when any weed species has started to flower and then mowing as low as possible when these species begin flowering again is another method of reducing, although not eliminating, seed production of summer annual weeds in wheat stubble.

For control of perennial weeds mow wheat stubble fields as soon as possible to allow maximum regrowth. Mowing holds back the annual weeds from producing large amounts of seeds, allows for good regrowth of perennial weeds, and gets rid of the wheat stubble that may interfere with spray coverage. While there is a temptation to apply glyphosate and/or 2,4-D in July or August to prevent seed production by annual weeds, this will actually reduce the control of perennial weeds, which are more difficult to control in the following year's crop. The best timing for perennial weed control is generally late September through late October depending upon the species. Herbicide effectiveness at that time will be maximized by applying after several days of warm weather when perennial weeds are fairly large and/or in the bud to flower stage. Perennial weed regrowth needs to reach a minimum height of 12 inches in order to obtain maximum control of the roots or rhizomes for next year. For the warm-season perennials such as johnsongrass, hemp dogbane, milkweeds and bindweeds a glyphosate and/or 2,4-D application in mid- to late September should provide the most effective control. Compared to cool-season weeds such as Canada thistle, quackgrass and Canada bluegrass that will be best controlled with a glyphosate application after a light frost in mid- to late October.

The most effective winter annual weed control in wheat, corn or soybean stubble is achieved by spraying herbicides as late as possible. Winter annual weeds, such as horseweed/marestail, mustards, fleabane, purple deadnettle and chickweeds are best controlled by fall herbicide treatments. This results in a more weed free no-till seedbed the following spring allowing the soils to warm up and dry out faster and have less trash getting in the way of the planter. If just using glyphosate and/or 2,4-D ester, an application in early December during a warm spell should control more winter annual weeds as compared to applying in September or early October. This is because new emergence of winter annual weeds may occur after early October if conditions are good for emergence. The use of residual herbicides such as Sencor or Canopy can help in providing effective winter annual weed control depending upon the species by providing residual control after application and into next spring. These residual products should not be put on until soil temperatures are below 50 degrees Fahrenheit but before the soil freezes.

Control of volunteer wheat should help in reducing the amount of wheat streak mosaic virus and possibly other diseases. To have the greatest impact in reducing wheat streak mosaic virus, the volunteer wheat should be controlled before September 1st. This presents a problem in targeting perennial and winter annual weed control using glyphosate. To selectively control the volunteer wheat and other grass species, an application of Select (8.0 oz/A) could be applied before August 15th and apply glyphosate and/or other products later to control other weeds. Glyphosate (1.5 pt/A) should effectively control volunteer wheat. Purple deadnettle, field pennycress and shepherd's- purse have been documented as hosts of the soybean cyst nematode and may be increasing the nematode populations, although not confirmed. Therefore control of these species may reduce soybean cyst nematode populations. The control of chickweed and other species may help in reducing cutworm and slug populations in the spring.

In a study conducted in 1993 through 1995, we examined the effect of Roundup/2,4-D applications in wheat stubble on annual weed populations the following year. The Roundup/2,4-D (1 quart + 1 pint/A) combination was applied in late July, late August, or late September, and weed populations were measured the following year. The July application controlled the foxtail, common ragweed, and other summer annual weeds most effectively and reduced seed production almost completely. This did tend to reduce annual weed populations the following year, but we saw little impact on control where an effective preemergence corn herbicide was used the next spring. Control of perennial and winter annual weeds was most effective with the late September application, which resulted in a much cleaner seedbed the following spring. Weeds in the 1994-95 study included horseweed (marestail), Philadelphia fleabane, curly dock, and mustards. Control of these weeds the following spring ranged from 83 to 100 percent with the September application, 30 to 64 percent with the August application, and 9 to 45 percent with the July application. In our opinion, control of the winter annual and perennial weeds in wheat stubble is more important than control of summer annual weeds. For this reason, we recommend that Roundup, Touchdown, or combinations of either product with 2,4-D be delayed until later in the fall. Mowing fields now will help reduce seed production by summer annual weeds.

Controlling weeds in wheat stubble is very important, but choosing the proper time to spray is difficult. To obtain the greatest benefit from the weed control in wheat stubble two trips across the field may be needed. We will discuss actual herbicide recommendations later to control perennial and winter annual weeds.

D) Kernel Development in Corn - Peter 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.

Drought, high temperatures, nutrient deficiency, disease or insect injury, shading, hail damage, overpopulation, and other stress factors 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 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.

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

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: David A. Jones        Web Editor: Tom Rosati

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868

  | C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |