In This Issue:
- Phytophthora Stem Rot is Prevalent in Many Areas
- Detecting and Managing Leaf Diseases of Corn
- Sprouting and Moldy Wheat
- Wheat Varieties Suitable for 15-inch Rows and for Relay Intercropping
- Time to Set Traps for Western Corn Rootworm Activity in Soybeans
- Insecticides for Use on Soybeans
- Soybean Aphid Update
- Controlling Weeds in Wheat Stubble
Authors: Anne Dorrance
Repeated summer downpours have kept Phytophthora going on the soybean crop this season. With every flooding or soil saturation event, more spores are ready to infect the soybean crop. With every flooding or saturation event, more and more plants of highly susceptible varieties will die. The stem rot phase is very easy to diagnose, the plants will yellow and wilt, with a chocolate brown canker expanding up the stem. This is the only stem disease that colonizes and discolors the stem from the base up the outside. There are a number of pictures on http://www.oardc.ohio-state.edu/ohiofieldcropdisease/
Rps genes – these are listed on the disease resistance package for your variety. In Ohio, most varieties have Rps1a, Rps1c, Rps1k or Rps3a. There are a few varieties that have 2 gene combinations of Rps1c+Rps3 or Rps1k+Rps3. In Ohio, with the exception of the 2-gene combinations, all fields have populations that can kill plants with the Rps1a gene while about 60 to 70% of the fields have populations that can kill plants with Rps1c or Rps1k. These genes provide an all or nothing resistance, so if you are seeing Phytophthora stem rot, you have a couple of options. If it is Rps1a – switch your variety to Rps1c or Rps1k. If your current variety has Rps1c or Rps1k – then look for a 2-gene combination OR keep your Rps1c or Rps1k – and add partial resistance.
Partial resistance is called many different things in the seed catalogues, tolerance or field resistance. This resistance can protect the plants – even when they are infected – so that the amount of root rot is reduced – and more importantly, when partial resistance levels are high enough (scores in the OSU performance trials of 5.0 or less) that the stem rot phase never develops. For those fields that are actually finding the stem rot phase, these fields need varieties with higher levels of partial resistance. Now the scoring system is different among the different companies. OSU uses a 1 to 9 scale, in which 1=no disease and 9=dead; others the scale is 9 is best and 1 is dead. In other words, you need to read the fine print.
Authors: Pierce Paul, Dennis Mills
The recent warm, wet weather conditions have had some Ohio corn growers concerned about possible foliar disease problems and the need to make preventative disease management decisions. Corn is currently between late vegetative growth stages and early reproductive growth stages (VT and R1) – growth stages at which foliar diseases normally begin to appear and fungicide application decisions have to be made. While it is true that most of the common foliar diseases of corn (gray leaf spot, northern corn leaf blight and rust) develop well under our recent weather conditions (warm temperatures, high relative humidity and wetness), with the exception of a few lesions of anthracnose on the lower leaves of some hybrids, we have not yet seen any major foliar diseases of corn in our plots nor have we had reports of any such problem in the state. Generally, disease problems show up first in no-tilled fields planted with susceptible varieties, and it is in these fields that yield reduction tends to be greatest and fungicide application most profitable.
Before applying fungicides, it is important to scout fields to determine whether the disease of concern is present. Scout no-tilled fields and fields with susceptible hybrids first and use this information as a guide as to whether you are likely to have a disease problem. Gray leaf spot, northern corn leaf blight, and eye spot fungi all survive in corn residue left on the soil surface from the previous crop, and as such, these diseases are likely to show up first in no-till fields. However, the presence of residue is no guarantee that disease will develop and reach economic levels. Fungal spores buildup in crop residue over time, and if disease levels were low last year (as was the case here in Ohio), the contribution of last year’s crop residue to disease development this year will more than likely be low. However, on-farm crop residue is not the only source of spores. Spores may be blow in from other location. For common rust, spores are generally blown in from southern states. So, it is always important to scout fields before making fungicide application decisions.
GRAY LEAF SPOT (http://ohioline.osu.edu/ac-fact/0038.html) – Typical lesions appear tan to brown in color, rectangular in shape, and flanked by the veins of the leaf. When fully expanded, individual lesions may be 3 to 4 inches long and 1/16 to 1/8 inch wide, depending on the distance between veins; however, if several infections occur near each other on the same leaf a broader lesion will result.
NORTHERN CORN LEAF BLIGHT (http://ohioline.osu.edu/ac-fact/0020.html) – Lesions are typically 1 to 6 inches long, cigar-shaped, gray-green to tan in color and appear first on the lower leaves. As the disease develops, the lesions spread to all leafy structures, including the husks.
EYE SPOT (http://ohioline.osu.edu/ac-fact/0021.html) - Affected leaves are covered with numerous small round spots. Spots are about 1/8 inch in diameter, oval to circular, and initially they appear water soaked. The central area of the spot soon dies, leaving a tan to cream-colored center surrounded by a distinct brown to purple border.
COMMON RUST (http://ohioline.osu.edu/ac-fact/0031.html) - Dark, reddish-brown pustules scattered over both the upper and lower surfaces of the corn leaves. Pustules appear oval to elongate in shape and are generally small (less than 1/4 inch long).
Foliar diseases of corn cause yield reduction when severe blighting of the ear leaf and the leaves above the ear leaf occur before and during grain fill. Most foliar diseases move up the plant from the lower to the upper leaves, with the lower leaves contributing spores for infection (penetration) of the upper leaves. If fields are scouted now and disease is detected on the lower leaves, growers will still have enough time to apply fungicides to protect the upper leaves of the plants from becoming infected. Depending on the weather and the susceptibility of the hybrid, it can take between 7 and 14 days for a lesion to produce a new crop of spores. Once these spores infect a new leaf, it may take another 7 to 14 days (or longer depending on the weather) for new lesions to develop. It will then require adequate temperatures (cooler temperatures for common rust and eye spot and moderate to high temperatures for gray leaf spot and northern corn leaf blight) and high moisture for lesions to expand and disease to spread to the upper leaves.
Several fungicides are available for use on corn to control foliar diseases. However, the economics of corn production have not permitted extensive used of these products. To achieve the desired effect, fungicides should be applied at the right time. The main goal of fungicide application is to prevent disease from spreading to the ear leaf and the leaves above the ear, since these leaves contribute a significant portion (about 75%) of the carbohydrates for grain fill. Applications should begin when the first few lesions are observed on the leaves below the ear leaf. Fields should be scouted regularly to determine the appropriate time for fungicide application. Research has shown that two applications may be more effective than one at reducing the level of disease; however, more than one fungicide application is generally not economical. Yield increase is often not enough to offset the cost of two applications.
The following should be taken into consideration when making decisions regarding fungicide application:
1- Susceptibility and yield potential of the hybrid
2- The amount of disease in the field
3- The growth stage of the crop
4- Weather conditions
5- Fungicide and application cost
6- Grain price
7- Directions and restrictions on product label
Authors: Dennis Mills, Pierce Paul
It has now been more that three weeks since wheat harvest began here in Ohio and some fields or sections of fields are yet to be harvested. The delay has been largely due to recent rains in some areas. With warm, wet conditions during harvest come two major problems – pre-harvest sprouting and moldy wheat.
Once the wheat matures to the harvest ripe stage, the moisture content of the grain drops to about 13 to 14%. This is the ideal time to harvest. However, frequent rains over the past few days have prevented growers (predominantly in the northern part of the state) from harvesting their wheat. This delay may lead to pre-harvest sprouting in some varieties in some areas. Sprouting is characterized by the swelling of kernels, splitting of seed coats, and germination of seeds (emergence of roots and shoots) within the wheat heads. Some varieties are more tolerant to sprouting than other, and for a given variety, sprouting may vary from one field to another depending on the duration of warm, wet conditions.
Sprouting affects grain quality (test weight). Once moisture is taken up by mature grain, stored reserves (sugars especially) are converted and used up for germination. The conversion of starch (the stored form of carbohydrate) to glucose leads to reduced test weights. Even before visual signs of sprouting are evident, sugars are converted and grain quality is reduced. Since varieties differ in their ability to take up water, their drying rate, the rate at which sugars are used up, and embryo dormancy (resistance to germination), grain quality reduction will vary from one variety to another.
In addition to sprouting, the growth of mold is another problem that may result from rain-related harvest delay. To fungi, mature wheat heads are nothing more than dead plant tissue ready to be colonized. Under warm, wet conditions, saprophytic fungi (and even fungi known to cause diseases such as wheat scab) readily colonize wheat heads, resulting in a dark moldy cast being formed over the heads and straw. This problem is particularly severe on lodged wheat. Like sprouting, moldy grain also results in low test weights and poor grain quality. In addition, some molds may produce toxins such as deoxynivalenol (DON), leading to further grain quality reduction and dockage. While DON contamination is generally higher in fields with high levels of wheat scab, it is not uncommon to find high levels of DON in late-harvested fields that have been exposed to excessive moisture. Even in the absence of visual scab symptom, the fungi that produce DON may still colonize grain and produce toxin. For more on sampling for DON and harvesting fields with potential toxin contamination problems, please refer to our recent newsletter article (http://corn.osu.edu/story.php?setissueID=140&storyID=839).
Very little can be done about sprouting once it has occurred. It is often minimized through variety selection. Growers are advised to harvest their fields as soon as possible. Try to harvest fields (or sections of fields) with sprouting problems and moldy wheat first, and keep problem grain separate from the rest of the lot. If grain is harvested with high moisture content (20%), it should be dried down to avoid further sprouting and mold growth in storage. Since sprouting also affects seed viability and vigor, growers are advised against keeping grain from fields with sprouting problems for seed (even from areas without visible signs of sprouting). Lower test weigh is a good enough indication that the quality of the seed has been compromised.
Authors: Jim Beuerlein, Rich Minyo
Some Ohio wheat producers are interested in producing soft red winter wheat in 15-inch rows which allows them to remove a grain drill from their machinery inventory and because production in wide rows reduces their seed cost by half. Other producers are interested in wide rows for the purpose of relay intercropping soybeans. We have evaluated wheat varieties in 15-inch rows for several years and have learned that some varieties will produce about as much yield in 15-inch rows as in narrow rows. For example, in 2001 six of 23 varieties produced a yield between 99 percent and 105 percent of the yield in 7.5-inch rows. That capability is due primarily to their growth habit, i.e. plant height and erectness of growth. Varieties with medium height and a very erect growth habit are needed for relay cropping and tall varieties with a non-erect growth habit perform well in 15-inch rows where intercropping is not planned.
We evaluated 55 varieties that were entered in the 2006 Ohio Wheat Performance Trial for height and growth habit and developed two lists; one list is of varieties that appear to be ideally suited for relay intercropping, and the second is for varieties that seem to be ideally suited for 15-inch row
production. Height measurements and growth habit ratings were made on each of the 55 varieties at each of five test sites and the data combined so we could select the appropriate varieties for each list. As seen in the following tables, only seven of the 55 varieties were found to be suitable for relay
intercropping and another 14 were found to be suitable for wide-row production. For wide-row production, wheat should be planted as soon as possible after the fly safe date, and the most profitable seeding rate is 20-25 seeds per foot of row. An application of 30 pounds of Nitrogen should be made at planting.
Table 1: Wheat varieties suitable for wide -row production
|Seed Consultants||SC 1343|
|Seed Consultants||SC 1337|
|AgriPro COKER||Coker 9553|
Table 2: Wheat varieties suitable for relay cropping
|AgriPro COKER||AgriPro Cooper|
|AgriPro COKER||Coker 9436|
Authors: Bruce Eisley, Ron Hammond, Curtis Young
The first year corn rootworm variant that lay their eggs in soybeans continues to be a concern to corn production, especially in western Ohio. Trap results from 2005 continued to show the presence in a number of fields sampled. One method to sample for this variant in soybeans is by using Pherocon AM unbaited yellow sticky traps. The traps are attached to fence posts slightly above the soybean canopy (6-8”).
Traps should be placed in the soybean fields this week. Place 6 traps at least 100 feet from the field edge and evenly distributed in the field. Also be careful not to get too close to an adjacent corn field. Maintain at least a 100 foot distance or more into the soybean field from any edge of the field. Twist tie wires are included with the traps, however we find it more convenient to use cable zip ties. The zip ties are thinner and faster to connect the traps to the posts. The traps should be changed each week for six weeks and a new trap put in its place just above the soybean canopy.
After changing the traps, count and record the number of rootworm beetles on each trap and the number of the days the traps were in the field. The number of beetles/trap/day is determined by adding the number of beetles found on the six traps and dividing that number by 6 and then dividing by the number of days the traps were in the field. Traps should be changed every 7 days if possible but if not be sure to record the exact number of days the traps were in the field.
Research from numerous universities indicates that catches in soybean of 5 or more beetles/trap/day during any trapping week indicates a potential problem with rootworm in the field the following year. The management in the field the following year can be to rotate to some crop other than corn, use a soil insecticide or a seed treatment, or plant one of the new Bt transgenic rootworm hybrids.
We are aware of a sampling program using these yellow sticky traps being sponsored by industry. While this activity is commendable, we wish to share a concern we have based on the information we have received. According to our information, the protocol for their program is to have the traps in the field for only a single week. If true, we strongly disagree with this limited time frame for sampling. From experience, we know that the western corn rootworm adults enter and leave the field at different times over several weeks, and it is impossible to know when their peak densities will occur. This is why our sampling is over a six week time period. If sampling for only a single week, there is a possibility that you will miss the peak number of adults. While you might get low adult numbers during that one week of sampling, you might have much larger numbers prior to or following that week. Although the variant rootworm is not that common in Ohio, you could end up with a false sense of security if only sampling for a single week and obtaining low numbers. If you are taking the time to sample for the western corn rootworm variant in an IPM fashion which is highly recommended, we urge you to do it properly and sample for the entire six week period.
Authors: Bruce Eisley, Ron Hammond
The article in last week’s newsletter about soybean defoliators referred to a table on the web that contained a listing of insects on soybean and insecticides labeled on those insects. However, the table was out of date and additional insecticides have been labeled since the table was originally posted. We have updated the table concerning soybean insects and have posted it on the web at: http://entomology.osu.edu/ag/articles/06update.html You can also find information about insecticides labeled on soybean insects at: http://entomology.osu.edu/ag/bull.htm
We try to keep information concerning labeled insecticides for field crops and insects up-to-date on the web but occasionally we miss an insecticide after it has been approved for a certain crop and insect. As always, the most up-to-date information is the insecticide label and should be consulted before any insecticide is applied.
Authors: Ron Hammond, Bruce Eisley
We continue to visit fields to determine the status of the soybean aphid in Ohio and have found few if any aphids. After sampling numerous fields in northeast and central OH, we found 2 aphids in one field, and 8 aphids in another. All other fields were zero. We are also sampling the soybean rust sentinel plot for aphids; aphids have been found in very low numbers in two of those fields. The finding of very few aphids is also being reported by surrounding states. Although we expect the aphid to increase somewhat over the next month or so, we will be extremely surprised to see them reach economic levels. For the time being, we recommend keeping a watch on the situation, and keep hoping for the best. As always, keep checking this CORN newsletter for further updates.
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. The timing of mowing and/or herbicide application will vary depending upon weed life cycle - summer annual, winter annual, warm-season perennials, or cool-season perennial. It is not possible to control all of these with a single herbicide application, so decisions must be made about which have the highest priority based on the history of weed problems in each field. Note – glyphosate is a key component of many herbicide treatments in wheat stubble. We recommend use of glyphosate in combination with 2,4-D in most situations, to reduce further selection for any glyphosate resistance that might be developing in weed populations.
Summer annual weeds (ragweeds, lambsquarters, marestail) - a primary goal is to prevent seed production, which reduces their infestations in subsequent years. This can be accomplished through mowing, tillage, or an herbicide treatment. Treatment with herbicide may be most effective for control of these weeds, because they can regrow following mowing. Tillage can control existing weeds, but 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). Use the higher 2,4-D rates where glyphosate-resistant marestail is present.
Winter annual weeds (chickweed, deadnettle, mustards, etc) – a primary goal is to control emerged weeds this fall, in order to start next spring with a relatively vegetation-free seedbed. Winter annual weeds emerge from late summer into fall, and are effectively controlled by an herbicide application between mid-October and mid-November. 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. Fields with summer annual weeds can be mowed now (before August in dry areas or before mid-August in areas with ample rainfall) to reduce seed production, and followed with a late-fall herbicide application for winter annual control.
Warm-season perennials (johnsongrass, wirestem muhly, hemp dogbane, common pokeweed, common milkweed, etc) – a primary goal is to apply herbicide at a time when it can move into roots or other underground structures, which reduces future infestations of these weeds. Growth of warm-season perennials will cease before or at the first frost, and they should generally be treated with herbicide by mid-September. Where warm-season perennials are small yet and few summer annual weeds are present, it may be possible to do nothing at this time, and treat with herbicide in early fall. Where perennial weeds are large and it is also necessary to prevent summer annual seed production, 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. The most effective treatments are usually combinations of glyphosate plus either 2,4-D or dicamba. Warm-season perennial broadleaf weeds such as milkweed, hemp dogbane, and perennial vines are not easily controlled, and control will often improve as herbicide rate increases.
Cool-season perennials (quackgrass, Canada thistle, dandelion, etc) – a primary goal is to apply herbicide in mid to late fall when herbicide can move into roots or rhizomes, which reduces future infestations of these weeds. Where control of summer annual weeds is also desired, the best strategy may be to mow the fields by early August, and then apply herbicide between mid-October and early November to control perennial weeds. It is also possible to apply a low rate of glyphosate plus 2,4-D now to control summer annuals, and then apply again later in fall for perennial control. However, it is possible that the first herbicide treatment will injure perennials and reduce their growth over the next several months, which could make them less susceptible to a late-fall herbicide application. Glyphosate or glyphosate plus 2,4-D is effective for control of most cool-season perennials. Glyphosate may be more effective than the combination of glyphosate plus 2,4-D for Canada thistle control. Where dandelions are the primary target, the herbicide options previously listed for winter annual weed control can also be effective, with the exception of Sencor plus 2,4-D.
State Specialists: Pierce Paul and Ann Dorrance (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science), Peter Thomison (Corn Production), Ron Hammond, Bruce Eisley and Curtis Young (Entomology), Robert Mullen (Soil Fertility). Extension Educators: Howard Siegrist (Licking), Greg LaBarge (Fulton), Jim Skeeles (Lorain), Mike Gastier (Huron), Keith Diedrick (Wayne), Jim Lopshire (Paulding), Gary Wilson (Hancock), Alan Sundermeier (Wood), Steve Prochaska (Crawford).