C.O.R.N. Newsletter 2009-33

Dates Covered: 
September 29, 2009 - October 5, 2009
Editor: 
Wesley Haun

Is it Really Anthracnose Top Dieback?

This year, top dieback has been the most widely reported problem in corn across the state of Ohio. However, symptom characteristics, timing of occurrence, and distribution of affected plants across the field have led to questions as to whether it is really Anthracnose top dieback. This is one of three diseases caused by the fungus Colletotrichum graminicola on corn; the others are Anthracnose leaf blight and Anthracnose stalk rot. Unlike the latter two diseases, top dieback is rarely ever a major concern in Ohio.

The most telltale symptom of top dieback, as suggested by its name, is death of the plant from the top down. It can occur at any time after tasseling, and affected plants are generally found scattered across the field. Symptoms commonly occur, and are most easily diagnosed, while the plant is still green. Most of this year’s reported problems are from fields at very late growth stages, beyond R5 (dent). At this stage, with almost all of the plants drying down, it is difficult to say whether the death of the tips is due to Anthracnose top dieback or natural senescence. In some cases, the problem is said to be occurring on almost every plant in the field. This is uncommon for Anthracnose top dieback, unless the hybrid is highly susceptible and the fungus is spread uniformly across the field.

Nothing can the done about top dieback at this time and it is difficult to diagnose at such a late grown stage. The Iowa State University Extension website has a very good publication on diagnosis of Anthracnose top die back at http://www.wittconsultinginc.com/images/E0092801/CornDieback.pdf.

Harvest Aids for Corn and Soybeans

Large weeds present at the time of crop harvest can slow the rate of crop maturation, slow the rate of harvest, and increase mechanical stress on equipment. There are essentially two ways to reduce the green weed biomass in the field in order to facilitate harvesting: 1) delay harvest until after a hard freeze (less than 25 degrees F for several hours); or 2) apply herbicides to control and desiccate the weeds. Waiting a week or so after the freeze or herbicide application will allow the weeds to desiccate to a greater extent and become more brittle.
Many glyphosate products can be used in a preharvest treatment in corn or soybeans – check labels for specific uses and rates. In corn, apply glyphosate at least 7 days before harvest when grain moisture is 35% or less. Corn should be physiologically mature (black layer formed) with maximum kernel fill complete. In soybeans, apply glyphosate after the pods have set and lost all green color, and at least 7 or 14 days before harvest, depending upon the glyphosate product. Most glyphosate product labels recommend avoiding preharvest application to corn or soybeans grown for seed, due to the potential for a reduction in seed germination or vigor.
Gramoxone and Parazone can be applied as preharvest treatments in field corn, seed corn, popcorn and soybeans. Apply when corn is mature, or after the black layer has formed at the base of the kernels, and at least 7 days before harvest. In soybeans, apply at least 15 days before harvest when at least 65% of the pods have reached a mature brown color or when seed moisture is 30% or less. Apply with crop oil concentrate (1.0 % v/v) or surfactant (0.25 % v/v). Use a spray volume of at least 20 gpa in ground applications, and 5 gpa for aerial application.
Aim is labeled as a preharvest treatment for corn and soybeans for desiccation of velvetleaf, morningglory, pigweeds, and other annual weeds. Apply at least 3 days before harvest, when the crop is mature and grain has begun to dry down. The rate is 1 to 2 oz/A, and it should be applied with nonionic surfactant (0.25% v/v) or crop oil concentrate (1 to 2% v/v). Rage D-Tech (premix of Aim and 2,4-D) can be used as a preharvest treatment in corn only. The rate is 16 to 32 oz/A, and guidelines for timing and adjuvant use are similar to Aim.
Other Considerations:
1. The greener the weeds, the more effective the treatments (including a freeze) and greater likelihood of reducing weed seed viability. Herbicides will not necessarily reduce the viability of seeds that have formed by the time of application.
2. In general, herbicides will be most effective when applied under warm, sunny conditions.
3. Glyphosate can control perennial weeds that are in the appropriate growth stage at the time of application.
4. Herbicide treatments and freezing weather will not necessarily force loss of fruit on black nightshade plants, or other fruit-bearing weeds.

Western Bean Cutworm Larvae Found

Having caught western bean cutworm moths for the past four years, with over 500 moths being collected this past summer, we finally located our first larva last week, along with evidence of additional ones. A caterpillar was brought into extension from a location in southeast Putnam County that was identified as a western bean cutworm larva. A subsequent visit to the location, while not finding any more larvae, did find a corn ear with characteristic holes in the husk with feeding injury immediately below it. Although over 100 ear tips were closely examined for larvae, it was by quickly examining corn ears for holes in the husk that revealed this other find. See the C.O.R.N. newsletter from August 17, http://corn.osu.edu/story.php?setissueID=307&storyID=1878 for a video showing how to look for western bean cutworm injury.
It should be noted that this infestation is extremely low, and hardly worth calling an infestation. Nonetheless, larvae, or at least an indication of them, were found. With such low numbers, it is questionable if this will lead to a resident population in this area next summer. However, we will plan on monitoring this area of the state closely with both adult pheromone traps and subsequent egg and larval sampling next summer for the possibility of a localized infestation.

Corn Ear Rot and Potential Mycotoxin

We have had several reports of ear rot problems in corn. This disease is caused by one or more of several fungi capable of infecting, colonizing and damaging the ear. The most common members of the ear rot complex are Gibberella zeae (causes Gibberella ear rot), Stenocarpella maydis (causes Diplodia ear rot) and members of the genus Fusarium (causes Fusarium ear rot).

Gibberella ear rot, is the most common of the ear rots this year, however, we have also received reports of Diplodia ear rot in some fields. Diplodia causes a thick white mass of mold to grow on the ear, usually initiating from the base of the ear and growing toward the ear tip. Diplodia infections can begin before tassel emergence up to silking. With Gibberella ear rot, the fungus enters the ear tips through the silk channel. A visible white to pink mold covering the ear tip or more of the ears is characteristic of this disease. Gibberella ear rot is generally most severe when rain and wet weather is prevalent during the 7 to 10 days after silking.

The Gibberella ear rot fungus produces mycotoxins that are harmful to animals. These include deoxynivalenol (Vomitoxin) and zearalenone and T-2 toxin, all of which may cause health problems in livestock. Therefore, suspect grain should be tested for these mycotoxins by chemical analysis before being fed to animals. As a general rule do not feed any grain with 5% or more Gibberella moldy kernels. Hogs and young animals are particularly sensitive to these mycotoxins. Diplodia ear rot is less of a concern from a mycotoxin standpoint. There have been no reports of Diplodia producing mycotoxins that are harmful to animals in Ohio, but animals do refuse to eat grain with high levels of Diplodia-damaged kernels. Additionally, severely affected grain has low nutritional value.

Certain hybrids are more susceptible to one or more ear rots than others. Examine ears to determine the presence of ear molds. Make a note of which ear rots are present and hybrids that are most affected. Make future hybrid choices based on this information.

Growers are advised to follow certain harvest and storage guidelines to minimize problems associated with kernel rots and mycotoxin contamination:
1. Harvest at the correct moisture and adjust harvest equipment to minimize damage to kernels. Mold and mycotoxins tend to be higher in (machine or insect) damaged kernels.
2. Dry harvested grain to 15% moisture and below to prevent further mold development in storage.
3. Store dried grain at cool temperatures (36 to 44 F) in clean, dry bins. Moderate to high temperatures are favorable for fungal growth and toxin production.
4. Periodically check grain for mold, insects, and temperature.
5. If mold is found, send a grain sample for a mycotoxin analysis to determine if toxins are present and at what level. For more on moldy grain, mycotoxins, and mycotoxins sampling and analysis visit the following websites: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/Mycotoxins/mycopagedefault.htm.

Soybean Aphids Going to Buckthorn in Large Numbers

A few weeks ago we commented on the large number of soybean aphids being found on late season soybeans, especially in southern Ohio. This past week we have been finding, again with many other states, large numbers of aphids already on buckthorn, their over-wintering host. These large numbers of aphids suggest the potential of a large over-wintering population. Being that this summer has been considered a “soybean aphid year” in much of the eastern Corn Belt, we would normally expect that next year would be an off-aphid year. However, significant over-wintering could mean soybean aphid problems next year and perhaps an end to the two-year aphid cycle that we have been in since the aphid’s arrival. Growers should keep abreast through this C.O.R.N. newsletter on what happens this fall and then in the spring of 2010. As of now, we are seeing something unexpected that could lead to aphid concerns next summer. We will continue to modify our prediction for aphid problems next summer as conditions change.

Weather Update

Last week was generally wetter and warmer than average. The exception was northwest and west-central Ohio which received less than 0.33 inches of rain. The rest of the state generally obtained 1-2 inches.
It appears that a series of storms the next two weeks might impact harvest somewhat. Expect some showers in the north to start this week. It will also be a colder than average week. Light showers will move in Friday - Sunday with more showers by middle to end of next week.
Overall, it looks active but progressive in the weather pattern. We expect most places to see an inch of rain over the next 2 to 2.5 weeks. Nothing real heavy but a series of events may interrupt the crop harvest process.

Soil Sampling This Fall

Once the crops are off for the year, it seems like we’re preparing for the next season already. If you don’t have a soil sample from a reputable lab in the last three years, it may be time to check up on your field’s nutrient contents.
First of all, try and take soil samples at about the same time of year, either in fall following the harvest or in the spring before applying any fertilizer, lime, or manure. Soil samples shouldn’t be taken while the crop is growing (with the exception of PSNT, or presidedress nitrate test).
When soil sampling, do use clean probes and buckets. We are not sampling a great amount of soil to represent the field, so a few contaminants in a probe or bucket can make a big difference when the analysis is done. Move residue off of the surface of the soil where the soil probe will be used. For conventionally-tilled fields, 8 inches is the depth to sample. In no-tillage fields, take a separate set of samples at 4 inches depth, as well as the 8 inch samples, to make a more precise assessment of soil pH (which tends to stratify, that is, show more acidity in the top couple of inches in no-tillage fields that use ammonia fertilizers), and thus lime recommendations. Take about 20 soil cores per 25 acres, mix thoroughly for a uniform sample.
Ohio State University does not run a soil testing laboratory; a list of labs can be found here: https://agcrops.osu.edu/tools/testlabs.pdf
Once you have a report and work on a plan for inputs, interpretation help can be found in the Ohio Agronomy Guide: http://ohioline.osu.edu/b472/0004.html. Depending on the lab, soil test values may be in either parts per million (ppm) or pounds per acre (lb/ac). To convert ppm into lb/ac, multiply ppm by 2 (since there are 2 million pounds of soil in an acre furrow slice).

Late-Season Northern Corn Leaf Blight

Late, but rapid development of Northern Corn Leaf Blight (NCLB) has some Ohio corn producers asking: why are we seeing such high levels of NCLB this year and how will this affect our yields? At the time of silk emergence (R1), foliar disease levels were very low in corn fields across the state, but shortly after R3 (the milk stage), lesions of NCLB began showing up on the middle and upper leaves of the plants. As the season progressed, lesions continued to develop, blighting as much as 20 to 30% of the ear leaf in some fields by the time the crop reached the dent stage (R5). On most of the hybrids with the problem, the symptoms were very characteristic of a susceptible reaction to the disease, with one-to-six inch long cigar-shaped gray-green to tan-colored lesions on the leaves.

Since the 2001 growing seasons, we have seen a fairly steady increase in the occurrence of northern corn leaf blight in the state. This may be due in part to an increase in the number of acres planted to NCLB susceptible hybrids. The relatively late occurrence of the disease this year was probably due to favorable weather conditions late in the season. NCLB develops best at temperatures between 66 and 80 F, accompanied by extended periods of surface wetness (due to rainfall, dew, and high relative humidity).

For an epidemic of northern corn leaf blight (and any other plant disease for that matter) to occur, three basic conditions must be met: 1) the fungus which caused the disease (Exserohilum turcicum) must be present; 2) the hybrid planted must be susceptible to the prevalent races of the fungus; 3) and the environmental conditions must be favorable. Quite often the fungus is present because it survives in crop residue in the field or can be easily blown from one field to another. So, if a susceptible hybrid is planted, then the only thing that prevents NCLB from developing is unfavorable weather conditions. We had favorable weather after R3, with average temperatures were between 67 and 74 F and, depending on the location, between 1.5 and 6 inches of total rainfall during the month of August. However, the disease developed late in most fields and as a result will likely have little or no impact on grain yield. Make a note of the susceptible hybrids and avoid planting them again next year. Planting resistant hybrids is the most effective method for control of NCLB. Partial resistance, which protect against all four of the known races of the fungus, is common among hybrids and should used to minimize problems in the future. On hybrids with partial resistance, lesions are smaller, of a lighter color, and produce fewer spores than lesions on susceptible hybrids

Archive Issue Contributors: 

State Specialists: Pierce Paul and Dennis Mills (Plant Pathology), Ron Hammond, Andy Michel, and Bruce Eisley (Entomology), Peter Thomison (Extension Corn Specialist), Mark Loux (Extension Weed Specialist), Keith Diedrick, David Henry, and Robert Mullen (Soil Fertility), James Noel (NOAA/NWS/OHRFC). Extension Educators and Associates: Glen Arnold (Putnam), Roger Bender (Shelby), Mike Gastier (Huron), Wes Haun (Logan), Harold Watters (Champaign), Greg LaBarge (Fulton), Les Ober (Geauga), Bruce Clevenger (Defiance), Alan Sundermeier (Wood), Ed Lentz (Seneca), Mark Koenig (Sandusky), Steve Prochaska (Crawford), Curtis Young (Hancock), Jonah Johnson (Clark).

About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.