Night 3 – “Corn Disease Update”
Dr. Pierce Paul, Chair and Professor
Department of Plant Pathology
Pesticides
Fall-applied Herbicide Considerations
Now that harvest is finally winding down, our thoughts change to fall weed control. This is the best time of year to control winter annuals and some of the more difficult to manage overwintering weed species. Biennial and perennial plants are now sending nutrients down to the root systems in preparation for winter. Systemic herbicides like glyphosate and 2,4-D applied at this time will be translocated down into the roots more effectively than if applied in spring when nutrients are moving upward. This results in better control. In addition, the increasingly unpredictable spring weather patterns we have experienced in recent years can influence the timing and efficacy of spring burndown applications. Fall-applied herbicides can lead to weed free situations going into spring until early emerging annuals begin to appear in April, and are an essential component in the control of marestail and other overwintering species.
Here are some reminders when it comes to fall-applied herbicides:
- Evaluate weed emergence and growth post-harvest to help determine if an application is necessary.
- Fall-applied herbicides should primarily target weeds that are emerged at the time of application.
- Species present in large quantities late-season that would necessitate the application of an herbicide include (but are not limited to): marestail, dandelion, wild carrot, poison hemlock, common chickweed, purple deadnettle, henbit, annual bluegrass, and cressleaf groundsel.
- OSU research has not found much of a benefit from adding metribuzin or other residual products late in the fall. The exception to this is chlorimuron, which can persist into the spring. The recommendation here has generally been to keep costs low in the fall and save those products for spring when you will get more bang for your buck.
- Herbicides generally work across a range of conditions, though activity can be slower as temperatures drop. Foliar products are most effective when daytime temperatures are in the 50s or higher and nighttime temperatures remain above 40.
Table 1 in the Weed Control Guide for Ohio, Indiana, Illinois, and Missouri provides ratings for various overwintering weed species in response to fall-applied herbicides.
Field Observations Thru July 21
Corn
Corn growth varies greatly throughout the county, but, more fields are beginning to tassel. As corn begins to tassel, nutrient (K > N > P) and water (0.30 inch per day) demands for the crop are close to maximum. Heat and drought will affect potential number of kernels. Scout for insects (e.g., corn leaf aphid, western bean cutworm, corn earworm, fall armyworm) and diseases (e.g., gray leaf spot, southern rust, northern leaf
blight). Total leaf defoliation severely affects final yields.
VT (Tasseling) – Stage VT occurs two to three days before silking, when the last branch of the tassel is completely visible but silks have not emerged yet from the ear shoot. The plant has reached full height and the pollen shed begins. The time between VT and R1 can vary with different hybrids and due to environmental conditions.
Pollen shed (pollen drop) normally occurs during the late morning or early evening. Hail damage is more serious at this time than for any other growth period. All leaves have emerged and the complete loss of a pollen source would result in no grain formation.
We are nearing a point in the growing season where it is time to scout your corn fields and make a decision regarding fungicide applications. Click here to view the 2023 corn fungicide ratings.
Soybeans
Postemergence herbicide applications – Don’t forget the preharvest intervals (PHI) for grazing or harvest. Read more here.
Soybean Postemergence Weed Control – Grasses
Soybean Postemergence Weed Control – Broadleaves
Soybean Growth & Development – R2: Full Bloom
- Open flower at one of two uppermost main-stem nodes
- About 50% of the total mature node number has been established.
- Very rapid nitrogen (N), phosphorus (P), potassium (K) and dry-matter accumulation is occurring and will continue through R6.
- Defoliation of the plant of 50% at this stage will reduce yield by 6%.
- Approximately 60 days away from beginning of physiological maturity (R7).
County Rainfall Update
Mount Vernon, OH
Preharvest Intervals for Soybean Herbicides Applied Postemergence
Preharvest intervals indicate the amount of time that must elapse between the herbicide application and crop harvest. Failure to observe the preharvest interval may result in herbicide residue levels in the harvested portion of the crop in excess of established limits. Also, livestock grazing or foraging treated soybean is not allowed on the labels of many postemergence soybean herbicides. Table 1 contains information regarding preharvest intervals and grazing restrictions for a number of postemergence soybean herbicides.
Table 18 from the OSU Weed Control Guide shows the PHI and feeding restrictions for postemergence soybean herbicide applications.
Soybean Postemergence Weed Control – Broadleaves
After planting, the second biggest challenge we face is timely weed control. If you haven’t already made a postemergence application, it might be time to check your fields. Most beans (and weeds) are at stage that might warrant an application.
Table 16 below from the 2023 OSU Weed Control Guide compares the relative effectiveness of herbicides on individual weeds. Ratings indicate the level of control of weeds present at the time of application, and are based on labeled rate and weed size or growth stage.
Why are my soybeans yellow? Part 1
Soybeans throughout the county just don’t look good right now. Development has been slowed and many fields have areas (sometimes large areas) with a light green or yellowish tint to the beans. What is causing this yellowing? One possibility is:
Yellow Flash (glyphosate)
Temporary yellowing of newly emerged soybean leaves due to glyphosate application to glyphosate-resistant beans. More common in:
- Conditions conducive to rapid soybean growth (high temperatures and/or humidity)
- High rates/areas of overlap
- Areas with micronutrient deficiency
Injury often appears one to two weeks after application and often disappears by 21 days after the area was treated. There is no evidence of yield loss from this phenomenon.
Soybean Postemergence Weed Control – Grasses
After planting, the second biggest challenge we face is timely weed control. If you haven’t already made a postemergence application, it might be time to check your fields. Most beans (and weeds) are at stage that might warrant an application.
Table 15 below from the 2023 OSU Weed Control Guide compares the relative effectiveness of herbicides on individual weeds. Ratings indicate the level of control of weeds present at the time of application, and are based on labeled rate and weed size or growth stage.
Click on table to enlarge
Dicamba Deadline Reminder
Source: Dr. Alyssa Essman
Crop progress is highly variable this year. Some early planted crops have emerged and have had decent growth following recent rains. Dry weather has delayed the emergence of some later planted crops. This means there will also be variability in growth stages, which can have implications for POST application timings. The deadline to apply dicamba over-the-top (OTT) in Ohio is coming soon. For OTT applications of the dicamba products labeled for this use, the cutoff is up to and through June 30th, or the labeled growth stage. Below are the cutoff dates and/or soybean growth stages for the different products.
- XtendiMax – R1 growth stage or through June 30th, whichever comes first.
- Engenia – no later than June 30th
- Tavium – V4 growth stage or through June 30th, whichever comes first.
A soybean plant is at the V4 growth stage when there are four open trifoliates. The R1 growth stage occurs when there is one open flower somewhere on the main stem. See here for detailed descriptions and pictures of soybean growth stages. Table 8 and table 18 of the weed control guide have information related to the maximum crop size and harvest intervals for POST applications in corn and soybean, respectively. As always, it’s important to scout fields before planning applications and follow the herbicide label guidelines.
Weed Response to Postemergence Herbicides in Small Grains
When determining your herbicide program for spring applications, the stage of the wheat crop should be considered. The majority of wheat herbicides labeled for application at certain wheat growth stages have very short windows in which they can be applied. The popular broadleaf weed herbicides 2,4-D and MCPA are efficient and economical, but can only be applied for a short period of time between tillering and prior to jointing in the early spring. Wheat growth stages and herbicide timing restriction are outlined in a in a post last week (Herbicide Applications on Winter Wheat).
Another consideration you should take into account when planning a spring herbicide application is the plant back (or recrop) restrictions to double crop soybeans. Many of the herbicides listed in Table 19, have soybean plant back restrictions greater than the typical three month time period between spring applications and double crop soybean planting. The soybean plant back restrictions greatly reduce the number of options available to wheat producers who double crop soybeans after wheat.
Herbicide Applications on Winter Wheat
When determining your herbicide program for spring applications, the stage of the wheat crop should be considered. The majority of wheat herbicides are labeled for application at certain wheat growth stages and some commonly used herbicides have very short windows in which they can be applied. The popular broadleaf weed herbicides 2,4-D and MCPA are efficient and economical, but can only be applied for a short period of time between tillering and prior to jointing in the early spring. Wheat growth stages and herbicide timing restriction are outlined in Figure 1.