C.O.R.N. Newsletter: 2017-31
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Are Fall Herbicide Treatments Necessary in Xtend Soybeans?
There is obviously a lot going on with the dicamba issue, resulting in uncertainty as to where exactly we are headed with regard to future labels, restrictions, and stewardship. However, we are fairly confident that the label for early-season use of dicamba in preplant/premeergence burndown programs will be preserved into next year at least (“dicamba” in this article refers to the three dicamba products approved for use in Xtend soybeans – Engenia, XtendiMax and FeXapan). Dicamba is more effective than 2,4-D on marestail in the spring, and has a good fit in burndown programs to help with this weed. And using dicamba early-season is one way to minimize the risk of off-target movement and injury to surrounding vegetation, compared with later-season POST applications when temperatures are higher, inversions are more prevalent, and non-target plants are more developed and sensitive.
As we move into the fall herbicide application season, one question coming up is whether the spring dicamba burndown treatment is good enough on overwintered marestail plants to eliminate the need for fall herbicides. The answer is yes, maybe, and no, depending. In OSU research, inclusion of dicamba in burndown treatments prior to early May has resulted in effective control of small emerged marestail. Most of these treatments have also included residual herbicides and glyphosate. When application is delayed past early May and marestail get taller and older, we have occasionally observed reductions in control. It appears that among other reasons, the residual herbicides can antagonize and reduce the activity of the dicamba on larger marestail sometimes. Large marestail can be tough to control – period – and cold weather increases the difficulty. We have also observed this with 2,4-D so it’s not unexpected.
Our research results indicate that where the spring burndown treatment is applied early enough and includes comprehensive residual herbicides that control later-emerging marestail, there should be little need for a POST application of dicamba (for marestail at least). In a year when burndown applications and planting get delayed and overwintered marestail plants are large, expect there to more variability in the effectiveness of dicamba on existing marestail, and an increased need for follow up POST dicamba treatments. Hence the “yes” and “maybe”. Certainly one way to minimize the potential for problems with spring marestail control in Xtend soybeans is to keep using fall-applied herbicides. The fall treatment will accomplish the same thing in Xtend soybeans that is does for all other soybeans – control the marestail that emerges in late summer and fall so that fields are devoid of weeds in spring. The spring burndown then just has to take care of small spring-emerged marestail. This is still the most consistently effective way to manage marestail for not a lot of money.
With regard to the “no”, we need to consider all of the reasons why fall herbicide treatments became commonplace. While they became an essential component of marestail management programs over the past decade, fall treatments first gained traction in the decade before that for management of dense infestations of winter annual weeds and dandelion. Comments from growers at that time were that spring infestations of chickweed and deadnettle and dandelion were interfering with tillage and planting, and spring-applied burndowns just did not cause the weeds to die and dessicate fast enough. Dandelion and other cool-season perennials are more susceptible to herbicides in the late fall compared with spring. Dandelion in particular became extremely problematic for a few years. Some winter annual weeds also serve as a host for soybean cyst nematode and other insects in late-fall and/or spring. The adoption of fall herbicide treatments resolved many of these issues. So moving forward, the omission of fall-applied herbicides is likely to mean the return of some of these problems, even if spring-applied dicamba can handle the marestail adequately.
The bottom line here is that applying herbicides in fall still results in a weedfree seedbed well into spring that allows for maximum soil warming and drying, greatest ease of tillage and planting, and the most consistently effective marestail control. And fall treatments help take the pressure off of spring herbicide programs in a year when spring weather is less than ideal. Risk management 101.
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Should we add Diaporthe stem canker and Cercospora leaf blight to our list of disease ratings for Ohio in 2018?
Improving soybean yields in 2018 begins first with the selection of the cultivars that have the best resistance package for Ohio’s notorious pathogens and pests. Any grower that’s slacked off on the Phytophthora package gets a quick reminder of the damage that this pathogen can continually cause in a vast majority of Ohio’s production regions. Same thing with soybean cyst nematode; while the symptoms may not be present, planting a susceptible variety and getting half the yield that the neighbors got leaves some farmers scratching their heads. We finally have resistance to Sclerotinia that is effective for those regions that deal with it on an annual basis. Over the past decade, we’ve added frog eye leaf spot to that list as it can overwinter and if infections get started at flowering it can cause substantial damage. Why put another $30+/Acre for a fungicide to control frogeye or Sclerotinia when the cultivar resistance can hold that disease in check?
Now it may be time to begin to think about some additions to this list as in the past 3 years, I’ve been called to fields that have been severely affected by some pathogens that are typically rare in this part of the U.S. Diaporthe Stem Canker and Cercospora Leaf Blight (Figures 1 and 2). Let’s take each of these separately and explain a bit more.
Diaporthe stem canker:
The symptoms that are occurring in Ohio this year are large patches of early dying plants that still have their leaves attached. The canker is not as well defined but can occur from the third node at the bottom of the plant up to the top 1/3. There may be rows of small pin dots in the center of the canker or on pods that are the actual fungus. The bottom two nodes of the plant are still green – which can separate it from Phytophthora. If the stem is bleached white and has white mold – it is Sclerotinia. For Diaporthe, the internal tissue – both the pith and the stem are degraded.
Diaporthe is caused by several different fungi and can also lead to Phomopsis seed rot. The fungus survives very well on residue. And in fact the most affected fields that I have walked in 2017 – were in fields where the disease may have occurred to a smaller extent the previous year. Diaporthe also infects a number of weed hosts that can also contribute to the survival in fields including: black nightshade, morning glory, northern joint vetch, and spiny amaranth are a few and most do not show symptoms.
Infections for this disease occur in the early vegetative growth stages of the plant and these are favored by long periods of warm (72 to 86F) wet weather. Rain splash of spores from plant residue can facilitate these infections. Does this sound like our summer where some areas had 15” or more during June-July? This fungus takes its time, and the symptoms tend to coincide when the seeds begin to fill in the pods.
Management is highly successful with two tactics, planting resistant cultivars and reducing inoculum. The screening for resistance to this pathogen is fairly straightforward for Diaporthe, so most companies should be doing this. Tillage and rotation both are effective in reducing inoculum. A study in Georgia reported in 1988 indicated that a soybean-wheat double crop was not as effective as a soybean-fallow rotation.
Cercospora leaf blight-and Purple Seed Stain
Another foliar and seed pathogen that is not well known in Ohio and is much more prevalent in the Southern US. The reddish discoloration and leathery appearance begins to appear on the top leaves as the plants begin to fill out the seeds. Purple lesions on the petioles or stems also develop. The infected petioles remain attached to the plants while the infected leaves fall off the plants. Warm temperatures and frequent rains also contribute to this disease which can be spread by rain and wind. This can also be residue born – so for farmers that have this in their fields in 2017 – TAKE NOTE- residue management and planting varieties with resistance will be essential in 2018.
Infections for Cercospora can begin at flowering and repeat throughout the season, but symptoms do not develop until pod fill. Seed can also become infected and develop the same purplish-red coloration and contribute secondary losses if they are food grade soybeans. The pathogen that causes Cercospora leaf blight and Purple seed stain produces a light-activated plant toxin, which contributes to the purple discoloration of the diseased tissue. Spores are produced on the residue or infected tissue and are dispersed by wind or rain onto nearby soybean plants. Only susceptible varieties will develop symptoms and these will be more pronounced when dry, warm conditions occur at pod fill.
Disease management strategies include planting high-quality disease free seed, tillage to break down infested residue and crop rotation to prevent inoculum build-up, and planting resistant varieties. For fields that are affected in 2017 – a timely harvest to ensure the fewest number of seeds develop purple seed stain. Secondly, for fields in 2018 that go back into soybean, if susceptible varieties are planted, a fungicide application of a triazole at R3 may provide some protection. However, infections can occur during the vegetative phase and most of the data to date is from the southern states with determinant later maturity groups. This is a big hole in our data set for providing recommendations for fungicide timing for Cercospora blight in northern areas.
The bottom line, when you are at Farm Science Review this week talking with the seed companies, ask for the resistance packages on the varieties. Yes, yields are important – but the resistance and savings in additional mid-season inputs is key to profits in years with narrow margins. The risk for disease development continues to increase with mild winters that favor overwintering and the soil-saturating rains that seem to occur weekly. These conditions make those resistance scores more and more important each year.
For some additional reading while you are on auto-steer during harvest check out these resources:
https://agcrops.osu.edu/sites/agcrops/files/imce/StemCanker_CPN1006.pdf
http://soybeanresearchinfo.com/diseases/cercosporaleafblight.html
http://plantpathology.ca.uky.edu/files/ppfs-ag-s-20.pdf
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Harvest Weather Outlook
After a first half of September which was 5-10 degrees below average, the second half of September will average 5-10 degrees above average making September in the end a near average month but marked by significant differences in the month. Temperatures the week of September 19-25 will run 10-15 degrees above average with no risk of frost.
Rainfall will remain limited in most areas for the rest of September as well. Some rainfall will occur Tuesday September 19 through Wednesday September 20. Rainfall will average less than a tenth of an inch in the southeast half of the state to 0.10 to 0.50 inches in the northwest half with isolated higher totals. After September 20, the next chance for rain does not come up until around Sept. 26 or 27.
October Outlook
Temperatures are likely to relax closer to normal in October after the warm late September. Rainfall is also expected to increase some especially in the second half of October. We expected October rainfall to be near or slightly below average which is close to 2 inches for the month on average.
Tropical Outlook
Tropical activity looks to stay east of Ohio in the coming weeks. In fact, with storms well east it is enhancing high pressure and drier conditions over the region locally. Historically, storms tend to shift into the Caribbean and Gulf of Mexico once we get to October and November. We will have to wait and see if some of that moisture would make its way back into our region.
Frost/Freeze Outlook
There is really no risk of frost and freeze conditions for the rest of September. At times we do see historically late September frosts in Ohio but none are expected this year.
We have been talking in recent months that data suggest a normal to later than normal frost/freeze in Ohio and that looks still to be the case. Sometime in October we will likely see our first widespread frost and possible freeze and typically that arrives the first 2-3 weeks of October but chances are growing it will be in the middle to end of the month of October.
La Nina Watch issued by NOAA Climate Prediction Center
Confidence is still low to moderate but the NOAA Climate Prediction Center has issued a La Nina watch for cooler equatorial Pacific Ocean water this winter.
This could lead to a winter and early next spring that is wetter than normal with temperatures starting winter warmer than normal and turning normal to colder than normal. It is too early to tell, but those are some of the early indications.
Two week rainfall outlook
The outlook for the next two weeks is normal to below normal rainfall for early harvest in the eastern corn, soybean and wheat areas with much above normal rainfall in western areas as the NOAA/NWS/OHRFC graphic shows.
Crop Observation and Recommendation Network
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.
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