Waterhemp Woes – Part 1
Originally posted in the VegNet Newsletter on May 17, 2020
We have detected an extremely large population of armyworm moths in Columbus during the past week. This pest prefers to feed on grasses, including corn, wheat, rye, and grassy weeds, but if those plants are in shortage and if populations of armyworm are large, it can infest other crops including alfalfa, beans, cabbage, cucumbers, lettuces, onions, peppers, and radishes. Infestation can be worse in no-till fields than in tilled fields. Any early-planted fields of these crops should be scouted for presence of armyworm. Scouting is best done near dawn or dusk because armyworm larvae are nocturnal and hide in the soil during the day. The name armyworm is given because of the ability of older larvae to form large aggregations that move together from field to field. Infestations can appear quite suddenly in a field, and much damage can occur in a short period of time.
Though it seems like spring has been slow to come this year, we have actually accumulated enough degree days to see potential outbreaks of alfalfa weevil in some locations. Ohio experienced its 5th warmest winter on record (1895-2020) and March temperatures averaged 2-8°F above average. Overwintered adults begin laying eggs when temperatures exceed 48°F. Peak larval activity and feeding damage occurs between 325 and 575 heat units (based on accumulation of heat units from January 1 with a base of 48°F). Current (Jan. 1 – Apr. 11, 2020) heating units range from near 100 in far northeastern Ohio, 100-200 across much of northern Ohio, and 200-300 units across much of central, southwest, and southeast Ohio. South central Ohio has currently eclipsed 300 units as evident at OSU South Centers in Piketon.
In short, now is the time to start scouting. Alfalfa fields should be scouted weekly for weevils until at least the first harvest. Don’t let your guard down with the recent turn to cooler weather! We’ve seen significant weevil infestations in past years when early warm weather pushed weevil development earlier than normal, followed by cooler weather later that slowed alfalfa growth. Continue reading
As you know The Ohio State University has closed all Campuses and Extension offices. While our office is closed, we are working from home and will continue to do so until we are able to return. You can reach us by phone (740-397-0401) Monday through Friday from 8 – 5. You can also reach us anytime by email:
In the meantime we are working diligently to create new options to stay in contact with everyone. With this in mind, beginning Monday April 6 we will begin VIRTUAL OFFICE HOURS – Knox AgChat
Knox AgChat will provide us the opportunity to utilize video and/or audio conferencing on your computer or cell phone. You can join us online here: https://osu.zoom.us/j/3927263521 or join by phone 1-253-215-8782 and enter Meeting ID: 392 726 3521.
We will focus on Ag questions from 7:30 – 8 and Horticulture questions from 8 – 8:30.
Additionally, we plan to periodically invite guest speakers to our chat. We will post that schedule each week.
According to our network of sources, the effectiveness of new soybean trait systems has some growers once again thinking about omitting preemergence residual herbicides from their weed management programs. Some people apparently need to learn the same lessons over and over again. Having gone through this once in the early 2000’s when Roundup Ready soybeans had taken over and we all sprayed only glyphosate all day every day, we think we’re pretty sure where it leads. We’re sensitive to concerns about the cost of production, but the cost-benefit analysis for residual herbicides is way in the positive column. We’re not the ones who ultimately have to convince growers to keep using residual herbicides, and we respect those of you who do have to fight this battle. Back in the first round of this when we were advocating for use of residuals, while the developers of RR soybeans were undermining us and telling everyone that residuals would reduce yield etc, we used to have people tell us “My agronomist/salesman is recommending that I use residuals, but I think he/she is just trying to get more money out of me”. Our response at that time of course was “no pretty sure he/she is just trying save your **** and make sure you control your weeds so that your whole farm isn’t one big infestation of glyphosate-resistant marestail.” And that answer probably works today too – maybe substituting waterhemp for marestail.
We need to state here that a good number of growers kept residual herbicides in their programs through all of this, and we assume they aren’t tempted to omit them now either. For everyone else – maybe interventions are called for. Where the recalcitrant person is repeatedly thumped with a stick while being reminded of what happened last time, until they change their minds.
Weed scientist: so you’re going to use residual herbicides right?
Soybean grower: no
WS: remember what happened last time – lambsquarters became a problem when every residual herbicide would have controlled it. Change your mind yet?
WS: remember when the weather didn’t cooperate and you ended up spraying 2 foot tall weeds because of no initial control? Do you want this again?
WS: so you’re going to use residuals?
SG: not sure
WS: and you expect your local dealer to clean up whatever mess occurs when you don’t use residuals?
WS: remember when you burnt out the FirstRate on marestail and then the glyphosate wouldn’t work? Do you want this to happen with dicamba, 2,4-D and glufosinate?”
WS: well then
Gentler persuasive tap
WS: You know how bad a weed waterhemp is right?
WS: what if residuals will help prevent waterhemp infestations
SG: Ok then – yes
WS: ok then
Note: we considered a number of sound effects here – thump, zap, whack…. Thump won out for no particular reason. We could not decide whether getting hit by a stick was more or less acceptable than getting shocked in this context.
The bottom line is that residual herbicides provide both short- and long-term risk management in weed management for a relatively low cost. A non-inclusive list of these:
– reduces weed populations overall and slows weed growth, resulting in more flexibility in the POST application window.
– Reduced risk of yield loss if weather interferes with timely POST application. In the absence of residual herbicides, soybean yield loss can occur when weeds reach a height of 6 inches.
– increases the number of different sites of action used within a season, slowing the rate of resistance development
– reduces the number of weeds that are treated by POST herbicides, which also slows the rate of herbicide resistance development
– residuals control lambsquarters which is not well-controlled by POST herbicides
– the most significant weed problems in Ohio soybean production – waterhemp, giant ragweed, and marestail – cannot be consistently controlled with POST herbicides alone. They require a comprehensive herbicide program that includes residual and POST herbicides. It may be possible to make a total POST system work some years or for a while, but in the end this approach will result in problems with control and speed up the development of resistance.
This whole subject of omitting residual herbicides makes us cranky because we don’t have to guess what will happen. We’ve made our best case here. It’s up to you of course, but we suggest that we not have to come back and have this discussion again. Because next time we’re bringing a few friends, a bigger stick, and a gorilla.
Disclaimer: Parts of this article are meant in pure jest. We would certainly never advocate in earnest the use of physical harm or other methods of persuasion to change the behavior of herbicide users. This goes against everything that the discipline of weed science stands for, and also OSU. Plus – we don’t even know where to rent a gorilla.
Are you interested in learning how to make the most of a few acres? If so, this eight-week course is just for you! Filled with practical knowledge on a variety of topics – you won’t be disappointed! Licking County will host this college starting January 22, 2020 and meeting for eight consecutive Wednesday evenings. See the flyer for further details and registration information.
Source: Anne Dorrance
Harvest is well underway and once the soybeans are off the fields this provides some time to sample soil for the SCN populations. The SCN Coalition theme for the next few years is What’s your number? Do you know which fields have SCN and what the current population is sitting at? If its high, then there is a second number – what is the SCN type? Which addresses the bigger question can it reproduce on the SCN resistance source PI 88788 or Peking. All of these numbers can impact management of this root pathogen and future losses.
The situation in Ohio: We know that the state is now “polluted” with SCN, fortunately most of those fields are at very low levels – which is where they should be kept.
From samples received to date of a statewide survey for Ohio of 50 counties as part of the SCN Coalition sampling, here are the numbers from 378 fields.
Yield losses have been measured as high as 25% with no above ground symptoms in populations of 2,000 and higher.
Summary to date:
If your SCN report in the past has come back as:
SCN is picky about what it feeds and reproduces on but it does like a few weed hosts and cover crops as well as soybean. If you have SCN in your fields , it is important to also control winter annuals such as purple deadnettle, but also avoid cover crops such as several of the clover’s, cowpea and common & hairy vetch.
So it is time to sample! We recommend sampling in the fall – because in most cases this is what the population will be in the spring. With the warmer weather this year and hopefully no frozen ground should give ample time to collect and process the samples in plenty of time for spring planting. Processing of samples does cost time and money, so here are a few thoughts on how to sample or how to target your sampling to get the best information for your money.
There is still some free sampling available. Contact your John at 740-397-0401.
Source: Pierce Paul, Felipe Dalla Lana da Silva, OSU Extension
Tar Spot, a new disease of corn caused by the fungus Phyllachora maydis, was reported for the first time in Ohio at the end of the 2018 growing season. At that time, it was found mostly in counties close to the Indiana border, as the disease continued to spread from the middle of country where it was first confirmed in 2015. Over the last few weeks, there have been several new, confirmed report of Tar Spot in Ohio, this time not only in the northwestern corner of the state, but also from a few fields in central and south-central Ohio. As was the case last year, disease onset was late again this year, with the first reports coming in well after R4. However, some of the regions affected last year had more fields affected this year, with much higher levels of disease severity. It could be that Tar Spot is becoming established in some areas of the state due to the fungus overwintering in crop residue from one growing season to another. This is very consistent with the pattern observed in parts of Indiana and Illinois where the disease was first reported. We will continue to keep our eyes out for Tar Spot, as we learn more about it and develop management strategies. You can help by looking for Tar Spot as you walk fields this fall, and please send us samples.
What does it look like? Even though corn is drying down, if Tar Spot is present, you can still detect it on dry, senescent leaves almost as easily as you can on healthy leaves. So, please check your fields to see if this disease is present. “Symptoms of tar spot first appear as oval to irregular bleached to brown lesions on leaves in which raised, black spore-producing structures call stroma are formed… giving the symptomatic areas of the leaf a rough or bumpy feel to the touch… resembling pustules on leaves with rust. Lesions … may coalesce to cause large areas of blighted leaf tissue. Symptoms may also be present on leaf sheaths and husks.” As the name of the disease suggests, symptoms look like the splatter of “tar” on the leaves. In some cases, each black tar-like spot may be surrounded by a necrotic halo, forming what is referred to as “fish-eye” lesions.
What causes Tar Spot and how damaging is it? In the past, the greatest impact of this disease in terms of yield loss were observed when P. maydis-infected plants were co-infected with a second fungus called Monographella maydis. In other words, the damage tended to be much more severe when the two fungi worked together to affect the plant. So far, only the first fungus, P. maydis, has been reported in the US, but based on work done in Illinois, this pathology alone is capable of causing substantial yield reduction on highly susceptible hybrids when conditions are favorable and infections occur early.
Where did it come from and will it survive and become established? At this point it is still unclear as to how Tar Spot got to the US in the first place and how it continues to spread. The fungus is not known to be seed-borne or infect other plant species, so corn seeds and weeds are unlikely to be the sources of inoculum. However, the fungus can survive and be moved around on fresh and dry plant materials such as leaves and husks. In addition, since spores of the fungus can be carried be wind, it could be blowing in from neighboring states/counties/fields. Although not yet confirmed through survival studies, it appears that the fungus could be overwintering in infected crop stubble between growing seasons.
What should I do if I find Tar Spot? If you see anything that fits the description of, or resembles (Picture) Tar Spot, please inform your state specialist, field specialist, or county extension educator, but most importantly, please send samples to my lab (1680 Madison Ave, Wooster, OH) for confirmation. We will also be using your samples to study the fungus in order to develop effective management strategies.
Read more about Tar Spot of Corn at:
Source: Peter Thomison, Pierce Paul, OSU Extension
2019 may be an especially challenging year for corn stalk quality in Ohio. Stress conditions increase the potential for stalk rot that often leads to stalk lodging (Fig. 1). This year persistent rains through June caused unprecedented planting delays. Saturated soils resulted in shallow root systems. Corn plantings in wet soils often resulted in surface and in-furrow compaction further restricting root growth. Since July, limited rainfall in much of the state has stressed corn and marginal root systems have predisposed corn to greater water stress.
Source: Dr. Mark Loux, OSU
If you have never applied herbicide in fall to burn down winter annuals, or done it only infrequently, this might be the year to make an investment in fall herbicides. Fall treatments are an integral component of marestail management programs. They also prevent problems with dense mats of winter annuals in the spring, which can prevent soil from drying out and warming up, interfere with tillage and planting, and harbor insects and soybean cyst nematode. 2019 was a generally tough year for weed control, leading to higher end of season weed populations in some fields. A number of acres were never planted, and growers got to experience the difficulty in obtaining season-long control in the absence of a crop. Reminds us all how important the crop canopy and shading of the soil is during the second half of the season. Bottom line – there was substantial production of weed seed in some fields, and a replenishment of the soil seedbank by both winter annual and summer annual weeds. The seed of winter annuals and marestail lacks dormancy so above-average weed seed production can lead to an immediate increase in fall-emerging weeds. Applying herbicides this fall can compensate for increased weed populations and make life easier in the spring.
We have published information on fall herbicides fairly frequently, and our suggestions for fall treatments have not really changed much. There is plenty of information on fall herbicide treatments in the C.O.R.N. newsletter archive and on other university websites. Our philosophy on this has not changed much over the past decade. A few brief reminders follow:
1. When to spray? Anytime between now and Thanksgiving will work, and possibly later. We have applied into late December and still eventually controlled the weeds present at time of application. Once hard freezes start to occur, there is usually a substantial change in the condition of certain weeds, such as dandelion and thistle, that renders them less sensitive to herbicides. We discourage applications during periods of very cold weather which can occur starting about Thanksgiving, and also (obviously) when the ground is snow-covered. The generally dry conditions we are experiencing have limited weed emergence so far this fall. We anticipate that rain occurring now that leads to some sustained soil moisture near the surface will likely result in germination and emergence of the weeds that have been missing until now. Our recommendation is to wait for rain and the additional weed emergence before applying any herbicide this fall. The risk in this is that the weather turns wet, making it difficult to apply herbicide. So it’s also possible to apply now and include a residual component to help with later fall emergence (which is the exception to the “no residual” recommendation in #4 below), such as simazine, a low rate of metribuzin or Canopy, or a Sharpen rate higher than 1 oz.
2. What about all of the crop residue on the ground after harvest – won’t that cause problems? We have not worried about this, and the herbicides seem to work regardless. Most agronomists I have asked have the same impression. On the other hand, it probably wouldn’t hurt to wait a while after harvest to let the residue settle down, and the weeds to poke through. Dense crop residue usually prevents marestail from emerging anyway. Continue reading