State of the state in potato weed management: current threats and future opportunities

Jed Colquhoun, Professor, University of Wisconsin-Madison

Every once in a while it’s worth stepping back and taking a big picture look at where we’re at and what’s coming for Wisconsin potato production, and when it comes to weed management we’ve had success this year in gaining new tools but the near future poses significant challenges. The intent of this article certainly isn’t to cry wolf or paint an overly gloomy picture but simply to provide a realistic gut-check. The short version: we’ll likely be ok for a while except for where herbicide-resistant weeds take over, but in the relatively near future we’re going to need to be prepared to make big changes in our weed management practices.

After years of research, we’ve been reveling in the rare addition of two new herbicide registrations for Wisconsin potato production in the past year: Sonalan and Zidua. These tools will be very useful to control problematic weeds such as nightshade and pigweed species. And it’s likely that we’ll see at least a couple more potato herbicides added to the arsenal in the next few years.

So why the dark clouds on the horizon? There are several reasons but for the sake of driving the situation home let’s focus on the top three:
1) Herbicide resistance among weeds is out of control. Globally, herbicide resistance has now been documented in 259 weed species and with 23 of 26 herbicide sites of action (Heap, 2019, This year University of Illinois colleagues documented waterhemp resistance to the site of action that includes s-metolachlor, the active ingredient in Dual and several other herbicide trade names commonly used in potato and vegetable production. Why is this noteworthy? This is the first time where a broadleaf weed has been found to be resistant to that important herbicide group, and to make matters much worse, waterhemp has now been found to be resistant to seven herbicide sites of action.
2) Weed species that are almost always found with herbicide resistance have spread at an amazing pace across Wisconsin. Waterhemp is the unfortunate poster child for the spread of herbicide resistant weeds. My colleagues in the UW-Madison Agronomy Department have now found waterhemp in 61 of Wisconsin’s 72 counties with glyphosate and other herbicide resistance traits common across those populations.
3) We haven’t seen a new herbicide site of action since 1988 and that won’t change soon. Hence the “recycling” of some of the first commercial herbicides from the 1940’s with 2,4-D and dicamba in herbicide-resistant soybean and other crops to address weed resistance. Even if a new herbicide site of action were to be discovered, it takes at least 10 years and hundreds of millions of dollars to get from the lab bench to a label, and potatoes won’t be first in line to say the least.

Without new herbicides on the way and the potential to lose control with our existing tools, where do we go? Our solutions are going to be vastly different and creative, and hopefully practically integrated into current management programs in smooth transition and without significant economic impact. Our recent work in carrot is a great example, where we were rapidly driven to creative solutions with the restrictions on linuron herbicide use on coarse-texture, low organic matter soils.

We’re focused on strategies that require few if any additional inputs, including grower time, but instead focus on overemphasizing inherent crop traits that improve competitiveness with weeds. In contrast, most integrated weed management work to date has included adding inputs, like cultivation, cover crops, or more herbicides. So, what traits are valuable in our potato and vegetable crops in terms of competitiveness with weeds? We’re looking for:
• Rapid and uniform crop emergence. This not only gives the crop a head start in the race against weeds but also decreases the time needed to get to a point where post-emergent herbicides and cultivation are less injurious. In our current situation crops like carrot and potato emerge slowly and rather inconsistently. We’re changing that equation by adding very low doses of natural plant growth regulators that stimulate growth as either seed treatments or applied to young crop foliage. These plant hormones already occur in all plants – we’re just tweaking them. However, just like my observations of my teenage children, one needs to be cautious about messing with hormones! Recent work in potato by western US colleagues has shown that some of these same plant growth regulators can shift potato tuber set and size distribution. In electricians’ terms, think of it as a three-way switch and not a single pole – flipping one switch affects others – so we’re working to make sure that we don’t cross the plant’s wires.
• Planting timing that not only optimizes yield but also early crop canopy closure. Each crop has a “sweet spot” for temperature and photoperiod that enhances early-season growth. Our recent work with carrot is a great example – shifting the planting timing two weeks later enhanced early carrot emergence and growth so much that very few weeds survived, and yield wasn’t compromised compared to earlier plantings.
• Planting populations and configurations that lead to earlier canopy closure while maintaining or increasing crop yield. If you could increase your marketable crop yield by 10 to 20% without increasing water, fertilizer, pest management, time or other inputs, would you take it? Probably so if there weren’t significant side effects. We’ve been able to do that in crops like carrot by adding two rows to each bed, in essence filling in areas that are still fertilized, water and sprayed to get to a competitive closed canopy earlier. Side effects could include increased risk of foliar diseases with less air movement in the canopy, and most significantly, equipment changes for the seeder and harvester. These will need to be balanced with the need to control herbicide resistant weeds with fewer tools.
• Competitive crop varieties. From the standpoint of added energy and time, it can’t get more efficient than just filling the planter with a more competitive variety with suitable end use characteristics. We’ve evaluated this extensively in potato with varieties dating back to some of the original Russet Burbanks from the late 1800’s to recent introductions. The general trend was that older varieties tended to have faster developing and more complete plant canopies, likely a result of breeding for many years for higher yield at the cost (intentional or unintentional) of above-ground growth. In related work in carrot, breeders are now doing both – selecting for disease-resistant heavy top growth that outcompetes weeds as well as high yield and quality. More recently, this has become a renewed subject of interest of those developing and evaluating potato varieties. Similarly, work is also underway to identify and select traits that naturally resist Colorado potato beetle feeding, such as “sticky” hairs on the potato leaves.

For some, this may seem like quite a shift toward more “natural” or “organic” means of pest management that could be less reliable and consistent as current control strategies. The intent of this work isn’t to abandon what’s working but to add to it in a way that makes the season-long management system more resilient and long-lasting – all of the tactics outlined above can be utilized alongside traditional herbicide programs for conventional growers. And while we’ll certainly face significant challenges with rapidly increasing populations of herbicide-resistant weeds, progressive Wisconsin growers will be able to adopt creative and effective solutions moving forward.