Cranberry operation worker Brian Allen spot spraying weeds

Mar 21, 2022
Cranberry weed impact studied by researchers

Precision weed management offers cranberry growers the potential to increase yields and reduce input costs by using decision-making tools to decide when herbicide applications are warranted, said Jed Colquhoun, professor and integrated pest management director in the Department of Horticulture at the University of Wisconsin-Madison.

Speaking at the 2022 Wisconsin State Cranberry Growers Association annual winter meeting, Colquhoun shared research from studies he and his colleagues in Massachusetts and New Jersey are conducting to develop weed thresholds for some of the major weed species impacting cranberry yields.

Colquhoun and his collaborators are creating a methodology to document weed impacts on both cranberry yield and quality. The long-term goal is to establish what he called a “weed impact library” with specific weed thresholds that growers can use in making weed management decisions.

“Some weeds simply may be aesthetically unpleasant but may have very little impact on berry yield or quality. Other weeds may have adverse impacts beyond yield, such serving as host plants for insect pests that affect berry quality.

With that in mind a decision support system can help us prioritize which weeds we go after on a marsh,” Colquhoun said.

In most crops, it is rare for a single weed species to dominate a cropping system. Cranberries are an exception however, oftentimes having only a few weed species growing in cranberry beds.

“I would say in the two dozen or so horticultural crops that I work in that cranberries have the situation most commonly where there are patches of single weed species versus the crop,” Colquhoun said.

One example is bristly dewberry, a low-growing perennial shrub commonly found on Wisconsin cranberry marshes. If present dewberry tends to form dense colonies in the corners of cranberry beds.

Data collected over the past two seasons showed that 20% dewberry groundcover caused a loss in cranberry yields of 4,143 pounds (41.43 barrels) per acre. The shading of vines by dewberry also resulted in an 18% loss in fruit color.

Besides dewberry, the weeds that Colquhoun and his counterparts in Massachusetts and New Jersey are focused on at present include slender-leaved goldenrod, seedling maple trees, yellow loosestrife, Carolina redroot and haircap moss.

The researchers collected data on 40 random quadrants per weed species, including some plots without weeds present and some with very high weed densities.

Colquhoun and others took weed counts, weight of both fresh and dried biomass and made visual estimates of weed ground cover within each quadrant. Data was also gathered on cranberry fruit quality including yield, berry color, percent of fruit rot, amount of insect damage and in some cases total anthocyanins and brix readings.

Wisconsin cranberry grower Jack Potter
Wisconsin cranberry grower Jack Potter uses pliers to try to pull a seedling maple tree. Photos: Lorry Erickson

In addition to testing their methodology across a spectrum of weed species, the collaborators are looking to see if the relationship between weed growth and cranberry yields are consistent from season to season.

The data on dewberry in Wisconsin from the past two growing seasons showed that all of the weed growth parameters measured were good predictors, but that visual ground cover estimates alone provided an easy – and very accurate – prediction of cranberry yield.

“We don’t have to weigh anything. You don’t even have to count plants. You can just throw down a quadrant and estimate the percent cover and we can use that to predict yield loss compared to where there is no dewberry. It’s a very simple approach to at least get an estimate of what you’re dealing with,” Colquhoun said.

Studies in New Jersey on Carolina redroot found insect-damaged fruit had a strong relationship with weed density. Colquhoun suspects Carolina redroot might be a host of some of the insect pests that damage cranberries.

“This offers us another reason why we need new tools to control problematic species like Carolina redroot,” he said.

Preliminary results from studies in Massachusetts on whether haircap moss has an impact on yield were less consistent, Colquhoun said.

“We’re collecting data because this hadn’t been done extensively before. As a matter of fact in the literature there is only a single paper on weed impacts on cranberries conducted by (University of Washington Extension Professor Emeritus) Kim Patton and his group in 1994,” Colquhoun said.

Collaborating on the research with Colquhoun is Hilary Sandler, director of the University of Massachusetts Cranberry Station, East Wareham, Massachusetts; Katie Ghantous, Integrated Pest Management weed scientist at the UMass Cranberry Station and Thierry Besancon, Extension weed scientist for specialty crops at the Philip E. Marucci Center for Blueberry and Cranberry Research and Extension in Chatsworth, New Jersey.

In addition to helping develop weed thresholds, Colquhoun is also running trials on new herbicides being tested for use on cranberries.

“As is the case with most of our pesticide development, the newer herbicides that are coming along are targeted, narrower spectrum herbicides. We currently have four herbicides in the registration process and they really have very targeted weed control spectrums,” Colquhoun said.

Two of the herbicides being tested are very safe when used on dormant cranberry vines and give persistent weed control. But if the applications were off by just a few days, vines showed significant damage that persisted through harvest, Colquhoun said.

Colquhoun also discussed the rapid advances being made in robotics for weed control, particularly in Europe. While costly now, automated weed control systems may become more feasible in the future.

Noting that cranberry beds in Wisconsin tend to be uniform laser- leveled rectangular sections, the use of robotics may be feasible one day, Colquhoun said.

“If there is any production system where I can imagine this working, it’s cranberries because we are already so precise. This just adds another layer and mechanizes something that right now requires significant labor input,” he said.

More than 250 registrants from 10 states and four Canadian provinces participated in this year’s Wisconsin Cranberry School. Organizers had planned to hold an in-person event this year, but due to a surge in COVID-19 cases when conference plans were being finalized, the decision was made to change to a virtual event.

– Lorry Erickson, FGN correspondent


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