Nov 15, 2018
Modes of action, product solubility key in fall weed control

A multi-year plan of control is the starting point for fall herbicide applications in tree fruit. The weed control plan should include tank mixing herbicides with multiple modes of action, rotating modes of action, evaluating herbicide water solubility, matching rates to soil type, and, of course, knowing your weeds.

“Know your weeds and have a three-year plan,” said Bernard Zandstra, Michigan State University (MSU) Extension specialist and professor in the Department of Horticulture. A well thought-out, written plan is the best way to stay on track with a complicated herbicide program.

Fall herbicide applications generally outperform the same treatments applied in the spring. Herbicides degrade slowly over winter in the cold soil and are moved into the soil by rain. This puts them in position to control germinating seedlings in the early spring.

A fall weed control program should always be a tank mix of at least two residual herbicide active ingredients with different modes of action to prevent herbicide resistance. The fall application also should include a foliar-active herbicide to kill emerged weeds. “Make sure you’re always using a combination of products with active ingredients with at least two modes of action,” Zandstra said. The first treatment next spring should include at least two products with different modes of action than were used in the fall.

Modes of action are identified using different systems. The Weed Science Society of America (WSSA) uses a number system. The Herbicide Resistance Action Committee (HRAC) uses an alpha-numeric designation.

For example, the WSSA classifies glyphosate as a Group 9 herbicide while the HRAC classifies it as a Group G. The WSSA is a non-profit, professional society that promotes research, education and outreach related to weeds. The HRAC was founded by the agrichemical industry to support efforts for controlling herbicide-resistant weeds.

A herbicide assessment should also consider the product’s water solubility. “Solubility is always an issue,” Zandstra said, “because it relates to movement of the herbicide in the soil. The higher the solubility in water, the more rapidly the herbicide may move into the tree root zone. The primary safety factor for most tree fruit residual herbicides is that the tree roots never come into contact with the herbicides.”

A product’s solubility in water is measured in parts per million (ppm).

The lower the ppm, the less water soluble the product is and the less likely it is to leach.

Prowl (pendimethalin), with a water-soluble of 0.3 ppm, is one of the least water-soluble products on the market. Some other products have water solubilities in the hundreds or thousands of ppm.

“You want something with less than 10 or 20 ppm water solubility,”

Zandstra said. “Most of the residual herbicides in fruit crops have relatively low solubilities.” Herbicides with water solubility above 200 ppm should be used only once in two years on coarse soils, and once per year on medium and fine-textured soils. Some of the other herbicides with low water solubilities include Goal (oxyfluorfen) with a water solubility of 0.1 ppm, Princep (simazine) with a water solubility of two and Surflan (oryzalin) with a water solubility of three.

Some herbicides have soil type restrictions. As examples, Sinbar’s (terbacil) label says it cannot be used on light, sandy soils. Surflan’s (oryzalin) label says to use a lower rate on lighter soil and Solicam’s (norflurazon) label says to use lower rates on sandy soils. The Alion (indaziflam) label says to use a higher rate on heavy soil.

Product labels use different terms to describe the timing of fall applications in tree fruit. Some say make applications after dormancy, some say in the fall and some say in the late fall.

Generally, the timing of fall applications in Michigan is November or early December in more southerly areas. Applications are usually made after several frosts have killed most annual weeds and fruit trees have lost most of their leaves.

Problem perennials can also be controlled to some extent with fall applications. “Glyphosate will kill a lot of dandelions and plaintains,” Zandstra said. “Stinger is effective for dandelions, thistles, horseweed and clovers.” Another labeled option is 2,4-D amine. These herbicides will kill perennials, biennials, winter annuals and fall-germinated annuals. Application of these foliar-active herbicides in the fall greatly improves weed control the next season. Herbicide injury can be an issue with young trees. “Growers should use shields or trunk protectors in young trees, especially if the trees have any green bark showing,” Zandstra said.

“If there’s good, brown bark, injury is much less of a concern. Trees under three years old in the orchard are susceptible to herbicide injury. Highly soluble herbicides should not be used on young trees.”

Some herbicides are only labeled for apples, some only for cherries, while others may be labeled for both cherries and apples and other tree fruit.

Some may only be labeled for mature trees. As examples, Sinbar’s (terbacil) label says it can only be used on apple trees three years old or older and Starane Ultra’s (fluoxpyr) says it can only be used on apple trees four years old or older. Alion’s (indaziflam) label says it can only be used on trees established for more than three years while Zeus Prime’s (sulfentrazone + carfentrazone) label says to use it only on trees established for at least three years.

Product labels can vary by state, especially if they’re a 24(c) Special Local Need Label. A good place for the latest information on labels is Crop Data Management Systems, at www.cdms.net.

“The label is always the starting point,” Zandstra said. “Control is a matter of the rate you use, the timing, the soil and your weeds. As we move into summer applications, the next issue that comes up is the pre-harvest interval. There are a lot of issues that go into making a herbicide control plan. But remember, it’s just a plan. It can be changed as issues arise.”

– Dean Peterson, FGN Correspondent 

Above: A well thought-out, written plan is the best way to stay on track with a complicated herbicide program in tree fruit, said Bernard Zandstra, Michigan State University Extension. Photo: Dean Peterson

All references to specific product labels in this story were taken from MSU Extension’s Michigan Fruit Management Guide for 2018, Extension Bulletin E154.





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