257-Weed Resistance Prevention Practices for Wild Blueberries

Fact Sheet No. 257

Prepared by David E. Yarborough, Extension Blueberry Specialist and Jennifer D’Appollonio, Assistant Scientist. The University of Maine, Orono, ME 04469. Revised January 2018.

How weed resistance occurs

Figure 1. Fineleaf sheep fescue (Festuca filiformis), an introduced perennial grass in Maine wild blueberry fields was resistant to Velpar (group 5), Sinbar (group 5) and Poast (group 1) herbicides and was taking over blueberry fields. — **Figure 1**. Fine-leaf sheep fescue (*Festuca filiformis*), an introduced perennial grass in Maine wild blueberry fields, was resistant to Velpar (group 5), Sinbar (group 5) and Poast (group 1) herbicides and was taking over blueberry fields.

Herbicides do not cause resistance within a plant through mutations. It is the repeated exposure of plants to the same herbicide site of action that will select for the very small proportion of individuals that already are genetically resistant within the population. These genetically different plants survive the herbicide application whereas other genetically susceptible plants die. These surviving plants then produce seed and over time this proportion of resistant plants increases among the overall population. Continued repeated exposure to herbicides with the same plant killing mode of action further selects for increased levels of resistance among the survivor population and the cycle continues. This selection pressure results in a resistant plant population with the inherited ability to survive an application of the herbicide used (and others within the same herbicide mode of action group) to which previous or other populations of the same species had been susceptible. These populations will usually be susceptible to other herbicides with a different mode of action unless they have been subjected to continued combinations of two groups and then could develop multiple resistances.

Why resistant weeds are a concern

Unlike planted crops, wild blueberries are perennial and so we can’t rotate different crop types to suppress weeds. We do, however, have a two-year cropping cycle that disrupts weeds and allows us a “fallow” (prune cycle) year to apply herbicides. Because of this, weed resistance has been slower to occur in our crop.

Wild oat grass (Danthonia spicata) was the first weed described to be resistant to hexazinone in wild blueberry fields in Canada and fine-leaf sheep fescue (Festuca filiformis) is now resistant to hexazinone, terbacil and sethoxydim in Maine, so we know that weed resistance does occur in wild blueberry fields. If weeds are not controlled, then yields have been shown to decrease from 5,000 pounds per acre on weed-free fields to 1,000 pounds per acre on fields with 100% dogbane (Apocynum androsaemifolium) or bracken fern (Pteridium aquilinum) weed cover. Weeds also hinder harvest and crush and split berries, greatly reducing their value.

How to prevent herbicide-resistant weeds

First, it is important to scout fields in order to detect weeds not controlled and eliminate small populations of suspected resistant weeds. This is also effective in preventing new weeds from becoming established in your fields.

Practice Integrated Crop Management (ICM), which uses a combination of cultural and chemical strategies to take advantage of the tools available to control weeds, and which results in a more effective control program.

Cultural practices

Include the application of sulfur to reduce soil pH to 4.0; this reduces the availability of nutrients for weeds and reduces their growth.
Cut seed heads of annual and perennial weeds when in flower and before they go to seed to prevent future infestations.
Mulch between clones will suppress weed seeds and will encourage the spread of wild blueberries, which will, in turn, suppress weeds and increase yields.
Sanitize equipment with a pressure washer to remove weed seeds before moving between fields to prevent the spread of resistant seed or plant parts.
If planting pollinator plantings, only select non-weedy seed for planting.
Sample blueberry leaves at the tip die backstage to determine nutrient requirements of blueberry to minimize fertilizer inputs, and only add fertilizer when necessary as fertilizer greatly increases weed growth.

Chemical practices

First, determine if there were other causes of herbicide failure to control weeds present.
Application Rate: check calculations and confirm gallons/acre to see if the intended rate is correct.
Application Method: need to have even distribution and good coverage (smaller spray particles) for foliar treatments; boom sprayer gives the best coverage but an air blast sprayer is not suitable as it gives an uneven distribution.
Calibration: confirm that your output matches your intended rate when applying your first tank load.
The timing of application: if applied too soon, soil herbicides such as hexazinone and terbacil can be washed from the weed root zone and not be available for uptake by the weeds; too late will defoliate blueberry leaves which will delay growth and create openings for weeds.
Environmental conditions: excessive rain with herbicides that leach readily, such as hexazinone and terbacil, and with the foliar application if rain occurs before the spray is absorbed will greatly reduce the effectiveness of control. But other herbicides such as pronamide, carfentrazone or sulfentrazone require rainfall to be activated.
Determine the best herbicide(s) for target weeds based on scouting fields and past weed populations. Whenever possible, combine or rotate herbicides with a different mode of action both within and between years (see Table 1). Combinations have been effective; combining hexazinone + rimsulfuron + diuron will give broad-spectrum control with multiple sites of action to prevent weed resistance.
Targeting weeds post-emergence with directed applications of herbicides such as mesotrione will control weeds such as dogbane more effectively than a pre-emergence application at a higher rate. See 2017 24(C) State local use label for details.

Table 1. Wild blueberry herbicides by group number, application timing and weeds controlled.
	Active Ingredient	Product	Mode of Action	Application Timing	Target Weeds
Group Number
1	Clethodim	Select, Arrow, Shadow	ACCase inhibitors (lipid synthesis)	Postemergence	Selective grass only
	Fluazifop-P Butyl	Fusilade	ACCase inhibitors (lipid synthesis)	Postemergence	Selective grass only
	Sethoxydim	Poast	ACCase inhibitors (lipid synthesis)	Postemergence	Selective grass only
Group Number
2	Rimsulfuron	Matrix, Solida	ALS inhibitors (amino acid inhibitor)	Pre/early Postemergence	grasses, sedges, broadleaf
	Tribenuron methyl	Express	ALS inhibitors (amino acid inhibitor)	Fall after harvest	bunchberry only
	Halosulfuron-methyl	Sandea	ALS inhibitors (amino acid inhibitor)	Fall or early spring	grasses, sedges, broadleaf
Group Number
3	Pronamide	Kerb	Seedling root growth inhibitor	Fall after harvest	resistant grasses
Group Number
5	Hexazinone	Velossa, Velpar	Photosystem II inhibitor	Preemergence	herbaceous/woody broadleaf, grasses
	Terbacil	Sinbar	Photosystem II inhibitor	Preemergence	grasses, sedges
Group Number
7	Diuron	Direx, Karmex	Photosystem II (different site)	Preemergence	grasses, herbaceous broadleaf
Group Number
9	Glyphosate	Roundup, many others	EPSP synthase inhibitors	Postemergence	non-selective herbaceous broadleaf
Group Number
10	Glufosinate	Rely, Glufosinate	Glutamine synthase inhibitors	Postemergence	non-selective herbaceous broadleaf, grasses
Group Number
14	Carfentrazone & Sulfentrazone	Zeus Prime	PPO inhibitor (cell membrane inhibitor)	Preemergence	grasses, sedges & herbaceous broadleaf
	Flumioxazin	Chateau	PPO inhibitor (cell membrane inhibitor)	Fall after harvest	mosses, marestail, herbaceous broadleaf
Group Number
18	Asulam	Asulox	Dihydropteroate synthestase inhibitors	Selective Postemergence	bracken fern only
Group Number
27	Mesotrione	Callisto	Carotenoid biosynthesis inhibitors	Preemergence/ selective Postemergence	grasses, sedges, rushes & herbaceous broadleaf

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