Cooperative Extension: Cranberries

• There is high concern about using Callisto® repeatedly year after year, as weed resistance occurred in some other crops after just 7 years of using it. Growers are being encouraged to rotate with herbicides from other classes of chemistry, and not to use Callisto® each year if at all possible.
• Cranberries are highly tolerant of the active ingredient in Callisto® (mesotrione) – Cranberry is highly tolerant because it is capable of rapidly metabolizing the mesotrione, but some growers out west have reported ‘some’ cranberry blossom damage as a result of the particular surfactant that might be used in conjunction with it, if used during bloom.
• 88% of the applied herbicide is absorbed within just 3 hours, and the dry-time (rainfast time) is 4 hours.
• It has pre and post-emergent activity on a wide range of weed species.
• Callisto® is absorbed by roots, stems and leaves, so it is translocated throughout the plant. Be careful not to use too much water when applying it, so that it does not soak into the ground to a depth deeper than the root zone of the weeds you are targeting.

Activity: Callisto® is a bleaching type herbicide, comprised of the active ingredient, mesotrione, which is a naturally-derived compound produced by a plant called Callistemon.  Mesotrione inhibits the building of a yellow plant carotenoid (pigment) which functions as a sun-screen to protect a plant’s chlorophyll — so without that sun-screen protection, susceptible plants treated with Callisto® will turn white or yellow. Callisto® has both pre-emergent and post-emergent weed control activity. When applied before germination of annual weeds the seedlings take up the herbicide from the soil resulting in death.  When applied post-emergent the herbicide is taken up through the foliage. Thus, irrigation immediately following a post-emergent application is not recommended. In bearing beds, Callisto® can be applied after the bud-break stage but before the fruit set stage. In non-bearing beds, Callisto® can be applied after the bud-break stage but not less than 45 days before flooding in fall or winter, according to the rules established for the Wisconsin label.  Now that the full label is available, some of these time and rate rules could be different, so that label will need to be examined thoroughly before using the product.

Weeds controlled: Callisto® is effective against a large array of weeds, including: smartweed (ladysthumb), carpetweed, ragweed, pigweed, birdsfoot trefoil, violet, marsh St. johnswort, buttercup, and many others.  Also, on the Section 18 label for Washington State, the “cranberry application directions” paragraph includes rushes, sedges and yellow loosestrife as some of its target weeds, suggesting that the material must have some noteworthy activity against those plants as well. Yellow loosestrife is one that has been particularly problematic on cranberry beds in Maine in recent years, and for which there has been a shortage of materials to use against it. According to Dr. Kim Patten, at Washington State University Extension, Callisto® will not kill yellow loosestrife outright, but will weaken it and stunt it, so perhaps after continued annual use of Callisto® for two or three years, the loosestrife could be eliminated.

Rates: As of 2018 (always check the label), the maximum application rate for Callisto® is 8 fl oz/acre/application with no more than two applications per season. Thus, the maximum annual application is 16 fl oz/acre. Split applications are to be at least 14 days apart.  As a spot treatment, use it in 20-30 gallons of water, as follows:  Mix 0.8 tsp in 1 gallon water for 4 oz/30-gallon-water rate, 1.9 tsp in 1 gallon of water for the 8 oz/30-gallon-water rate, or for a very concentrated solution for targeting woody weeds such as poison ivy, use 5 Tbsp of Callisto® per gallon up to a maximum of 5.3 gallons of solution applied per acre per application.  Add nonionic surfactant (0.25% v:v or 1.9 tsp per gallon) or COC (1% v:v or 2.5 Tbsp/gal) with all postemergence applications, regardless of Callisto® rate.

Application: Since Callisto® is a very active herbicide it is critical that application be made with a carefully calibrated sprayer. Also, because Callisto® is expensive, many of the applications are going to be spot applications with a backpack or hand-held sprayer. These sprayers must also be calibrated. Calibration of hand sprayers is determined by the walking speed of the applicator and the discharge rate from the nozzle along with the concentration of the material in the tank. Use great care in spot-treating. The difference between an 8-oz application and an 80-oz application is only seconds on the trigger. Spot treatments made to runoff will exceed the maximum application rate.

Worker Protection Standards: These are not listed on the various state Section 18 Exemptions (Wisconsin’s and Washington’s, for example). That is one reason why it is critically important to read the package label. Callisto® has a 12-hour re-entry interval and a 45-day pre-harvest interval. Applicators and handlers must wear at least:

• Long-sleeved shirt and long pants.
• Shoes plus socks.
• Chemical resistant gloves (Category A).

Pesticide interactions: Wisconsin’s Section 18 exemption did not mention tank-mixing Callisto® with any other pesticides. In addition, the recommendation from Syngenta was that Callisto® not be applied within 7 days (either before or after) of an organophosphate or carbamate insecticide such as Lorsban, because, in corn, insecticide interactions have been seen. This recommendation is expected to be removed from the label soon, however, because this was tested in cranberries in Massachusetts with no evidence of any harmful interactions.

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Summary: Research plots in Wisconsin (and in Washington State) showed Callisto® to be a safe and effective herbicide for control of weeds in cranberry plantings. In those test trials, researchers did not observe a reduction of cranberry yield from Callisto® application. However, as with any new pesticide, it would be advisable to test this product in a small area first, given that research plots cannot account for all climatic conditions, management practices, cranberry varieties and bed age, and other variables that differ greatly among beds.

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Disclaimer: Pesticide registration status is subject to change and varies from state to state; therefore the author and the University of Maine Cooperative Extension cannot assume liability for any pesticide recommendations. It is the responsibility of the pesticide applicator to verify the registration status of any pesticide before applying it. The label is the law! Always read and follow the label when applying pesticides. Use of product names does not imply endorsement.

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