Good Fruit Grower / April-15th 2011 / Tree fruit ­herbicides

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Tree fruit ­herbicides

A bandolier of bullets, but none silver.

As fruit growers move to ever less vigorous trees planted closer and closer together, the need to control competing vegetation becomes more and more important. In a long-term study by Dr. Ian Merwin at Cornell University in New York, modern apple trees grown without suppressing ­vegetation did not become fully competitive with grass cover until they were 17 years old.

Nowadays, growers need vegetation-free conditions for most of the life of their orchards—and probably ­supplemental irrigation even with weedfree strips.

Luckily, over the 50 years that growers have been shrinking the vigor of their trees and increasing their density, more and more herbicides have been developed to control competing vegetation.

Growers would love to find a silver bullet, of course—some one product, inexpensive, that kills everything but is gentle on trees and has a long residual effect so weeds don’t come back soon. Sometimes, they come close—as with the herbicide Roundup (glyphosate)—but issues generally arise, like resistant weeds, or, as is currently suspected, greater damage to trees than was earlier thought (see “Tree safety is key issue with herbicides”).

While no bullets are silver, growers have a bandolier full of bullets, and it’s up to them to figure out which ones to shoot with and how many they need to shoot.

Dr. Bernard Zandstra, the horticultural weed control authority in fruits and vegetables at Michigan State University, lists 27 different active herbicide ingredients in the 2011 version of the Michigan Fruit Management Guide—and three more are newly labeled or may be labeled yet this year.

So, the challenge for growers is to choose among these tools, combine products where needed, and put together a weed control program that fits their situation, dealing with the weeds that cause them problems—and not ­creating new problems—while keeping costs under ­control.

“In preparing the weed guide, I assume the grower has some knowledge of the basics,” Zandstra said. “If they’re totally green, they should get advice from their extension agent or other professional advisor. There are a lot of ­factors that enter into choosing which products to use.”

For many orchardists, the weed control program they choose remains pretty much in place from year to year, and new products help them overcome problems that emerge. Weed resistance is a potential problem when herbicides with the same mode of action are used over a long period of time. 

Pre and post

“In my opinion, growers should use both pre- and postemergence herbicides each season,” Zandstra said. Moreover, he says, it would be useful to develop more than one weed control regimen and alternate between them.

Growers should apply a good residual herbicide in the fall or early spring, then follow after May 1 with postemergence applications as needed. Once trees are past their active growth stage, weed control is less important from a competition aspect, he said, but there are other considerations as well. “Big, tall weeds later in the season make it difficult to operate,” he said. “Young trees need clean ­culture all year.”
Young trees cannot tolerate many of the herbicides that can be used later when the trees have developed deeper roots and thicker bark.
Zandstra runs weed control trials at three Michigan State University experiment station locations and also on growers’ farms. He works with chemical companies to test new products, and with the IR-4 project to provide data to support labels for specialty crops.

In his testing, his plots are seldom weedfree because he is evaluating how well various herbicides, alone or in combination, control weeds—and those that escape have a field day. In the weed control guide he prepared for 2011, one large table rates the effectiveness of 26 ­herbicides for the control of 32 weeds—832 separate entries.

The Michigan Fruit Management Guide costs $25 and is not available online as a downloadable file. Other management guides, such as the 2011 Crop Protection Guide for Tree Fruits in Washington and the 2011 Midwest Tree Fruit Spray Guide, are available on the Internet and can be downloaded as pdf files. Neither of these two have the weed-by-weed efficacy data that Zandstra provides in the Michigan guide.

Zandstra’s weed control recommendations, including the table on herbicide efficacy, can be found at his Web site www.msu.edu/~zandstra by clicking on “Weed Control in Fruit Crops” and then “2009 Weed Control on Fruit Crops Excerpt.”

Programs

For apples and pears in the year of planting, growers have the following preemergence materials to choose from: Surflan (oryzalin), Prowl H2O (pendimethalin), Kerb (pronamide), Solicam (norflurazon), Gallery ­(isoxaben), Casoron (dichlobenil), and Sinbar (terbacil). Then for postemergence, they have Gramoxone (paraquat), Roundup and Touchdown (glyphosate), and for grass control only, Fusilade (fluazifop-P) and Poast (sethoxydim).

Once trees are established, the choices are broader. For preemergence, they can use Princep (simazine), Karmex (diuron), Solicam, Surflan, Goal (oxyfluorfen), Kerb, ­Sinbar, Casoron, Chateau (flumioxazin), Prowl H2O, and Matrix (rimsulfuron). Matrix and Chateau are the newest in this group.

For postemergence grasses, they can use Poast. For annual and perennial weeds, they can use Gramoxone, Roundup, Touchdown, Weedar (2,4-D), Rely (glufosinate), Aim (carfentrazone), Starane (fluorxypyr), Treevix (saflufenacil), Venue (pyraflufen-ethyl), and Sandea (halosulfuron). In this group, Treevix and Sandea are new or have new labels for tree fruit.

Most of these materials can also be used on stone fruits. One herbicide that is cleared for use on stone fruits but not on apples and pears is Stinger (clopyralid).

Applied postemergence, it controls some hard-to-­control weeds—composite weeds like sunflowers and dandelions, clover, nightshade, plantain, and smartweeds.In general, stone fruits tend to be more sensitive and need greater protection when Roundup or Touchdown is used.