Tree and vine growers are fortunate to have a fairly well stocked arsenal of fungicides and insecticides, but they are not quite so fortunate in the weed control department. “There just aren't that many new herbicides on the horizon,” says Kurt Hembree, Fresno County University of California Cooperative Extension farm advisor.

Additionally, factors such as herbicide resistance, groundwater protection rules and herbicide costs are making it difficult for growers to establish a good economical weed control program and stick to it. The widespread use of glyphosate in particular is leading to increasing resistance problems in certain weeds, and the problem is expected to worsen unless growers bite the bullet and adopt aggressive resistance management programs. Unfortunately, a good resistance management program does not come cheap — at least in the short-term.

“The use of post-emergence herbicides in trees and vines has increased greatly over the past 10 years as opposed to pre-emergence materials,” Hembree says. “About 80 percent of that is directly related to the increased use of glyphosate.”

As a result, glyphosate resistant weeds have become not only a reality in some fields, but also a formidable threat on the horizon in many other fields. A grower who is not currently experiencing glyphosate resistance problems could find himself in a totally different ball park in the very near future — particularly if he does not practice resistance management before there is resistance.

“We have at least 17 biotypes in California with documented resistance now to various herbicides,” Hembree says. “In reality, I think we actually have more than that.”

Horseweed, also known as mare's tail, was first identified in California as being resistant to glyphosate in 2005 near Parlier. The resistance was found in weeds where only glyphosate was permitted due to groundwater contamination concerns. It was in a Groundwater Protection Area (GWPA). identified by the California Department of Pesticide Regulation (DPR) where the use of simazine, diuron, norflurazon and other compounds is not permitted. There are now 2.4 million acres in GWPAs.

New challenges

These regulations have ironically created a new set of challenges for growers who began turning increasingly to glyphosate for weed control where little else could be used

Since 2005, resistant horseweed has been identified elsewhere. Researchers also suspect hairy fleabane has developed resistance to glyphosate. These two weed species produce large numbers of seeds that are easily dispersed by wind. A few escape weeds can result in a much wider infestation of new, resistant weeds in a very short period of time.

Horseweed and hairy fleabane have become major weed pests for tree and vine growers because of this issue, particularly in the southern San Joaquin Valley. In addition to infesting fields, they are also common along roadsides, irrigation ditches and other areas adjacent to cultivated fields. That proximity to commercial agriculture has contributed to the rapid spread of both weed species. Researchers also attribute the increased infestation to several other factors including reduced weed control inputs in vineyards and incorrect timing of post-emergence herbicides.

The focus has now switched to utilizing the available herbicides more effectively and heading off resistance before it becomes unmanageable. One post-emergence option growers have to combat resistance is Rely a new herbicide from Bayer CropScience. Rely's active ingredient is glufosinate as opposed to glyphosate.

“My first experience with the compound was in 1994 on cheeseweed and stinging nettle,” Hembree says. “It was very impressive. It's a relatively expensive herbicide compared to the alternatives, but I think it has a fit in weed management programs. Rely is a good rotational option to help minimize resistance. It gives excellent burn down of tough-to-control weeds such as horseweed, hairy fleabane, nettle, malva, henbit and purslane. It also has reduced risk for crop injury if the application is off-target for some reason. It also gives excellent sucker control.”

The key to using Rely most effectively is timing and coverage, according to Hembree. Both horseweed and hairy fleabane are much easier to control when the weeds are in the small rosette stage with less than 10 leaves.

“You want to apply the product when the weeds are very small,” he says. “You also want to make sure that you get thorough coverage over the entire plant. Don't scrimp on the rate to try to cut costs even if you're dealing with small weeds.”

Nozzle importance

Nozzle selection is also an important consideration, according to Hembree. One option he has been evaluating is a boom-less flat spray nozzle. “I think it might be a good choice with Rely to improve control,” he says.

Unfortunately, there are no easy inexpensive solutions for tree and vine growers when it comes to effective, long-term weed control. Resistance is a reality or at least a very real threat. Dismissing it for a quick, cheap fix is quite likely to be a long-term economic disaster.