The Vegetable Growers News

IPM coordinator discusses methods to fight tough strawberry pests

By Karen Gentry Associate Editor

Annemiek Schilder, Michigan State Uni- versity small fruit pathologist, takes part in a strawberry field day in Comstock Park, Mich.

Joe Kovach, IPM coordinator for Ohio State University, spoke at two strawberry field days hosted by Michigan State University (MSU) Extension on May 23 and 24. During the field day at Krupp Farms in Comstock Park, Mich., Kovach spoke on strawberry pests including the strawberry bud weevil and the tarnished plant bug and using bees to deliver a biological agent to control gray mold of strawberries.

Also taking part in the field day in Comstock Park were Annemiek Schilder, MSU botany and plant pathology; Rufus Isaacs, MSU small fruit entomologist; Bob Tritten, Phil Schwallier and Amy Irish-Brown, MSU Extension agents; and Paul Krupp from Krupp Farms.

Bee delivery of trichoderma

Bee delivery of trichoderma harzianum to control gray mold of strawberries could reduce fungicide inputs, according to Kovach. “The best time to control botrytis fruit rot is to spray one or two applications at bloom,” he said. Trichoderma is an antagonist to other soil fungus and also has some effect against botrytis, he said. In trials at four sites, researchers found bees can deliver enough of the trichoderma to the flower without any harm to the bees. The experiments were done a couple of years at the Experiment Station in Geneva, N.Y. while Kovach was at Cornell, and at 10 growers’ farms.

“It seems bees are able to carry it and deliver it right to the spot,” Kovach said. This method of delivery seems to increase yields by 20% without any additional fertilizer use.

In these tests of whether bees can be used to control gray mold in strawberries, four different treatments at each demonstration site were replicated six times. The four treatments included 1) a grower standard spray program (usually several applications of Ronilan, Rovral or Captan/Benlate); 2) honeybee delivered trichoderma; 3) a trichoderma spray and; 4) an untreated check.

In growers’ fields, the tests showed in 1997 that the grower standard spray program and the bee delivered trichoderma had the lowest percentage of botrytis. The yields in 1997 also showed the bee visited and grower standard spray and the bee plus trichoderma had the highest yields and the highest weight per berry.

Trichoderma is not registered yet for use on strawberries and may be several years away. “I’ve had a lot of interest from strawberry growers and beekeepers,” said Kovach.

Tarnished plant bug

The tarnished plant bug attacks 300 different plants and is the number one pest of fruit for strawberries, according to Kovach. He said that the tarnished plant bug overwinters as an adult. “The nymph actually causes the damage on strawberries,” he said. Growers need to watch for the tarnished plant bug at blossom. He said the bugs, easily confused with aphids, are really quick. He advised growers to keep grass mowed to reduce overwintering. Controlling weeds also reduces tarnished plant bug pressures.

Kovach told growers that the tarnished plant bug flies fairly low and it may be possible to trap them out. He said there are fewer plant bugs on strawberry varieties such as Honeoye.

Strawberry clipper

Research has shown that the strawberry bud weevil (clipper) is no longer the significant pest that it was once thought to be, according to Kovach. He said the strawberry clipper has an edge effect and growers may just need to apply a border spray.

“They move eight-10 meters per year. We looked at a number of cultivars. They don’t seem to prefer clipping primary buds,” Kovach said. The strawberry clipper, which clips strawberries before they open, overwinters as adults and lays eggs in the clipped strawberry buds.

Kovach recommends that growers sample their borders for the clipper using a 3:30 threshold; if three primary buds are clipped or 30 secondary or tertiary buds are clipped, then the field is over the threshold and sprays are needed.

Some strawberry varieties are more compensating than others. “Plant your most compensating varieties on border edges,” Kovach said.

New option for gray mold

Schilder distributed information at the field day on the fungicide Elevate 50WDG which is now registered for control of botrytis bunch rot in grapes and gray mold in strawberries. Elevate is considered a reduced-risk fungicide because of its low mammalian toxicity and is sold as a 50% water dispersable granule.

Elevate 50WDG’s active ingredient is fenhexamid, which has a different mode of action from Rovral, Ronilan and Vangard, according to Schilder. Elevate can be alternated with these chemicals for fungicide resistance management.

“Since fenhexamide is a protectant fungicide, thorough coverage of all flowers and fruiting parts is essential for control of blossom and fruit infections caused by botrytis cinera,” she reported. Elevate cannot be applied through any type of irrigation system and must be applied using ground equipment only.

Elevate’s recommended rate is 1.5 pounds of product per acre to control gray mold of strawberries. Applications should begin at early bloom prior to disease establishment. Schilder reported that applications should be continued every seven to 14 days or when conditions favor disease development. She said to avoid making more than two consecutive applications of Elevate, after the second application, use an alternative (Captan + Benlate appears to be the only option here) for two consecutive applications before reapplying Elevate.

Schilder said a maximum of six pounds of product may be applied per acre per season. Elevate will be tested in Michigan this year for control of gray mold in strawberries. Indications from other states show that Elevate provides excellent control of gray mold in strawberries, she said.