Researchers test alternative ways to protect crops

Methyl bromide was the go-to fumigant for strawberry growers until January 2001, when a phase-out period was initiated. This is the last year that even a critical use exemption will be available for methyl bromide, according to Gerald Holmes, director of the Cal Poly Strawberry Center in San Luis Obispo.

Research for alternatives to methyl bromide has been going on ever since the phase-out was announced, and Holmes is working on finding disease-resistant strawberry cultivars.

Holmes is evaluating germplasm for its ability to resist disease, and there are excellent sources of resistance to most of the soil-borne pathogens, he said.

"What we're doing that's unique is that ours is a naturally infested soil, and we're doing it under field conditions. So it's kind of an acid test," he said.

Holmes said the research is starting with the three main diseases:

• Verticillium wilt
• Macrophomina crown rot
• Fusarium wilt.

These diseases are very common, and they are becoming much more important as the fumigants are phased out, Holmes said.

"Last year, we looked at verticillium wilt, and looked at four different varieties. Two were quite resistant and two were quite susceptible," Holmes said, adding this year he will be looking at about 90 different genotypes for resistance.

Before the phase-out of methyl bromide, macrophomina was not considered an important disease, but now it has become a worldwide problem, Holmes said.

With the fusarium wilt, Holmes is finding some genotypes are very susceptible and others are very resistant.

"We don't expect to find quite as wide a variance in the macrophomina, just because of the way that pathogen attacks strawberries," he said.

"But in the case of verticillium and fusarium, there are really big differences between resistant and susceptible," Holmes said.

"We have a really unique situation with macrophomina because it's a heat-loving pathogen, and we're a little warmer than they are in Santa Maria and Monterey. So, we're kind of a worst-case scenario, and that's what you want for your screening. You want a worst-case scenario," he said.

Holmes is in the second year of the research, and it is ongoing.

"I see us doing it for the next decade at least, and then we'll see, because every year there's new material to be evaluated and breeding is a continuous process. They're always releasing new varieties and bringing a new germplasm along that needs to be evaluated, so I see us doing it on a continuous basis," Holmes said.

Mark Bolda, a University of California Cooperative Extension strawberry and caneberry farm advisor for Santa Cruz County, is working with Steve Fennimore from UC Davis, the lead on a steam project, and Joji Muramoto from UC Santa Cruz, the lead on an anaerobic soil disinfestation project. Both projects are looking at alternatives for methyl bromide for strawberries.

ASD is simply flooding the soil for extended periods of time with a type of carbon. Rice bran is being used as the carbon source, but the researchers also tried alcohol and onion residues, Bolda said.

"What happens is that the microbial activity in the soil gets a boost, and it consumes all the oxygen. You create an anaerobic condition. It's worked; not always, but it's worked," Bolda said, adding it releases "tremendous amounts" of nitrogen.

ASD does use water, and during a drought, that can be an issue, he said.

Another research project is applying steam to the soil instead of a chemical fumigant. This is also very promising technology, and steam has been used for more than 100 years, Bolda said.

"We all know that steam is going to kill pathogens, but it is also releasing certain levels of nitrogen beyond what one gets from methyl bromide or something else," he said.

As far as control, the steam is unquestionable, Bolda said.

"The technology continues to be modified. There are some issues with it, such as how to transmit the steam into the soil and things like that," Bolda said. "We also have an up-and-coming material called Dominus. It's derived from Allyl isothiocynate. It's really more or less a biological fungicide. It acts differently in the soil than your conventional fumigants. For example, it doesn't travel around that much in the soil."

Good results have been found by fumigating with something like Dominus or chloropicrin, and after the fumigation, incorporating an amendment like rice bran or mustard seed meal into the soil, Bolda said.

The amendments would be applied at high rates. By doing this, it recreates the ecology of the soil, he said.

"You've eliminated a lot of stuff through the fumigation, but now you're putting in the amendment so that soil may be recolonized with things that are more beneficial," Bolda said.

"There are different populations of different things following these fumigations, and especially when you put in that mustard seed meal, you're changing it that much more. You're making it a richer population, a more diverse population. We're seeing some good things in there with the use of the mustard seed meal," he said.

Another important aspect is creating an integrated pest management program, Bolda said.

"We haven't done that. We've just gone in with fumigants, cleaned it up and then we're done—we're good to go. This is different. This is IPM—understanding what's going on in the soil, and then taking the steps to address those issues," Bolda said.

"If we're going to make a system of integrated pest management of soil pathogens, we need to know what's going on down there," he said. "And we don't. We just know, 'Hey, there's a lot of fusarium, the plants are sick.' Well, why is that fusarium flourishing in that soil and not in another one? What's the difference?"

Other important aspects of an IPM program would be crop rotation and cleaning equipment before moving to another field, he said.

"We're at zero hour now with methyl bromide alternatives," Bolda said. "This is a very critical time for the industry, and we are doing a lot. We're not at the end point yet where we've got it all figured out, but we have good researchers working on it."

(Kathy Coatney is a reporter in Bend, Ore. She may be contacted at kacoatney@gmail.com.)