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New tomato disease control tool ready

"We’re trying to reduce pesticide usage wherever we can and find alternatives to traditional chemistries to meet strict customer specifications," explains Roger Scriven of The Morning Star Packing Co. in Stockton, Calif.

"California feeds the United States and the world, so processors in California are very cautious about the products we produce," Scriven continues. Residue standards are mandated by law, but individual processors often go beyond those, for example, extending the preharvest interval of some materials."

In short, customers are calling the shots.

Scriven, who has direct contact with growers as a buyer, advises growers on chemical use restrictions. This also entails keeping growers informed of new technologies.

Processing tomatoes are among the very first spring planted crops, and growers are concerned about getting a good spring plant establishment and avoiding early-season insects and disease. However, the two most popular processing tomato varieties -- 3155 and 8892 -- have no disease resistance. "There are new varieties on the horizon that have some disease resistance built in," Scriven observes. "There is also some new chemistry that helps the plant protect itself."

This includes harpin, a new biopesticide that recently received a California label to help manage bacterial speck (Pseudomonas syringae) in tomatoes. Sold under the trade name Messenger, harpin is a protein that stimulates biochemical pathways in the plant that, in turn, trigger the plant’s natural defense mechanism through specific genes.

Fend off diseases

These biochemical responses, including systemic acquired resistance, have been shown to help numerous crop plants fend off a broad spectrum of fungal, bacterial and viral diseases. It also activates the plant’s natural ability to overcome environmental stresses by increasing the rate of photosynthesis and stimulating nutrient uptake.

Unlike other materials, the harpin protein does not act directly on the target pest. Rather, it activates the innate plant genetic systems for resistance and growth. It doesn’t disrupt predatory mites or beneficial insects. It has low use rates (4.5 to 9 ounces per acre). It’s also nontoxic, leaving no detectable residue on treated crops, and it poses no threat to the environment. Fields treated with Messenger can be re-entered in four hours, and there is no preharvest interval.

The new California label includes grapes and fruiting vegetables such as tomatoes, peppers and eggplant. Messenger had already been registered in California on strawberries.

Messenger is not recommended as a stand-alone product for disease control, but as part of an Integrated Pest Management (IPM) program that integrates fungicides and good management practices.

The idea is to alternate applications of Messenger with standard fungicides to allow growers to save their most effective materials for heaviest disease pressure. This would help reduce the pesticide load and take selection pressure off standard fungicides, avoiding or delaying resistance.

Scriven notes that the last two years have been light disease years in tomatoes. However, there was significant crop loss due to late blight and bacterial speck in 1999, and with early plantings, disease is always a threat if cool, wet weather persists. Preventive treatments with copper (for bacterial speck) are common in early-season transplants and seedlings. But the number of additional fungicide treatments needed depends on weather and type of disease pressure.

Vascular disease

Mark Richter of Richter Brothers Farms in Knights Landing, Calif., says the greatest disease pressure they’ve encountered recently has come from "a soilborne fusarium-type vascular disease." Last year, Richter Brothers cooperated in several field trials with Messenger under an experimental use label because of reports that the harpin protein benefited Florida tomato growers with this same problem.

"But even if the material provides protection against bacterial speck alone, we’d be interested in that," Richter adds. "Something like that is evidently a non-issue in terms of pesticide residues like proven fungicides, which we are trying to get away from. And since Messenger goes on early, the timing may be a factor in terms of keeping plants healthy and preventing disease development."

The first trial at Richter Brothers was conducted in four 5- by 6-foot subplots in treated and untreated blocks. The processing tomato variety was 9663, planted on 60-inch beds. Messenger was applied at a low rate of 4.5 ounces per acre in 25 gallons of water on May 31, June 13 and July 2. (No fungicides were applied.) While there was no significant disease pressure, yield measurements were nevertheless taken on Sept. 4 and showed a significant difference between treated and untreated plots.

Yields, weights

Messenger-treated plots yielded a total 100,369 pounds per acre while untreated plots yielded 84,942 pounds. Weight of red fruit was 84,760 pounds per acre in Messenger plots compared with 69,696 pounds in untreated plots. Weight of green fruit was 5,808 pounds per acre in Messenger plots, 4,719 in the untreated.

The design of the second trial was identical to the first. So were application rates and timing. In this case, yield measurements taken on Sept. 4 showed 122,222 pounds per acre in the Messenger-treated plots compared with 101,399 pounds in untreated plots. Weight of reds was 101,458 pounds per acre in Messenger plots vs. 94,743 pounds in untreated plots. Weight of greens was 17,859 pounds in Messenger plots vs. 7,841 in untreated plots.

Richter says he was surprised by these results in the absence of significant disease pressure. "Certainly, we are going to take a closer look at this. Preliminary trials last year showed us enough (in yield increases) that we’re interested in doing more replicated field trials with Messenger this year.

"There may be other factors going on out there that we don’t know about besides disease control that account for the yield differences," he continues. "In some years, prevention of bacterial speck would account for that increase. But, this year, without that much disease, it appears something else is going on.

"The bottom line is that when you have healthier plants with no disease pressure, they continue blooming and producing fruit," he points out. "That’s probably what we’re seeing, especially in the greens in the one trial. Healthier plants continue to produce and size."

Robin Ross, field development scientist for Eden Bioscience, the manufacturer of Messenger, says that these results are consistent with those from 13 large (10-acre plus) comparison trials conducted last year in Colusa, Fresno, Kern, Merced, Stanislaus, Sutter and Yolo counties. In these trials, growers applied Messenger in addition to their standard fungicide programs.

"In some cases," Ross notes, "there was little disease pressure, so growers actually ended up applying Messenger alone. In other cases, they applied Messenger with, or in rotation with, standard fungicides."

The result, she says, was a significant increase of marketable fruit in treated plots. "On a per-acre basis, the Messenger-treated plots yielded an average tonnage increase of 18 percent over the grower standard," Ross reports. "In three of these trials, in which measurements of soluble solids were also taken, there was an average 6.6 percent gain in soluble solids over the untreated plants."

Ross adds that trials conducted over the last three years in which disease pressure was significant indicate efficacy of Messenger in the reduction of early-season economic damage from bacterial speck.

Cites field trial

She cites a typical replicated field trial conducted last year in Glenn County in which Messenger was applied at a rate of 4 ounces per acre with Kocide at a rate 2 pounds per acre in 20 gallons of water. This was compared with Kocide applied alone at the same rate, and an untreated check.

"In the Glenn Country trial, the average disease incidence in the untreated plots was 30 percent; in the Kocide-treated plots 12.5 percent, and in the Messenger plus Kocide treatments, 6.3 percent," she reports.

Ross concludes that such trials indicate that "using Messenger in a tank-mix or in rotational program with standard fungicides can reduce incidence and severity of disease."

"We must be interested in new technology because the industry demands it," Scriven reiterates. "The bottom line is we have to be interested in chemistry like this because of pressure on the industry to reduce pesticide usage."

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