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is a correlation between high residual populations and potato damage, but not always,” Wilson says. “That is why the effectiveness of your management program should not be judged at the end of one year, but after three or four years. By then, you’ve gone through the crop rotation.” Wilson recommends soil sampling according to university guidelines. WSU trials will continue in 2010. SOIL SAMPLING Should a grower pull soil samples for nematodes prior to planting? According to Phil Hamm, Oregon State University extension plant pathologist, the answer is unequivocable: “Always, always, always sample for nematodes.” That recommendation is based on 20-plus years of working in potatoes in the Columbia Basin. “Given the value of the investment—it costs at least $2,500 per acre to grow potatoes—growers need to be knowledgeable about what they are planting in to lessen the risk,” he says. “Is it a bazillion nematodes or just one? The only way to find out is with a soil sample.” In many potato-growing areas, including the Columbia Basin, the presence of certain nematodes alone justifies treatment. The results from soil sampling can help guide growers to the most effective treatment for that crop in that field—from fall fumigation to in- season products or a combination of both. Nematode samples should be taken during the late summer or early fall the year prior to potatoes being planted. “Give yourself enough time to get the results back, and make an educated treatment decision,” Hamm says. “For root knot nematode, sampling can happen any time so long as the field is being cared for with the crop still in it. You do not want to sample after wheat harvest when the soil is dry from no water application for a month. Getting an adequate estimate of the population is impossible.” Fall and spring are better times to sample for stubby root nematodes based on where they are in the soil profile. “In the summer, stubby root nematodes tend to move deep and are difficult to find,” he says. How many soil samples? “More sampling is better,” Hamm says. “But you have to be reasonable based on the time and expense it takes to sample. www.potatogrower.com 19 I’d like to see growers split a circle (120 acres) in quarters, then take 20 cores from each area. That would give a good representation of root knot and lesion nematodes. “Stubby root nematodes can be more spotty, and even that many samples could miss a stubby root hotspot. Keep in mind that 120 samples are equivalent to one sample per acre. An acre is still a big place, which is why you often hear the phrase ‘zeroes don’t mean zeroes.’” The spottiness of nematodes within a field is why Hamm cautions against variable-rate application of fumigants. “A grower has too much at risk to use variable-rate fumigation,” he says. “If yields were the only issue with these pests, then a grower might be able to a stand a small decrease in yield. But nematodes and soil-borne fungal pathogens can lead to rejections or substantially reduce grade, respectively. Growers cannot stand that. Besides, there is just not sufficient research data to support this kind of application.” Hamm also encourages growers to maintain a record of nematode issues in a field, and refer to that “field history” when preparing a nematode management plan. PG