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30 POTATO GROWER | FEBRUARY 2022 DIGGIN' IN FUMIGANTS | By Jim Crants & Carl Rosen In North American potato agriculture, chemical fumigants such as metam sodium and chloropicrin are often used to control soil-borne diseases. However, this approach to disease suppression may be a double-edged sword. On the one hand, fumigation is highly effective at controlling soil- borne diseases like common scab and potato early dying, at least for one growing season. With fewer soil-borne pathogens, potato plants may develop healthier, more robust root systems, making them better at capturing soil nutrients and decreasing nitrogen input requirements. Alternatively, soil-borne disease suppression may increase nitrogen input requirements by increasing the maximum yield the crop is capable of achieving. On the other hand, soil fumigants have effects on the soil microbial community beyond their effects on pathogens. For example, fumigants have been found to reduce rates of soil nitrogen cycling and microbial respiration. They may also decrease populations of some disease- suppressing microbes, increasing the degree to which the grower must rely on fumigation to control pathogens in the future. In short, chemical soil fumigation may have a negative effect on soil health. Our Approach to This Question We used the word "may" a lot in the last two paragraphs. To date, there has been very little research on whether fumigation changes the optimal nitrogen rate in any crop. It is also unknown how long it takes for the microbial community to recover from a fumigation treatment, or if it ever does. We set out to address these questions in a field study on Russet Burbank potatoes conducted in 2016 and 2017. In each year, we had study plots fumigated in the fall before potatoes were planted with metam sodium or chloropicrin. A non-fumigated control was also included. In the following year, each plot was divided into five subplots, each receiving nitrogen at a different rate. All subplots received 40 pounds of nitrogen per acre as DAP (18-46-0) at planting, and each received either 0, 80, 140, 200, or 260 pounds per acre as ESN (44-0-0) at emergence, for total nitrogen rates of 40, 120, 180, 240, or 300 pounds per acre. We measured crop responses to these fumigation and nitrogen rate treatments in terms of tuber set, yield, size and quality, symptoms of potato early dying, the amount of nitrogen taken up per pound applied, and the increase in yield per pound of nitrogen applied, among other things. We measured soil microbial responses in terms of Verticillium propagule density, respiration rates, and soil nitrate and ammonium concentrations. Fumigation Suppresses Disease, Improves Yield As expected, fumigants successfully served their primary purpose of controlling soil-borne pathogens. They both decreased populations of viable Verticillium dahliae propagules in the soil and the severity of foliar symptoms of potato early dying. Chloropicrin also decreased the prevalence of common scab in tubers. Using these fumigants to control soil-borne diseases had positive effects on the crop beyond disease control. Total and marketable yield and tuber set were higher in plots treated with either fumigant than they were in the non-fumigated control plots at all of the nitrogen rates tested. This effect on yield did not meaningfully change the agronomic optimum nitrogen rate at which yield was maximized. The amount of nitrogen the crop took up per acre and per pound of nitrogen applied were both higher in plots fumigated with chloropicrin or The pros and cons of soil fumigation in potatoes Double-Edged Sword Marketable yield in each fumigation treatment at each nitrogen rate tested. Lines are best- fit curves, and vertical arrows indicate the point on each curve where yield is maximized. The horizontal, double-headed arrows indicate the error around that point. There is a 90% chance that the true location of the yield peak is within the range of nitrogen rates indicated by the double-headed arrow for each peak. 0 100 200 300 400 500 600 0 50 100 150 200 250 300 350 400 Marketable yield (CWT·ac -1 ) Total N applied (lbs·ac -1 ) No fumigant Chloropicrin Metam sodium c b bc d bc b a a a a a b a a a Microbial respiration rates over 24 hours in each fumigation treatment. "NA" indicates that researchers did not test for an effect of the fumigation treatments before we had applied them. "NS" indicates that fumigation treatment had no effect on microbial activity.