Glyphosate residues affect granddaughter seed potato growth in commercial potato fields Submitted to MN Area II and NPPGA

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1 Glyphosate residues affect granddaughter seed potato growth in commercial potato fields Submitted to MN Area II and NPPGA Andrew P. Robinson Department of Plant Sciences North Dakota State University and University of Minnesota Executive Summary Seed potato plants exposed to glyphosate can translocate the herbicide to the daughter tubers causing emergence problems the following year. Symptoms of glyphosate carryover in clones include erratic and slow emergence, malformed leaves, multiple shoots from a single eye, and/or enlarged shoots. The purpose of this study was to determine the effect of glyphosate residues in seed pieces on yield. Ten plant-to-plant comparisons were made to compare a normally growing plant to a glyphosate-affected plant. Seed pieces with glyphosate residues there were delayed in emergence by 3 weeks had a 67% reduction in yield, a 50% reduction in tuber number, and a 38% reduction in tuber weight. Introduction Seed potato (Solanum tuberosum) plants exposed to low levels of glyphosate during the growing season can store the glyphosate in the daughter tubers resulting in delayed emergence when they are planted the next growing season (Worthington, 1985). Glyphosate is the one of the most widely used herbicide in the United States because of the rapid adoption of genetically modified crops, low cost, and effective control of weeds. In North Dakota 31% of crop acreage was treated with glyphosate in 2008 (Zollinger, McMullen, et al., 2009). Seed potato fields can unintentionally come into contact with glyphosate by contamination of spraying equipment, inversions, physical drift, or misapplication. The level of glyphosate that comes into contact with potatoes will vary, but often the low levels of glyphosate during bulking do not cause visible foliar symptoms. This can make early detection of glyphosate toxicity in daughter tubers difficult to determine. Because glyphosate is phloem mobile, it will translocate throughout the plant reaching highest levels within four days in the meristematic tissues (Smid and Hiller, 1981). The amount translocated will vary by the amount of glyphosate coming in contact with the potato plant and the temperature, with greater absorption of glyphosate at higher temperatures (Masiunas and Weller, 1988). Symptoms of glyphosate carryover in seed pieces include an erratic and slow emergence; bending, twisting, and yellowing of new leaves; multiple shoots coming from a single eye; candelabra formation of shoots; cauliflower formation of shoots around an eye; enlarged shoots; and reduced rooting (Figure 1 and 2) (Worthington, 1985). Less in known about the effect glyphosate residues in potato seed have on the yield of potatoes planted the following year. Research Objectives 1. The purpose of this study was to compare normally growing plants with plants affected by glyphosate residues in the seed. Materials and Methods

2 Three commercial fields planted with confirmed glyphosate contamination were identified in North Dakota and Minnesota. Glyphosate contamination was suspected based on symptomology in the field and confirmed in samples from two fields sent to a commercial laboratory for analysis using a liquid chromatography with tandem mass spectrometry detection. Levels ranged from to ppm glyphosate. The potato clones from each field were grown in North Dakota in The potato cultivars were Dark Red Norland, Yukon Gold, and Red LaSoda. In each field 10 adjacent plants were flagged to compare a normally growing plant to a glyphosateaffected plant that was delayed in emergence by approximately three weeks (Figure 3). After vine kill potato hills were hand harvested and yield and tuber number were recorded. Data was analyzed using the SAS MIXED procedure with field as the replication. Means were separated using Tukey s pair-wise comparison (P 0.05). Results Yield in plants from seed pieces affected by glyphosate had a 67% (from 2.25 to 0.75 lb/hill) reduction in yield, a 50% reduction in tuber number (10 to 5 tubers/hill), and a mean tuber weight reduction of 38% (3.92 to 2.40 oz/tuber) (Figure 4). Summary and Conclusions The amount of glyphosate in the tubers can determine the degree that the potato tubers are affected, with higher levels preventing or reducing emergence for a greater period of time. However, a low incidence of seed with glyphosate residues may not reduce field production significantly, because the affected plants may have little delay in emergence. Additionally, potato plants affected by glyphosate can be weak and may be easily attacked by pests and diseases, and allow weeds to germinate. It is unknown if glyphosate residues in seed pieces will effect tuber initiation, or if the reduction in tuber number and mass is a result of plant-to-plant competition. Literature Cited Masiunas, J.B. and S.C. Weller Glyphosate activity in potato (Solanum tuberosum) under different temperature regimes and light levels. Weed Sci. 36: Smid, D. and L.K. Hiller Phytotoxicity and translocation of glyphosate in the potato (Solanum tuberosum) prior to tuber initiation. Weed Sci. 29: Worthington, T.R The effect of glyphosate on the viability of seed potato tubers. Potato Res. 28: Zollinger, R., M. McMullen, J. Knodel, J. Gray, D. Jantzi, G. Kimmet, et al Pesticide use and pest management practices in North Dakota NDSU Extension Service W-1446.

3 Figure 1. Effects of glyphosate residues in seed potato on potato foliage, (a) delayed and erratic emergence, (b) yellowing of leaves, (c) bending and twisting of leaves, and (d) swollen stems.

4 Figure 2. Effects of glyphosate residues in seed potatoes when planted the following year (1) multiple sprouting from a single eye, (b) delay in sprouting, (c) candelabra sprouts starting to develop, (d) cauliflower formation of sprouts.

5 Normal plant + glyphosate Normal plant + glyphosate

6 Figure 3. Effect of glyphosate residues in potato seed compared to a normally growing plant at (a) emergence and (b) harvested yield. Figure 4. Effect of glyphosate residues in potato seed on a single potato hill s (a) yield, (b) tuber number, and (c) mean tuber weight. Error bars represent one standard deviation.