Roundup (and generics) herbicide is now more widely used throughout Idaho and other potato-producing areas than ever before. Roundup's active ingredient is glyphosate, which is now sold under several other names. Glyphosate was patented in 1971 or 1972 and the first commercial use was about 1974. Thirty-one years ago, when Lloyd started research with glyphosate on potatoes at low (simulated drift) rates, glyphosate was used on a relatively limited basis. His first commercial experience with glyphosate on potatoes was talking with a grower in Madison County, Idaho, where Roundup had been sprayed by air to a grain stubble field adjacent to his potato field in the early 1980s. There was some drift to his potato field.
Glyphosate-containing products are now used for weed control in several Roundup Ready crops, such as corn, alfalfa and sugarbeets in Idaho and other states. It is also used extensively for weed control under various cropping (non-Roundup Ready crops) and non-crop systems.
Recently, we have had cases of glyphosate remaining in a spray tank after glyphosate application to another crop and then spraying potatoes with a fungicide, insecticide or other chemicals. Potatoes were injured. Potato seed quality has been reduced in certain instances where potato fields have had "drifted" glyphosate from application to adjacent fields during the year potatoes were grown for seed.
Idaho Research
My glyphosate injury in potato research started in 1982. Potato injury from glyphosate was documented in drift studies and subsequent potato seed grow-outs in 1983 and later. Rates used in simulated drift studies were about 1/5th to 100th a normal use rate (of 32 fl oz/A of a 4 lb ai/gal product). Timing of application of drift rates of glyphosate to potatoes has included prior to row closure, mid-season and late season (vine kill). Potato yield and quality (including tuber shapes and skin aberrations such as ulcer/elephant hide) occurred. Potato seed quality was also reduced as seen in emergence and yield the following year.
Each of the herbicides used in the studies were non-target applications (not for weed control in potatoes). The herbicides were commonly used in grain or non-crop or fallow areas. Studies started in 1982 have continued to the present, and the work we have conducted has been with the Russet Burbank variety. Recently, Drs. Pam Hutchinson (University of Idaho), Rick Boydston (Washington State University) and Joel Felix (Oregon State University) are now also conducting research with glyphosate on potatoes. They have included other varieties in some of the studies. More work needs to be done with other varieties because there is some recent information indicating some differences between varieties.
Symptoms
Leaf symptoms are not immediately observed at low rates, and that is true with other herbicides as well. Often times, the maximum foliar expression occurred about two to three weeks after the drift application. I have seen that phenomenon many times in grower fields, with glyphosate, 2,4-D, dicamba and other herbicides.
The first symptoms from glyphosate injury from a drift application are a yellowing or chlorosis of the newest leaves. Other foliar symptoms include a wilted and leaf cupping appearance. Such symptoms could be confused with some diseases or nutritional deficiencies.
Tuber symptoms have included white, grub-caused channels on the tuber surface, roughened skin surface (called "elephant hide") and tuber cracking. Usually, the cracking is on the bud end. Vine killing application timing at relatively high rates of glyphosate can cause tuber rot/break down during storage. Typically there is tuber yield reduction depending on rate and timing of glyphosate application to the growing crop. (See Table 1.)
Potato seed quality can be reduced with glyphosate, which includes a slower emergence of the plant, witch brooming (multiple stems-up to 10-15 stems from a tuber) and reduced yield. (See Table 1.) Usually, the seed piece remains firm during the early summer/growing season.
All growers and applicators should be aware that potatoes are sensitive to glyphosate and no application should be made within a mile or more of the nearest potato field without extreme care. Spray tanks and booms should be cleaned thoroughly before spraying a potato field after having sprayed glyphosate-containing materials on other fields. PG