The Schutter Diagnostic Lab at Montana State University has confirmed symptoms of herbicide damage in samples sent from gardens in 17 counties across the state, said MSU Pesticide Education Specialist Cecil Tharp.
Missoula County alone had 38 cases, Tharp said. Ravalli County had 10, and Gallatin County had six. Other counties with suspected damage were Big Horn, Broadwater, Choteau, Flathead, Garfield, Lake, Lincoln, Madison-Jefferson, Musselshell, Phillips, Pondera, Ravalli, Stillwater and Sweet Grass. Missoula County samples tested positive for aminopyralid exposure through contamination of composted material. This active ingredient, along with other pyridine chemicals, can be a threat to broadleaf vegetables if incorporated into compost or mulch.
Aminopyralid (product name: Milestone) controls broadleaf weeds along roadsides and in grass pastures, which are often used for grazing livestock or hay production. Some examples of other pyridine products are Tordon 22K "picloram" and Curtail M "clopyralid." Tharp emphasized that this represents a subset of pesticide products available. Discrimination or endorsement is not intended with the listing of commercial products by MSU Extension. Due to labels and registrations which are constantly changing, applicators must always read and follow the product label. MSU Extension cannot assume liability for the suggested use of chemicals.
Signs that garden plants may be exhibiting pyridine toxicity are leaf cupping or curling, and stunted growth, Tharp said. Tomatoes and potatoes are highly susceptible to aminopyralid, but lettuce, spinach, sugar beets, carrots, beans, peas and other broadleaf vegetables are susceptible, too.
One theory about how aminopyralid can get into manure and compost is that applicators/producers don't allow their livestock to move from herbicide-treated pastures to feed on an untreated pasture for three consecutive days prior to using manure. As a result, contaminated manure may end up in composts and natural fertilizers that are later used in home gardens.
Emphasizing the need for private and commercial applicators to read their pesticide product label before applying pesticides -- even if they've applied pesticide products for many years -- Tharp explained that all applicators should be aware of their pesticides' composting and recropping restrictions. Tharp said he is uncertain how long any pyridine chemical would remain in gardens currently contaminated. He indicated that there are reports of pyridine chemical persisting between two and four years, depending on factors like temperature, compaction and soil type.
In the meantime, he said gardeners may want to move their gardens.
To see if it's safe to resume gardening in the original spot, Tharp suggested that gardeners conduct an experiment with 10 pots. They should fill five pots with uncontaminated soil and five with soil randomly collected from the garden. Then they should plant intended garden vegetables in each pot and let the plants grow until they have at least three leaves. The appearance of the plants will indicate whether or not the soil is restored.
Anyone who suspects that his or her garden has been exposed to contaminated manure or compost can send samples to the Schutter Diagnostic Lab at MSU, Tharp said. For more information, see the Montana Pesticide Bulletin article on "Residual Herbicide Can Damage Sensitive Garden Plants" (page two).
Evelyn Boswell, (406) 994-5135 or firstname.lastname@example.org