Eastern Black Nightshade Cross-Resistance to Pursuit, Raptor, and Beacon in Wisconsin

Dean Volenberg*, Dave Stoltenberg, and Chris Boerboom

Eastern black nightshade is a very troublesome weed for soybean and pea producers. The weed can cause substantial yield reductions, but harvest problems and reduced crop quality are probably of greater importance, even at low levels of infestation. The contents of eastern black nightshade berries are released during harvest and cause seed, soil, and other foreign material to stick together, making harvest operations difficult. Mature berries often stain soybean during harvest, reducing grain quality. In addition, immature berries of eastern black nightshade are similar in size, shape, and color of pea making separation between species difficult. During processing the immature berries turn black and reduce pea quality. Pursuit and Raptor are herbicides commonly used throughout the Midwest for management of eastern black nightshade in soybean, and Pursuit is used in pea. Both herbicides belong to the imidazolinone family and inhibit acetolactate synthase (ALS).

In 1999, for the first time, the number of weed species resistant to ALS-inhibiting herbicides exceeded that of any other herbicide class. Currently, 63 weed species have evolved resistance to at least one of the four commercialized classes of ALS-inhibiting herbicides. Within the north central region of the United States, 16 weed species are reported to be resistant to ALS-inhibiting herbicides. For more information on herbicide resistant weeds, visit the international survey of herbicide-resistant weeds web site at http://www.weedscience.com.

In August 1999, a grower in south central Wisconsin, reported lack of eastern black nightshade control in two soybean fields in which Pursuit DG had been applied postemergence at 1.44 oz / acre and a second time at 0.72 oz / acre. In 1995 and 1997, the fields had been planted in soybean and Pursuit was applied preemergence at 4 fl. oz / acre plus pendimethalin. ALS-inhibiting herbicides were not applied in 1996 or 1998. Pursuit use before 1995 could not be confirmed.

Greenhouse and laboratory experiments confirmed and quantified resistance of eastern black nightshade. Whole-plant experiments with three- to four-leaf stage eastern black nightshade indicated that resistant plants were 150-, 120-, and 5.9-fold resistant to Pursuit, Raptor, and Beacon, respectively, compared to the susceptible plants. ALS enzyme assays determined that the enzyme was 170- and less than 20-fold resistant to Pursuit and Beacon, respectively, and indicated that resistance was associated with an insensitive ALS enzyme.

These results indicated that eastern black nightshade was highly resistant to Pursuit and Raptor, but was much less resistance to Beacon. This differential response of resistant eastern black nightshade to three ALS-inhibiting herbicides can be explained by the chemical classes in which these herbicides are classified. Pursuit and Raptor are imidazolinone herbicides, whereas Beacon is a sulfonylurea herbicide. The selection of eastern black nightshade cross-resistance to these herbicides occurred in a field with a history of only three known annual applications of Pursuit. Reports of eastern black nightshade resistance to Pursuit or Raptor have also been received from two locations in west central Wisconsin. The occurrence of eastern black nightshade resistance has several important implications for management, especially because this weed affects both crop yield and quality. Integrated weed management practices that include alternative herbicides, cultural, and mechanical methods need to be implemented to manage these resistant eastern black nightshade populations, and to delay development of additional resistance problems.

*  Graduate Research Assistant