The genetics and physiology of a dicamba resistance trait in kochia (Kochia scoparia L. Schrad.)

Abstract

Kochia is a problem in cultivated fields, waste lands and gardens throughout the western United States. Herbicides that mimic auxin are important for the control of kochia. In 1993, kochia was collected that did not respond as expected to dicamba an auxinic herbicide. In 1995, Montana and North Dakota reported auxinic herbicide resistant populations. From 1994 to 1998, 250 kochia samples from Colorado, Kansas, Montana, and Nebraska were tested for dicamba tolerance. Beginning in 1999, studies were conducted to investigate the genetics and physiology of the dicamba resistance trait identified in these populations and to evaluate resistance to fluroxypyr another auxinic herbicide used for kochia control. Four kochia accessions with different dicamba tolerance levels were evaluated under irrigated field conditions without competition for cross resistance to fluroxypyr. Five rates of dicamba and fluroxypyr were applied to the four kochia accessions. Dicamba control of dicamba resistant accessions was reduced. Fluroxypyr controlled all dicamba susceptible and resistant accessions tested. Auxinic herbicides have been shown to induce ethylene production in susceptible plants. The ethylene production of dicamba susceptible and resistant kochia when treated with dicamba and fluroxypyr was measured as a way to identify a resistant kochia plant and determine if it was cross resistant to fluroxypyr. The ethylene produced was measured over time and herbicide rate. Ethylene inhibitors and tobacco mutants were used to determine ethylene's contribution to observed herbicide symptoms of auxinic herbicides. Ethylene production by dicamba resistant kochia was lower over time and in lower amounts as dicamba rate increased than susceptible kochia. Resistant and susceptible kochia responded similarly to fluroxypyr treatments. The ethylene inhibitor AOA did not reduce dicamba or fluroxypyr symptoms. Senescence was delayed in ethylene insensitive tobacco when treated with dicamba but not fluroxypyr. The inheritance of the dicamba resistant trait was studied. Resistant and susceptible inbred kochia lines were crossed and segregation patterns of eight F2 families evaluated by treating them with 280 g ha-1 dicamba. Fourteen days after treatment intermediate phenotypes were identified but resistant, intermediate, and susceptible plants did not segregate in a 1:2:1 ratio. The dicamba resistance trait in kochia is not due to a single nuclear gene and does not consistently fit simple modified ratios for a two gene system. At least two genes interact to produce the dicamba resistance phenotype observed.