Herbicides marketed for selected crops were chosen on the basis that major cultivars of the specific crop were tolerant. Our work with soybeans [Glycine max(L.) Merr.] has demonstrated that variability exists with regard to sensitivity or tolerance to several herbicides. As a part of our breeding program to develop improved soybean cultivars, we have evaluated advanced breeding lines and germplasm for reaction to herbicides on the market as well as experimental herbicides. A breeding line was identified and released for production which showed very little injury from two over-the-top applications of a double rate of 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid]. Other strains showed severe injury and depressed seed yield. Variation in reaction to glyphosate [N-(phosphonomethyl)glycine] has been observed. Lines tolerant to 0.56 kg ai/ha in one growing season were severely damaged the following season. When 200 germplasm lines from eastern Asia were treated with glyphosate, 6% showed less than 15% injury while 21% showed over 80% injury. The cultivar ‘Tracy′, selected for tolerance to 2,4-DB and a high level of resistance to the soil-borne disease causing phytophthora rot, was found to be sensitive to metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one]. Tracy has two major genes controlling resistance to phytophthora rot caused byPhytophthora megaspermaDrechs. f. sp.glycineaKuan and Erwin. One of these genes is closely linked with a gene controlling reaction to metribuzin. A crossover type was identified. The cultivar ‘Tracy-M’ retains the high level of resistance to phytophthora rot and is tolerant to metribuzin. A small percentage of germplasm lines evaluated were found to be sensitive to bentazon. Reaction to bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] is simply inherited.
Protection of potatoes from phytophthora rot
