No genetic estimates for resistance to Mal de Rı! o Cuarto (MRC) disease in Zea mays (L.) are currently available in the literature. Therefore, the objectives of this investigation were (i) to estimate the variance and heritability of partial resistance to MRC disease and of other agronomic traits from maize families and (ii) to examine associations among MRC disease severity values across different environments and between MRC and other agronomic traits. These estimations, obtained in an endemic area, could contribute to the design of efficient enhancement programmes and evaluation activity for the improvement of MRC resistance. The research was conducted by testing 227 F $ derived-lines from a cross between a susceptible dent line, Mo17, and a partially resistant flint line, BLS14, for MRC disease at two Rı! o Cuarto locations in each of 2 years. The resistance of the lines, measured with a disease severity index (DSI), was normally distributed across environments. Genotypic variances were highly significant on all scoring environments. Estimates of genotypeenvironment interaction were also significant, suggesting that certain genotypes have little stability over different environments. For disease severity index all estimates demonstrated moderate heritabilities ranging from 0n44 to 0n56 and were similar when based on individual environments or across environment. Confidence interval widths ranged from 34n88 to 50n30 % as large as the heritability point estimate. The correlations between environments were small enough to indicate that families did not rank similarly in individual environments for MRC resistance. Disease severity index correlated significantly (P 0n01) with plant height, leaf surface, leaf border, leaf length and tassel type. Heritability estimates for plant height and tassel type were 0n48 and 0n38 respectively and for the various leaf traits heritability values were very low. On the basis of the substantial genotypeenvironment interaction and the little association between DSI values in the different environments, selection for an increased resistance to MRC disease would require evaluation of germplasm across multiple years and locations. Tassel type would be a useful predictor of DSI and can be used effectively to improve screening procedures.

Cotton (Gossypium hirsutum L.) cultivars resistant to different herbicides have generated important advances in crop management. There is a wide range of cotton resistance to herbicides, however, commercial imidazolinone (IMI)‐tolerant cultivars are not available. In this research, a new mutation that confers tolerance to IMI in cotton was isolated in the M2 generation after seed mutagenic treatments with sodium azide water solutions. The mode of inheritance of the trait was evaluated in the F2 generation by the application of imazethapyr. The inheritance corresponds to a semidominant type with a 1:2:1 distribution of tolerant/intermediate tolerance/not tolerant, in agreement with that observed in most IMI‐tolerant crops. The IMI‐tolerant line behavior was evaluated in the field nursery after the application of the herbicide in the form of foliar and pre‐emergence sprays. The tolerance level was high in pre‐emergence applications and intermediate in foliar applications compared with the nontolerant genotype, which showed a decrease in fiber yield and quality with both methods of application. After sequencing the acetohydroxyacid synthase (AHAS) gene, a point mutation corresponding to Ala205Val in the AHAS enzyme was found, which suggests it could be responsible for the increased IMI tolerance observed in cotton line SP 4172‐32. Based on results from field assays, it can be concluded that this mutant is a promising experimental line that can be used in obtaining IMI‐ tolerant commercial cotton cultivars.