Evaluation of commercial Upland (Gossypium hirsutum) and Pima (G. barbadense) cotton cultivars, advanced breeding lines and glandless cotton for resistance to Alternaria leaf spot (Alternaria alternata) under field conditions

Alternaria leaf spot (ALS, caused by Alternaria spp.) is one of the most common foliar diseases in cotton (Gossypium spp.) that occurs in most cotton-growing regions of the world including the United States. In New Mexico, ALS caused by A. alternata (Nees:Fr.) Keissler is becoming prevalent due to favorable weather conditions in the late cropping season; however, there is limited information on screening cotton for ALS resistance. In this study, a total of 125 Upland cotton (G. hirsutum L.) genotypes were separated into 5 trials each with 32 entries and 3–4 replications, together with one trial including 8 Pima cotton (G. barbadense L.) cultivars and lines with 4 replications in 2016. Three additional field trials each with 32 or 34 genotypes and 4 replications were conducted in 2017. Based on the high disease incidence (99.9% with a range of 90–100% in 2016 and 100% in 2017) and high disease severity index (DSI, 26.7–92.0 with a mean of 62.4 in 2016 and 42.0–89.0 with a mean of 66.7 in 2017) of foliar symptoms in the late growing season in both years, no cotton genotype was immune to ALS (A. alternata) under the natural infection field conditions. The analysis of variance showed that there were significant genotypic variations in six of the nine replicated trials, and the broad-sense heritability estimates for DSI ranged from 0.489 to 0.702 with an average of 0.566. No significant genotypic difference was detected among Pima cotton cultivars and lines in both years. An orthogonal contrast detected no overall significant difference between Upland and Pima cotton in both years, while 15 Upland glandless lines as a group, had a significantly lower DSI than the glanded group with 17 Upland lines in one replicated test. Glandless NuMex COT 15 GLS, NM 13P1117 and NM 12Y1002, and glanded commercial transgenic FM 2484 B2F, PHY 444 WRF and NG 4545 B2XF were consistently resistant to ALS in two or more tests. The results provide useful information for breeding cotton for ALS resistance.