Evaluation of sorghum germplasm used in US breeding programmes for sources of sugary disease resistance

Abstract

Ergot or sugary disease of sorghum has become an important constraint in North and South American countries that rely on F1 hybrid seeds for high productivity. The objective of this research was to determine the vulnerability of various germplasm sources and publicly bred sorghum lines to sugary disease (Claviceps africana) in the United States. Flower characteristics associated with sugary disease resistance were also studied. A-/B-line pairs, R-lines, putative sources of resistance and their hybrid combinations with an A3 cytoplasmic male-sterile source were evaluated using a disease incidence, severity, and dual-ranking system. Trials were planted in a randomized complete block design with three replications and repeated in at least two planting dates. Planting dates and pedigrees had significant effects on overall ranking for resistance. A-lines were most susceptible to sugary disease. R-lines were more susceptible than B-lines with respect to incidence and severity of the disease. Newer releases of A- and B-lines were more susceptible to sugary disease than older releases. Sugary disease reaction of A-lines was a good indicator of disease reaction of B-lines. Tx2737, a popular R-line, was highly susceptible to sugary disease in spite of being a good pollen shedder because the stigma emerged from glumes 2-3 days before anthesis. The combination of flower characteristics associated with resistance were least exposure time of stigma to inoculum before pollination, rapid stigma drying after pollination, and small stigma. An Ethiopian male-fertile germplasm accession, IS 8525, had good levels of resistance. Its A3 male-sterile hybrid had the highest level of resistance in the male-sterile background, IS 8525 should be exploited in host-plant resistance strategies.