Inheritance of partial resistance to black stem (Phoma macdonaldii) in sunflower

Two programmes to investigate the inheritance of resistance to black stem i n sunflowers were undertaken in a controlled-environment growth chamber. In the first, an experiment was performed using a randomized complete block d esign with 24 lines, six male-sterile (A lines), six maintainers (B lines), six restorers (R lines) and their six F-1 hybrids in six replications. Eac h treatment consisted of 12 seedlings. Twelve-day-old seedlings were inocul ated with a suspension of pycniospores, and 7 days later the two cotyledon petioles of each seedling were scored on a 1-9 scale for the percentage of necrotic area. Some alloplasmic lines (which have the same nucleus and diff erent cytoplasms) gave a significant cytoplasmic effect for improved partia l resistance to the disease. Deviation of F-1 hybrids from the mean of the parent values was significant for partial resistance to Phoma macdonaldii i n three of the six F-1 hybrids. Two further experiments with eight lines (r esistant and susceptible) at the fifth leaf-pair and flowering stages were carried out under the same conditions. At both growth stages the previous c lassification of lines at the seedling stage was confirmed. In the second p rogramme, five male-sterile sunflower lines were crossed with five fertilit y-restorers in a factorial mating design. The 10 inbred lines and their 25 F-1 hybrids were studied in two successive experiments under the same condi tions and with the same experimental design and isolate of Phoma as in the seedling-stage experiment in the first programme. Analysis of variance show ed that male-sterile and restorer lines possessed general combining abiliti es, and also that specific combining abilities of F-1 hybrids were signific ant. The estimates of general combining ability for partial resistance were significant in AS617A, AS618A and AS614R inbred lines. These lines are ava ilable for developing F-1 hybrids with improved resistance to Phoma in sunf lower-breeding programmes.