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Occurrence of the White Rust Pathogen, Albugo tragopogonis, in Seed of Sunflower

White rust of sunflower (Helianthus annuus L.), caused by Albugo tragopogonis (Pers.) S. F. Gray, appeared in South African fields not previously planted to sunflower. Spread to these fields from infested fields was unlikely, as some of the newly affected fields were planted out of season and were more than 300 km away from other sunflower production fields. Several reports of this nature led us to investigate the possibility of seed transmission of the causal organism. Extensive colonization of sunflower heads by A. tragopogonis was observed in field trials and breeding nurseries. Head infections consisted of two distinct lesion types. White rust pustules, typical of those formed on abaxial sides of leaves, were recognized on abaxial sides of involucral bracts. Grayish, localized lesions containing dark-colored oospores of the fungus, similar to those formed on stems and petioles (1), were produced in sub-epidermal tissue and extended 3 to 5 mm deep into receptacles. Colonization of seeds was found in only a few lines. Oospores were produced in the pericarps and testae of seeds. No oospores or hyphae, however, were observed in the embryo. This is the first report of A. tragopogonis being seed-borne. Since the incidence of seed infection is low, spread of disease to infested fields is expected to be insignificant. Of more concern, however, is the possible long-range dissemination of the fungus by means of infected seed into regions or countries where the disease has not been previously reported.

Reference: (1) P. S. Van Wyk et al. Helia 18:83, 1995.