RESPONSES OF ZINNIA-ANGUSTIFOLIA X Z-VIOLACEA BACKCROSS HYBRIDS TO 3 PATHOGENS

True-breeding lines of Zinnia marylandica Spooner, Stimart & Boyle [al lotetraploids of Z. angustifolia H.B.K. and Z. violacea Cav, (2n = 4x = 46)] were backcrossed with autotetraploid Z. angustifolia (2n = 4x = 44) and Z. violacea (2n = 4x = 48), Seed-generated, backcross (BC,) f amilies were screened for resistance to alternaria blight (Alternaria zinniae Pape), bacterial leaf and flower spot [Xanthomonas campestris pv,zinniae (Hopkins and Dowson) Dye], and powdery mildew (Erysiphe cic horacearum DC. ex Merat), All BC, families exhibited high levels of re sistance to alternaria blight and powdery mildew, BC, families derived from crossing Z. marylandica with autotetraploid Z. angustifolia were highly resistant to bacterial leaf and flower spot, whereas BC, famil ies derived from crossing Z. marylandica with autotetraploid Z. violac ea were susceptible to this disease, Our results suggest that one. ang ustifolia genome in BC, allotetraploids is sufficient to confer resist ance to A. zinniae and E. cichoracearum, but at least two Z, angustifo lia genomes are required in BC, allotetraploids to provide resistance to X. campestris pv. zinniae.