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.