Fifty-seven cultivars of zinnia (Zinnia elegans Jacq.) were studied for 17 weeks to determine their resistance to alternaria blight (Alternaria zinniae Pape), powdery mildew (Erysiphe cichoracearum DC ex Merat) and bacterial leaf & flower spot [Xanthomonas campestris pv. zinniae (syn. X. nigromaculans f. sp. zinniae Hopkins & Dowson)]. A disease severity scale was used to determine acceptability for landscape use. At week 4, all cultivars were acceptable. At week 10, eleven cultivars were acceptable. At week 17, all cultivars were unacceptable. Ten cultivars had been killed by one or more pathogens by week 17. Only two cultivars showed any tolerance to any disease (powdery mildew) at week 17.
Linda Gombert, Mark Windham, and Susan Hamilton
Thomas H. Boyle and Robert L. Wick
True-breeding lines of Zinnia marylandica Spooner, Stimart & Boyle [allotetraploids 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 (BC1) families 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 cichoracearum DC. ex Merat). All BC1 families exhibited high levels of resistance to alternaria blight and powdery mildew. BC1 families derived from crossing Z. marylandica with autotetraploid Z. angustifolia were highly resistant to bacterial leaf and flower spot, whereas BC1 families derived from crossing Z. marylandica with autotetraploid Z. violacea were susceptible to this disease. Our results suggest that one Z. angustifolia genome in BC1 allotetraploids is sufficient to confer resistance to A. zinniae and E. cichoracearum, but at least two Z. angustifolia genomes are required in BC1 allotetraploids to provide resistance to X. campestris pv. zinniae.
Thomas H. Boyle
Allotetraploid Z. angustifolia × Z. elegans hybrids (2 n =46) were reciprocally backcrossed to Z. angustifolia (2 n = 22 or 44) and Z. elegans (2 n = 24 or 48). Pollen germination and pollen tube penetration of the stigmatic surface were observed for all 8 cross combinations. At 14 days after pollination, the percentage of florets with embryos ranged from 0 to 69%, and some hybrid embryos exhibited developmental abnormalities. Seed-propagated backcross (BC1) populations were generated with Z. angustifolia (2 n =44)as ♀ or ♂, and Z. elegans (2 n =48) as ♀ BC1 progeny from these 3 crosses demonstrated low to high levels of resistance to bacterial leaf and flower spot (incited by Xanthomonas campestris pv. zinniae) and high levels of resistance to powdery mildew (incited by Erysiphe cichoracearum). BC1 hybrids derived from crossing allotetraploid hybrids as ♀ and Z. elegans (2 n =48) lines have commercial potential as disease-resistant, flowering annuals.