Blackspot disease, caused by the fungus Diplocarpon rosae Wolf, is the most serious disease problem of roses worldwide. From a breeding perspective, the documented occurrence of races in the pathosystem necessitates the characterization of the race structure for the pathogen, identification of resistance genes in rose germplasm, and determination of the nature of genetic resistance to breed stably resistant rose cultivars. The objective of this study was to characterize the diversity of eastern North American races of D. rosae and to simultaneously determine the degree to which partial resistances existed in rose hosts. Twelve rose genotypes were inoculated with 14 single-conidial isolates collected from eastern North America. Three distinct D. rosae races were identified based on presence/absence of infection in the differential array. There were five distinct reaction patterns across pathogen isolates among the 12 rose genotypes. The partial resistance components of leaf area with symptoms, lesion length and incubation period were evaluated for all inoculated plants. Variation for expression of partial resistance was noted, and partial resistance appears to behave in a race-specific manner for some rose genotypes. Early defoliation differed between genotypes and may prove to be another component of partial resistance.

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Ethylene-releasing agents (ethephon, ethanedial dioxime, etacelasil, 2-chloroethyl-methylbis(phenylmethoxy)-silane, and 1-aminocyclopropane-1-carboxylic acid) applied to dwarf mistletoe (Arceuthobiumpusillum Pk.) infections on black spruce (Piceamariana (Mill.) B.S.P.) in four separate tests (August 1982, 1983, and 1984 and May 1983) promoted premature abscission of mistletoe shoots. Concentrations of the chemicals ranged from 12 to 5000 μg/g. Surfactants (Ortho X-77 and PGFI-100) were evaluated for increasing the effectiveness of the ethylene-releasing agents. The treatment combination that consistently resulted in 74–100% abscission of dwarf mistletoe shoots was August applications of 2500 μg/g ethephon plus either surfactant. Treatments did not significantly affect growth of black spruce or subsequent growth of dwarf mistletoe shoots. Abscission of dwarf mistletoe shoots prevents spread of the disease in black spruce for 2 years and for at least 4 years in some western conifers.

Additional index words. Rosa hybrida, plant breeding, rose black spot disease, disease resistance, shrub rose Roses are popular landscape plants valued for their beautiful and diverse floral displays that can be enjoyed throughout the majority of the growing season. Most of the landscape rose cultivars available today are not reliably winter hardy in U.S. Department of Agriculture (USDA) Plant Hardiness Zones 3 and 4 (USDA Plant Hardiness Zone Map, 2012). Susceptibility to rose black spot (Diplocarpon rosae F.A. Wolf) and other foliar diseases such as spot anthracnose [Sphaceloma rosarum (Pass.) Jenk.] is also a limiting factor. Foliar diseases defoliate plants and reduce the ornamental value, vigor, and winter survival of susceptible cultivars. A breeding program was initiated at the University of Minnesota in 1990 to develop coldhardy, repeat blooming, black spotresistant landscape rose cultivars available for use in northern continental landscapes (Zuzek and Hokanson, 2007). Below we describe four polyantha rose cultivars developed by the Woody Landscape Plant Breeding and Genetics Project at the University of Minnesota.