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Ornamental flowering cherry trees are important landscape plants in the United States but are susceptible to several serious pests and disease problems. Cherry leaf spot, incited by the fungus Blumeriella jaapii, is characterized by defoliating susceptible trees in late summer, leading to weakening or even death of the tree. To identify resistant plants for use in landscape plantings and in our breeding program, we used a detached leaf assay to screen 69 diverse ornamental flowering cherry taxa for resistance to cherry leaf spot. We found clear differences in susceptibility among the accessions, with seven accessions developing essentially no symptoms at all. A variance decomposition showed that most of the variance (59%) occurred among accessions, indicating that genotype, even more than species, determined susceptibility. The detached leaf assay used in this study is an effective method for screening large numbers of plants for relative resistance to cherry leaf spot. These methods will be particularly useful to characterize germplasm and screen hybrids in breeding and selection programs.
Two simple and rapid in vitro bioassays using detached stems were developed for evaluating the susceptibility of boxwood genotypes to the blight disease caused by Calonectria pseudonaviculata (Crous et al.) L. Lombard et al. Individual leaves were inoculated on detached stems or entire detached stems were sprayed to assess susceptibility. Both assay systems were optimized for inoculum concentration and disease rating time. The assay methods described here require minimal plant material and inoculum, especially the leaf inoculation assay, which uses as few as six leaves per stem and 500 spores per leaf for inoculation. The stem spray inoculation produced less variable results and was easier for quantifying susceptibility but required more inoculum than the leaf inoculation assay. No differences between the assays were found for the cultivars tested. The leaf inoculation assay is best used when limited plant material or inoculum is available; the spray inoculation of detached stems is suitable when larger plants are available.
Five simplified DNA preparation procedures for polymerase chain reaction (PCR) amplification were tested for detection of phytoplasmas from infected herbaceous and woody plants. Thin freehand cross-sections made from infected plant tissues and stored in acetone were used as sources for DNA preparation. The tissue sections were treated by: 1) grinding in sodium hydroxide; 2) sonicating in water; 3) microwaving in water; 4) boiling in sodium hydroxide; or 5) placing directly in PCR tube. PCR amplification was performed with a universal phytoplasma-specific primer pair in a reaction buffer containing 0.5% (v/v) Triton X-100, 1.5 mm magnesium chloride, and 10 mm Tris-HCl. All five procedures provided phytoplasmal template DNA for successful PCR amplification from infected herbaceous plants {periwinkle [Catharanthus roseus (L.) G. Don (periwinkle)], carrot (Daucus carota L.), maize (Zea mays L.)}, while the grinding, microwaving, and boiling procedures also allowed positive amplification from a woody plant [green ash (Fraxinus pennsylvanica Marsh.)]. The quality of the resulting DNA was adequate for subsequent identification of the aster yellows and ash yellows phytoplasmas through nested-PCR using phytoplasma group-specific primer pairs. These methods provide remarkable savings in labor and materials, making disease testing and indexing of plant materials much more attractive.