Table of Contents Margaret Mmbaga Downy Mildew in Lilac .............................................................................................................................................. 296 Abstract. Syringa spp. ‘Old Glory’ is a disease-resistant selection of lilac that was developed from a controlled hybridization between Syringa ‘Sweet Charity’ and Syringa × hyacinthflora ‘Pocahontas’. ‘Old Glory’ is credited for high-level disease resistance to powdery mildew, bacterial blight, and other foliage diseases common in the southern region of the United States, but in 2005 and 2006,‘Old Glory’ plants developed symptoms of downy mildew in McMinnville, Tennessee, U.S. Symptoms started in late April as chlorotic lesions and later became necrotic. Upper leaf surface symptoms appeared similar to common leaf spots with necrotic lesions, but underside leaf lesions were covered with masses of sporangiophores and sporangia. Morphologic features of the sporangiophores and sporangia matched that of Plasmopara spp. The leaf lesions were circular or irregular and developed between veins. Coalesced lesions formed large necrotic patches; severely infected leaves defoliated prematurely. Surface- sterilized healthy leaves were spray-inoculated with sporangiospores and placed in Petri dishes over triple-layered wet paper towels at 23°C to 25°C (73.4°F to 77°F). Disease symptoms were reproduced in approximately 12 days. Noninoculated control leaves did not develop disease symptoms. Oospores were not observed. Downy mildew has not previously been reported in lilac and this is the first report of the disease in Tennessee. Key Words. Plant Health Care; Plasmopara spp. Leaf Spot Diseases; Syringa species. Richard H. Yahner, Richard T. Yahner, and Bradley D. Ross A New and Improved Automated Technology for Early Sex Determination of Ginkgo biloba............................300 Abstract. Random amplified polymorphic DNA (RAPD) technique with male associated decamer primer S1478 was used to amplify DNA from 72 leaf samples collected from Ginkgo biloba trees with known sexual determinism in the canton of Geneva, Switzerland. This marker was found to be male-spe- cific and was lacking in all female plants. Automated random polymorphic DNA analysis (ARPA), a new automated technology developed in the frame of this work, proved highly effective in distinguishing males and females with 100% efficiency and successful in male and female discrimination from a collection of young seedlings derived from a sexual cross. Our findings provide unambiguous evidence that ARPA combined with the male-associated decamer primer S1478 could be considered an efficient, rapid, and easy method to make an early sex determination in the dioecious tree Ginkgo biloba. Brian Kane, Robert Farrell, Shepard M. Zedaker, J.R. Loferski, and D.W. Smith Failure Mode and Prediction of the Strength of Branch Attachments .................................................................. 308 Abstract. Predicting the strength of branch attachments is important for arborists and urban foresters because branch failure can cause dam- age and injuries. Previous studies have shown that the ratio of branch to trunk diameter is a better predictor of strength than the angle of attachment. Aside from these two factors, however, few other morphological measures of the attachment have been investigated with respect to predicting the strength of attachments. Many branch attachments from three species were broken on a testing machine and breaking stress was calculated. Prior to breaking, various morphological measures and ratios describing the attachment were made. Breaking stress var- ied by form of the attachment (u-shaped or v-shaped), failure mode, and the presence of included bark. The best predictor of breaking stress was the ratio of branch to trunk diameters. Results are discussed in the context of previous studies and with respect to tree risk assessment. Key Words. Branch Attachment; Breaking Stress; Included Bark; Tree Risk Assessment. Albert E. Mayfield III, Edward L. Barnard, Jason A. Smith, Shawn C. Bernick, Jeffrey M. Eickwort, and Tyler J. Dreaden Effect of Propiconazole on Laurel Wilt Disease Development in Redbay Trees and on the Pathogen In Vitro ......................................................................................................... 317 Abstract. Laurel wilt is a vascular disease of Lauraceous plants caused by a fungus (Raffaelea spp.) that is vectored by a recently introduced, nonnative ambrosia beetle (Xyleborus glabratus). The disease is devastating to redbay (Persea borbonia) trees in forests, parks, and resi- dential landscapes in the southeastern United States, and management strategies for reducing its impact are needed. In this study, the sys- temic fungicide propiconazole completely inhibited mycelial growth of Raffaelea spp. in vitro at concentrations 0.1 parts per million (ppm) or greater and was fungitoxic at 1 ppm or greater, whereas the fungicide thiabendazole was less inhibitory. None of the ten mature red- bay trees that received root-flare injections of propiconazole developed crown wilt symptoms for at least 30 weeks aſter being inoculated with Raffaelea spp., whereas nine of ten untreated control trees wilted in more than one-third of their crowns. Propiconazole was retained in the stem xylem for at least 7.5 months aſter injection but was more frequently detected in samples from trees injected 4.5 months ear- lier and was not well detected in small-diameter branches. Results suggest that propiconazole may be useful in preventing laurel wilt in red- bay, but limitations and questions regarding duration of efficacy, rate of uptake, and efficacy under different levels of disease pressure remain. Key Words. Ambrosia Beetle; Fungicide Injection; Laurel Wilt; Persea borbonia, Propiconazole, Raffaelea; Redbay; Thiabendazole; Vascular Disease; Xyleborus glabratus. ©2008 | International Society of Arboriculture | ISSN:1935-5297
September 2008
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