Journal of Arboriculture 31(5): September 2005 257 HOST RANGE AND DISTRIBUTION OF THE PHYTOPLASMA CAUSING ARIZONA ASH DECLINE By Jerald S. Bricker1 and Jean C. Stutz2 Abstract. Ash decline (AD), caused by a phytoplasma, is a progressive dieback disease previously reported in Fraxinus velutina. The host range and distribution of AD is discussed based on samples collected from ash trees sampled from across the state of Arizona, U.S., including both wild-type F. velutina in riparian areas and F. velutina cv. Modesto, F. velutina cv. Rio Grande, and F. uhdei in landscaped areas. The results of the statewide survey indicate that AD is found in native and cultivated ash trees, with F. velutina cv. Modesto exhibiting the highest (100%) level of disease in the Phoenix metropolitan area. Wild-type F. velutina and F. velutina cv. Rio Grande also exhibited AD symptoms but with lower frequency and severity. Fraxinus uhdei did not exhibit symptoms of AD. Phytoplasma infection was detected in all tree types of F. velutina trees using DAPI staining and polymerase chain reaction (PCR) but was not detected in F. uhdei. Higher frequency of phytoplasma infection was detected in tree canopies versus roots in contrast to previous results reported in white ash. PCR was found to be more efficient at detecting the low-titer infection levels typical of F. velutina in comparison to DAPI staining. Key Words. Ash yellows; Fraxinus; Fraxinus velutina; Fraxinus uhdei; host specificity; shamal ash; velvet ash. Ash decline (AD) is a progressive dieback of stems and branches along with a necrosis of leaf tissue in Fraxinus velutina ‘Modesto’ (see Bricker and Stutz 1992, 2004 for a full description of AD symptoms). The causal agent of the condition has been determined to be a phytoplasma (Bricker and Stutz 2004). A number of ash species, including Fraxinus velutina Torrey, are native to Arizona, U.S., and at least two addi- tional species, F. uhdei (Wenz.) Lingelsh. (shamal ash) and F. greggii A. Gray (littleleaf ash), are cultivated as landscape trees in the Phoenix and Tucson, Arizona, metropolitan areas. Our initial observations of AD indicated that not all of the ash species and cultivars growing in the Tempe, Arizona, area displayed symptoms of AD. The purpose of this study was to (1) establish the host range for AD, (2) determine the most effective sampling strategy for detecting phytoplasmas associated with AD, and (3) compare the effectiveness of detecting phytoplas- mas using polymerase chain reaction (PCR) techniques versus the DAPI staining test. In order to address the species and cultivar susceptibil- ity to AD, native and cultivated populations of ash species were surveyed for symptoms of AD and screened for the presence of phytoplasmas to establish the geographic distribution of AD within Arizona. MATERIALS AND METHODS Phytoplasma Host Range in Arizona Sample collection. During September 1992, visual surveys were made for symptoms of AD of 57 Fraxinus velutina trees at 11 sites throughout its natural range in Arizona. In October 1992, 108 cultivated F. velutina cv. ‘Modesto’ trees in Tempe were examined for symptoms of AD and in November 1992, 50 trees each of F. uhdei and F. velutina cv. ‘Rio Grande’ were surveyed in the Tempe area (in areas where F. velutina cv. ‘Modesto’ was seen with AD) for symptoms of AD. In Septem- ber 1992, 26 F. velutina ‘Modesto’ trees were surveyed in the Tucson, Wickenburg, Miami-Globe, Safford, and Yarnell areas for symptoms of AD. Each tree included within the survey was rated for decline symptoms using the classification system of Silverborg and Ross (1968): class 1 = apparently healthy trees, class 2 = trees with a few dead branches, class 3 = trees with foliar symptoms but with less than one-half of the foliar crown dead, class 4 = trees with foliar symptoms and more than one- half of the foliar crown dead, and class 5 = dead trees. Addi- tionally, tissue samples were taken from each tree in the survey group and subjected to the DAPI and PCR tests. PCR Protocol Sample collection. Ash stem samples, with leaves attached, were collected from across Arizona and subjected to PCR analysis using methods described in Bricker and Stutz (2004). Nine trees of wild-type Fraxinus velutina, ten of F. velutina cv. ‘Modesto’ from outside the Phoenix metropoli- tan area, ten of F. velutina cv. ‘Rio Grande’, and ten of F. uhdei were screened using PCR. The test results from the above types of ash were then compared with those obtained for the 51 F. velutina cv. ‘Modesto’ from the Phoenix area previously published by Bricker and Stutz (2004). Comparison of DAPI and PCR Tests Stems, with the leaves attached, were collected on 14 September and 18 October 1990, as well as on 13 September 1991. For each tree, samples were collected from as many of the following types of branches that were present on the tree at the sampling time: (1) branches with healthy leaves, (2) branches with leaves ©2005 International Society of Arboriculture
September 2005
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