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Himalayan Rhododendron Spruce Rust -Chrysomyxa Himalensis

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Himalayan Rhododendron Spruce Rust -Chrysomyxa Himalensis U.S. Department of Agriculture, Agricultural Research Service Systematic Mycology and Microbiology Laboratory - Invasive Fungi Fact Sheets Himalayan rhododendron spruce rust -Chrysomyxa himalensis Chrysomyxa himalensis is a heteroecious rust fungus, an obligate parasite completing different stages of its life cycle on different plants. The sexual stage occurs on Rhododendron species in the Himalayan region of southern Asia. An asexual stage is reported on Picea species. Although not a major problem in its narrow native range, this rust fungus could be more damaging as an invasive on Picea and Rhododendron. The fungus is a Regulated Pest for the United States; it is considered potentially damaging to Rhododendron by CAST (2002). Because small amounts of perennial or latent infection may be overlooked, accidental introduction of the rust could occur through importation of infected germplasm by the horticultural industry or by flower enthusiasts. Chrysomyxa himalensis Barclay Uredinia generally hypophyllous, on petioles and midribs, pustulate, orange-yellow, subepidermal, erumpent, 1-2 mm wide, spores grayish-yellow, globose to ovoid, 26.5-43.5 x 12.5-31 µm, verrucose; epispore 0.5-1.0 µm, hyaline. Telia hypophyllous, on petioles and midribs; sori in a linear row, subepidermal, erumpent, round. May cover entire leaf. Spores catenulate, in loose chains of three to five, chains 93.0-127.00 µm. Spores oblong, 25.0-29.5 x 6.0-9.5 µm, oblong, red; epispore 1-1.5 µm. See Vattiprolu and Agarwal, 2002 as Melampsoropsis himalense. Notes: Barclay (1890) described only the teleomorph and its spores: Telia yellow, stalked, stalk 120 µm, with heads 2 mm diam, height 1.5 mm. Teliospores averaging 25 x 13 µm but variable, in chains of 3-4, germinating by elongate stalk bearing terminal 4-celled promycelium. Sporidia 9-12 x 9-10 µm, ovoid-globose, orange-red. On petioles and midribs, occasionally on leaf blades or stems. A number of species of Chrysomyxa occur on either Picea or Rhododendron or both (Hansen, 1997) are likely to be identified in areas where these plants are part of the native flora. The work of Crane (2005) and Crane et al. (2005) demonstrates the continuing need to clarify identities and relationships in the genus. Species are usually identified and distinguished by aeciospore and urediniospore size, shape and ornamentation, and by peridium wall form and cell ornamentation, characters that require close and careful examination. All teliospores in this genus are 1-celled and produced in chains; they do not provide many diagnostic characters. The stalked telia primarily on Rhododendron petioles and leaf midribs are a distinctive feature of C. himalensis. The description of the uredinial state by Vattiprolu and Agarwal (2002), on the other hand, does not permit distinction from uredinia of other Asian species, C. succinea (Sacc.) Tranz. or C. dietelii Syd., as described in detail by Crane (2005). Telia of the former species are stalked, but occur in circular groups on leaf blades, while the latter on R. arboreum, also a host of C. himalensis, has telia without stalks (Crane, 2005) In identifying this species, Barclay (1890) described characters of the teleomorph, with an illustration of the stalked fruiting body. Chen (1984) placed the species in a new genus, StilbeChrysomyxa, comprised of Chrysomyxa-type rusts with stalked telia. Crane (2005) does not support this genus, having observed a range of stalked telia among Chrysomyxa species. Vattiprolu and Agarwal (2002) provided a full description of the telial stage as well as a partial description of a uredinial form, in transferring Barclays species to Melampsoropsis. Melampsoropsis, nevertheless, is a genus synonymous with Chrysomyxa (Cummins and Hiratsuka, 2002). Distribution: As far as is known, this species has a limited distribution in the Himalayan region of southern Asia (Zhuang, 1993; Farr et al., 1996). Risk of Introduction: Because the life cycle and host range of this rust are not fully known, the risk of introduction is difficult to determine. Spaulding (1961) states that it is considered to be the cause of occasional serious defoliation of spruce in India and Pakistan, yet Vattiprolu and Agarwal (2002) assert that the aecia and spermagonia, which would be the stages on spruce, are unknown. The Council on Agricultural Science and Technology in the United States lists the species as a potentially damaging rust on Rhododendron (CAST, 2002), although few species of that genus are known to be susceptible (Farr et al., 1996). Rossman et al. (2006) also place the species in the category of major threat to crop plants and forest trees for the United States. While this fungus might be damaging as an invasive on species of Picea and Rhododendrons, most countries where spruces and Rhododendrons occur have not identified this species as one of concern. Detection and inspection methods: Leaves, particularly the petioles and midribs, of Rhodoendron plants from southern and central Asia should be examined under low power magnification for the presence of telia and uredinia. A period of post-entry quarantine should be sufficient to detect latent (overwintering) infections in such plants, though it may not be completely successful (Savile, 1973; Bennell, 1985). Perennial infections may be detected by the proliferation of shoots or witches broom symptoms. Other plants affected: Roane (1986) lists a few Himalayan or Tibetan Rhododendron species as susceptible, but other species from that region are not reported to be susceptible. Susceptibility of the species/hybrids of other origins is not known, and many, if not most, have not been tested. Farr et al. (1996) list four host species known from China, India, and Nepal. The rust may be limited in its host range to species in certain sections of the host genus, as other Chrysomyxa species appear to be (Crane, 2005). Symptoms-Description: This rust causes a witches broom of Rhododendron. Stunted shoots proliferate from infected stems. Leaves on these shoots are small but not otherwise distorted (Barclay, 1890). Spaulding (1961) states that it is considered to be cause of occasional serious defoliation of spruce in India and Pakistan, but the timing of the defoliation and any symptoms preceding it are not reported. Biology and Ecology: Teliospores on rhododendron leaves germinate in spring to produce basidiospores that are likely to infect young spruce needles (Barclay, 1890). Barclay was unable to obtain infection by inoculation of potential conifer hosts, including the common local spruce, Picea morinda (= P. smithiana (Wall.) Boiss. (USDA-ARS, 2009). On the other hand, Bagchee (1940) and Spaulding (1961) reported that pathogenicity to this species was established by inoculation tests. No description of the rust on the conifer host was provided. If this species behaves as do many others in the genus (Hansen, 1997), aeciospores produced on the spruce would infect young rhododendron leaves in summer, and the fungus would overwinter in these leaves. Telia develop on the previous years leaves in spring, but the fungus is perennial in the shoots, so does not require alternation to an aecial host each year (Barclay, 1890). Urediniospores are produced in the growing season, which spreads many Chrysomyxa species to susceptible new leaves of Rhododendron (Roane, 1986). Crane (2005) indicates that this species has aecia on Picea but that the uredinia are unknown. Barclay (1890) searched for uredinia without success. Conversely, Vattiprolu and Agarwal (2002) state that the aecia and spermagonia are unknown, and they describe the uredinia, apparently from preserved specimens rather than new collections. Natural dispersal: Rust aeciospores, urediniospores and sporidia (basidiospores) usually are distributed by wind (Alexopoulos et al., 1996). Vector transmission: Unknown Accidental introduction: Introduction of rust species can occur through importation of infected cuttings and plants by the horticultural industry or by flower enthusiasts (Bennell, 1985). Economic impact: Roane (1986) states that rusts on Rhododendron are not usually a serious problem, although some species and varieties may be severely damaged. Growth distortion in the form of a "witches broom" can have a more severe effect on growth and appearance than do the usual leaf spots and pustules of Rhododendron rusts, and thus reduce the ornamental value. Post-entry quarantine, which may require more than 6 months if infections are latent (Bennell, 1985), imposes an additional cost on trade in rhododendrons. Prevention: Given the possibility of latent infections in Rhododendron, phytosanitary post-entry quarantine of imported plants is inevitable (Roane, 1986). Clearly, in areas that are climatically favorable for the rust and where spruce and Rhododendron grow together, introduction of C. himalensis should be prevented (USDA/APHIS, 2009). Cultural control: Adequate aeration, achieved by spacing, pruning and location of plants in Rhododendron plantings will reduce the humidity and free moisture needed for germination and infection by aeciospore and/or urediniospores (Bennell, 1985; Roane, 1986). Planting should be avoided where the alternate host is growing or the alternate host should be removed in the vicinity of the more valuable planting, either of spruce or Rhododendron (Bennell, 1985; Hansen, 1997) Chemical control: Although Bennell (1985) discussed the use of various fungicides on rhodendrons, such as zineb, triadimefon and oxycarboxin, as a complement to eradication efforts,
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