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日 植 病 報 60: 109-112 (1994) 短 報 Ann. Phytopath. Soc. Japan 60: 109-112 (1994) Phytopathological Note Occurrence of Soybean Rust Caused by Phakopsora pachyrhizi in the Philippines and Its Wild Legume Hosts* Fe M. DELA CUEVA**, Antonio C. LAURENA**, Marina P. NATURAL***, Yuichi YAMAOKA•õ, Yoshitaka ONO•õ•õ and Makoto KAKISHIMA•õ Key words: soybean rust, Phakopsora pachyrhizi, Glycine max, Leguminosae, Uredinales. It was not until 1970s that soybeans became one of major crops in the Philippines although soybeans were introduced from southern China and land races were established in the Philippines as early as the first century or by not later than 15 16th century6). Recently soybeans have become an increasingly important crop in the country and their production has been encouraged by the government. Before early 1970s, land races of soybeans were cultivated in small scale by local farmers in the Philippines. But current soybean production is undertaken in a large scale farming system known as financed farms•h and •gcooperative farms•h11). A few cultivars with preferable agronomic characters are grown extensively in major growing areas. This leads to natural consequences of difficulties in large scale commercial agricultural production, and soybean rust is one of those obstacles. The soybean rust inciting fungus in Asia is Phakopsora pachyrhizi H. et P. Sydow8) and this fungus was first recorded on Glycine max (L.) Merr. in 1914 in the Philippines2). But the fungus did not receive attention by Filipino scientists until the mid-1960s7) perhaps because no epidemic of the rust had occurred due to small-scaled and scattered cultivation of soybeans. However, the rust incidence has become prominent in the past two decades in the Philippines, especially in Provinces of Davao, North and South Cotabato in the Mindanao and Province of Cagayan in the Luzon where soybeans are produced extensively in •gfinanced farms•h and •gcooperative farms•h. Although no reliable statistics for yield loss or economic impact due to soybean rust in the Philippines is available, as much as 80% reduction in gross yield would be expected in individual cropping fields as is in Taiwan5) and 20-40% yield loss would occur annually in the country as a whole4). To cope with the rust fungus in soybean production, employing cultivars that are resistant or at least tolerant against the fungus in the field is of primary importance. Meanwhile, eradication or reduction of populations of wild legume hosts of the fungus also is one of most promising practices to reduce the rust incidence. This perspective becomes tenable because the fungus urediniospores do not survive away from living host plants during hot and humid weather or dry weather in intercropping season in the Philippines and because the fungus is known to have an extraordinarily wide host range, which includes 31 species in 17 genera of the Leguminosae8). In addition to the natural and actual hosts, 60 species in 26 genera have been proved to support infection and sporulation of the fungus under greenhouse conditions. In the Philippines, the following wild hosts of P. pachyrhizi have been known: * Contribution No .114, Laboratories of Plant Pathology and Mycology, Institute of Agriculture and Forestry, University of Tsukuba. This work is a part of international cooperative research program under the core- university system in the field of agricultural science supported by the Japan Society for the Promotion of Science (JSPS). ** Institute of Plant Breeding , University of the Philippines at Los Banos, College, Laguna 4031, Philippines フ ィリピン大学植物育種学研究所 *** Department of Plant Pathology , University of the Philippines at Los Banos, College, Laguna 4031, Philippines フィリピン大学植物病理学科 † Institute of Agrichlture and Forestry , University of Tsukuba, Tsukuba, Ibaraki 305, Japan 筑 波 大 学 農 林 学 系 †† Faculty of Education , Ibaraki University, Bunkyo, Mito, Ibaraki 310, Japan 茨 城 大 学 教 育 学 部 110 日本植 物病 理 学 会 報 第60巻 第1号 平 成6年2月 Canavalia villosa Benthl5), Crotalaria albida Heyne1,16), Derris philippinensis Merr.16), Pachyrhizus erosus (L.) Urban1,3,12,16,17) Pueraria pulcherrima Merr.1), and Vigna cylindrica Skeels (specimens in BPI)8). However, question arises whether wild legumes that harbor P. pachyrhizi actually serve as the primary source of infection on G. max in the field resulting in soybean rust epidemics. To answer the question, both long-term field survey and experimental cross inoculation are inevitable. We have accordingly conducted studies on soybean rust in the Philippines. This is a preliminary report of field observations of soybean rust and inoculation experiments with the causal fungus in 1992-1993. Field survey. In the Philippines, most extensive soybean growing areas locate in Provinces of Davao, North and South Cotabato in the Mindanao and Province of Cagayan in the Luzon, and these and some additional localities in southern Luzon were surveyed for potential wild legume hosts for P. pachyrhizi. Most frequently encountered wild or weedy legumes in and around soybean fields were: Calopogonium mucunoides Desv., Centrosema pubescens Benth, Crotalaria anagyroides H.B.K., Crotalaria mucronata Desv., Desmodium sp., Mimosa pudica L., Mucuna pruriens (L.) DC., Pueraria pulcherrima, and a few other unidentified legumes. All these observed plants except P. pulcherrima were free from the rust infection. Vigna unguiculata (L.) Walp., Phaseolus vulgaris L., and Pisum sativum L., which are potential hosts of P. pachyrhizi, were also cultivated near soybean fields, but these legume crops were not infected by P. pachyrhizi. Pueraria pulcherrima was found infected at Polomolok, South Cotabato Prov. in the Mindanao (TSH-R1205) and at Oas (TSH-R1206, 1207) and Guinobatan (TSH-R1208), Albay Prov. in the Luzon. Minute uredinia were formed in small groups or scattered on abaxial surface of the leaves. The soni were characteristically paraphysate at the periphery (Fig. 2). The paraphyses were cylindric to clavate, incurved, apically thick-walled, and basally united to form pseudoparenchymatous layer characteristic to an anamorphic genus Malupa, which is anamorphic state of P. pachyrhizi8). The spores were borne from a basal sporogenous cell without appreciable intercallary cell or pedicel; 20-32•~16-24ƒÊm in size, thin-walled, colorless, and completely echinulate. Germpore (s) was hardly observable. These mor- phological characteristics were similar to those of P. pachyrhizi collected on soybean in the Philippines (TSH-R1190 -1204, Figs. 1, 3, 4) and led us to identify the fungus as P. pachyrhizi. Pueraria pulcherrima was reported as the host of P. pachyrhizi for the first time in the Philippines in 19361). Although no report had since been published as to P. pulcherrima as a wild host of P. pachyrhizi, the plant seems to be a widespread wild host for P. pachyrhizi in the Philippines. Inoculation experiment. Urediniospores of P. pachyrhizi formed on susceptible variety, TK5, of G. max was used as inoculum. The spores were inoculated onto healthy seedlings of Calopogonium mucunoides Desv., Centrosema pubescens Benth., Crotalaria anagyroides H.B.K., Desmodium sp., and Mimosa pudica L., which were commonest weedy legumes found in and around soybean fields. Inocula- tion and further observation for sorus development were undertaken at room temperature in a green house, and inoculation was repeated twice. Seven days after inoculation, uredinia started to appear on abaxial surface of the inoculated leaves of C. mucunoides, C. pubescens, C. anagyroides, and Desmodium sp., but not on M. pudica. These plants are newly found potential hosts of P. pachyrhizi in the Philippines, though they have been found to support sporulation of P. pachyrhizi elsewhere in Southeast Asia: C. mucunoides in Indonesia13) and Thailand9), C. pubescens in Thailand9,10), C. anagyroides in Taiwan4), three species of Desmodium in Taiwan4). Five wild legume hosts, one actual and four potential, for P. pachyrhizi were found in the Philippines in the two-year study. Our field survey was by no means exhaustive and more is needed to elucidate real biology of P. pachyrhizi. Most urgent need is to study whether each rust population on different wild legumes, which is identified as P. pachyrhizi, is equally infective and has ability to sporulate on other wild hosts, and whether P. pachyrhizi populations on wild legumes can equally infect and incite rust disease of cultivated soybeans. Species structure and biology of subspecific populations of P. pachyrhizi have to be analyzed before any ecological or integrated strategy of soybean rust control is developed and become feasible. All the specimens of P. pachyrhizi collected in the Philippines are deposited in the Mycological Ann. Phytopath. Soc. Japan 60 (1). February, 1994 111 Herbarium, Institute of Agriculture and Forestry, University of Tsukuba (TSH) and Institute of Plant Breeding, University of the Philippines. We wish to express our thanks to Dr. E.M. Tescon-Mendoza, Institute of Plant Breeding, University of the Philippines, Dr. K. Tomaru, Tokyo University of Agriculture, and Dr. K. Hirano, Chiba University, for providing us with an opportunity to participate in the JSPS international cooperative research program. We also sincerely thank Dr. V.M. Aquino, Dr. T.O. Dizon and Mr. E. Enicola, Institute of Plant Breeding, University of the Philippines, and Mrs. M.N. Estrella, Bicol University, for assistance in field survey of soybean rust. Literature cited 1. Arthur, J.C. and Cummins, G.B. (1936). Philippine rusts in the Clemens collection 1923-1926, II. Philippine J. Sci. 61: 463-488 & 4 pl. 2. Baker, C.F. (1914). The lower fungi of the Philippine Islands. Leaflets of Philippine Botany 6: 2065-2190. 3. Baker, C.F. (1931). Second supplement to the list of the lower fungi of the Philippine Islands. Philippine J. Sci. 46: 479-536. 4. Bromfield, K.R. (1984). Soybean Rust. Monograph No.11. American Phytopathol. Soc., St. Paul. 5. Hsu, C.M. and Wu, L.C. (1968). Study on soybean rust. Scientific Agric. (Taipei) 16: 186-188.

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