Fusarium oxysporum f. sp. colocasiae n.f. sp. Causing Dry Rot of Taro (Colocasia esculenta)

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1 Ann. Phytopath. Soc. Japan 60: (1994) Fusarium oxysporum f. sp. colocasiae n.f. sp. Causing Dry Rot of Taro (Colocasia esculenta) Norio NISHIMURA*,** and Kazuichi KUDO*** Abstract The forma specialis and cultural and morphological characters of a F. oxysporum, the dry rot pathogen of taro (Colocasia esculenta), were examined. No forma specialis which corresponded to the family Araceae has been reported. The host range of the dry rot pathogen differed from that of the following 17 formae speciales: lycopersici race 2, melongenae, cucumerinum, melonis, niveum, lagenariae, conglutinans, cepae, allii, asparagi, arctii, batatas, spinaciae, fabae, fragariae, radicis-lycopersici and raphani. As a result, it was proposed to establish a new forma specialis, Fusarium oxysporum f. sp. colocasiae, and rename the disease gwilt h to differentiate this disease from taro dry rot caused by F. solani f. sp. radicicola. (Received December 9, 1993) Key words: Fusarium oxysporum f. sp. colocasiae, new forma specialis, taro (Colocasia esculenta), dry rot, wilt. INTRODUCTION Dry rot of taro (Colocasia esculenta Schott) caused by Fusarium oxysporum Schl. has been causing serious losses in Miyakonojo, Miyazaki, Japan. It was known that a pathogenic F. oxysporum was isolated from diseased taro plants9,23), and the authors reported the occurrence of the disease in farmers' fields for the first time15). The symptoms and the disease development of the dry rot were reported in detail in a previous paper18). The purpose of this paper is the identification of the forma specialis of the pathogen. Cross inoculation tests, especially with plants belonging to the same family, are necessary for the identification, however, there was no report on a forma specialis of F. oxysporum which corresponded to the family Araceae. The authors selected 17 known formae speciales, which were confirmed in Japan, and compared the pathogenicity of the dry rot pathogen with them in cross inoculation tests. The disease has been called dry rot15-18), because taro plants parasitized by the F. oxysporum have some symptoms similar to the taro dry rot caused by F. solani (Mart.) App. et Wr. f. sp. radicicola (Wr.) Snyd. et Hans.7,14,25) Consequently, both of F. oxysporum and F. solani were registered as taro dry rot pathogens24). However, it is reasonable to give a disease name to a pathogen, and hence the authors proposed to change the disease name in this paper. MATERIALS AND METHODS Isolates and formae speciales used in inoculation tests. The following isolates of F. oxysporum were used as the pathogen of taro dry rot in inoculation tests: a single-spore isolate OX-17-7

2 Ann. Phytopath. Soc. Japan 60 (4). August, and a single-colony isolate Col-20 which were isolated from soil of naturally infested fields with a selective medium16). In cross inoculation tests, the following 17 known formae speciales were used: f. sp. lycopersici race 221), f. sp. melongenae10), f. sp. cucumerinum19), f. sp. melonis20), f. sp. niveum20), f. sp. lagenariae13), f. sp. conglutinans20), f. sp. cepae20,22), f. sp. allii12), f. sp. asparagi2), f. sp. arctii11), f. sp. batatas20), f. sp. spinaciae20), f. sp. fabae27), f. sp. fragariae26), f. sp. radicis-lycopersici5) and f. sp. raphani6). The sources of the isolates used were as follows: each isolate of the last 3 formae speciales and an isolate of f. sp. cepae were single-colony isolates from naturally infested soil, the other isolates were supplied by the late Dr. T. Matuo, Shinsyu University, Dr. Y. Sonku, formerly of Kurume Branch of Natl. Rec. Inst. Veg. Ornam. Plants & Tea, or Dr. F. Kodama, Hokkaido Central Agr. Exp. Stn. Each isolate was shake-cultured in flasks of potato-sucrose liquid medium at 27 Ž for 5 days. After filtration with paper gauze and centrifugation at 800 ~g for 10min, spores were resuspended in an aliquot of distilled water, and the spore density was adjusted to about 8 ~107/ml. Inoculation. Crops and their commercial varieties used are listed in Table 1. For most test plants, five to 20 seeds, bulbs or seedlings of the test crops were planted in a 1/5,000 a (are) wagner pot, and 100ml of a spore suspension was poured into the pot just after planting. In the case of taro, seed corms of variety Eguimo were surface-disinfected with about 1% NaOCl for 15min and rinsed in tap water. Then each 5 seed-corms were planted in a 1/2,000 a wagner pot, and 100ml of a spore suspension was poured into the pot after sprouting. In the case of strawberry, each seedling from runners was grown in a 1/10,000 a pot, and 20ml of a spore suspension was applied to the soil in each pot after rooting. In the case of sweet potato, 5 cuttings were planted in a 1/5,000 a wagner pot just after dipping the base of cuttings in a spore suspension for 15min. The inoculation tests with an isolate of f. sp. radicis-lycopersici to tomato were conducted in a phytotron controlled at 18 Ž, and the others were done in a greenhouse. In all inoculation tests, soil was disinfected with chloropicrin. Soil from an uncultivated field was spread to a depth of 20cm on polyethylene film. Soil was then injected with 3ml of chloropicrin at 30 cm intervals, and was wrapped with the film for more than 2 weeks. After the film was opened and the smell of chloropicrin no longer detected, the soil was used. Disease assessment. Disease severity in taro and other plants were investigated about 2 months and within 1 month after inoculation, respectively. All living plants were longitudinally cut with a kitchen knife or a razor blade, and all plants sprouted were grouped to 3 grades: no symptom, vascular browning or wilting, and dead. In the case of taro, the following disease index of the mother corm (DI) was adapted: DI-0: no symptom, DI-1 `5: proportion of lesion part per mother corm volume corresponded to <20%, <40%, <60%, <80% and <100%, respectively. Susceptibility of taro varieties. Seed-corms of 13 varieties listed in Table 3 were planted in a field and were inoculated with 100ml of inoculum, which was the isolate Col-20 cultured on soil-bran medium at 27 Ž for 7 days, around each mother corm after sprouting. In another test, seed-corms of the same varieties were planted in a field, in which taro dry rot naturally occurred in the preceding year. Disease assessment was conducted in the harvesting season. Cultural and morphological characters. The isolate Col-20, single-spore isolates, F.o-21-1, F.o-25-2, F.o-22-1 and Col-30-1, and other pathogenic isolates were cultured at 25 Ž on potato sucrose agar plates or Difco potato dextrose agar plates. Microconidia on mycelia and macroconidia in cultural stromata were collected from 2 and 4-6 weeks cultures, respectively. Sizes of each 100 spores were measured from photographs taken with a compound microscope ( ~400). RESULTS Inoculation tests Isolates of 17 formae speciales induced symptoms of vascular browning, wilting, one sided wilting and/or dead on their respective host plants in each inoculation test, and therefore the pathogenicity was confirmed (Table 1). In cross inoculation tests, only an isolate of f. sp. melonis induced symptoms on 1 of 30 taro plants inoculated (Table 1). On the other hand, taro isolates didn't induce any symptom on the

3 Table 1. Result of inoculation tests with isolates of 17 known formae speciales and taro isolates to the respective host and taro a) Average disease index. b) The sprouting ratio was 22%. host plants of 17 formae speciales except for 2 of 56 onion plants and 1 of 38 broad bean plants (Table 2). Susceptibility of taro varieties Variety Eguimo, Nakate-hasubaimo, Ishikawawase-maru and Daikichi are commonly planted by farmers in the area around Miyakonojo, Miyazaki. Eguimo and Nakate-hasubaimo were obviously susceptible in farmers' fields. In the tests, all 13 varieties developed the disease and Ishikawawase-maru was the most susceptible. However, the difference in susceptibility among varieties was not clear, because the disease severity was not high enough except for Ishikawawase-maru (Table 3). Cultural and morphological characters Aerial mycelia were white and floccose, cultural stromata were fragile and often showed blue or purple stains. Most of the isolates produced reddish-purple pigment on the selective medium, but in some isolates such as F.o-22-1 and Col-30-1, the pigment was absent (Table 4). Microconidia were abundantly formed in aggregation on short conidiophores. Macroconidia with more than 2 septa were rarely formed on conidiophores on agar media. They were formed in cultural stromata and most of them had 3 septa. Chlamydospores were formed readily. Microconidia were hyaline, elliptic, long elliptic or ovoid, and the average (range) size of 5 isolates was 7.6( ) ~2.5( )ƒÊm. Macroconidia were hyaline and sickle-shaped, and the average (range) size of macroconidia with 3 septa from 3 single-spore isolates was 25(18-38) ~3.5( )ƒÊm. Sizes of microconidia and macroconidia with 3 septa of each isolate are shown in Table 4. DISCUSSION The morphological and cultural characters of the taro isolate agree with those of Fusarium oxysporum Schl. emend. Snyd. et Hans.8). The results of cross inoculation tests demonstrated that the

4 Ann. Phytopath. Soc. Japan 60 (4). August, Table 2. Result of inoculation tests with taro isolates to host plants of 17 known formae speciales Table 3. Susceptibility of taro varieties a) Average disease index. b) Tests were held in a filed in which taro dry rot naturally occurred in the preceding year. Table 4. Sizes of microconidia and macroconidia of taro isolates a) +: Reddish purple pigment was produced on the selective medium. -: Pigment was absent. b) Average value (range) of conidium size (Đm). c) Sizes were not measured, because no cultural stroma was formed on culture media.

5 pathogenicity of taro isolates was different from that of 17 known formae speciales used in the inoculation tests. In addition, a forma specialis, which corresponded to the family Araceae, was not found in 25 formae speciales listed by Snyder and Hansen20), in 69 listed by Armstrong and Armstrong1), in 33 listed by Matuo8), in 40 listed in gfungi h published in 19893), in the reports recorded in greview of Plant Pathology h published in or in the gagris h database in As a result, a new forma specialis is proposed for the taro isolates, Fusarium oxysporum Schl. f. sp. colocasiae Nishimura et Kudo. The following isolates were deposited in Central Bank for Microorganism Genetic Resources of Ministry of Agriculture, Forestry and Fisheries: MAFF (Col-20), MAFF (F.o-25-2), MAFF (F.o-22-1), MAFF (Col-30-1) and MAFF (F.o-21-1). The authors concluded in a previous paper18) that it was possible to differentiate symptoms of this disease from ones of the F. solani dry rot on the following points: the reddening of vascular bundles from leaf scars or roots, necrosis between veins of mother corm leaves and subsequent wilting of their leaves and stems. Since the pathogens and symptoms are different and wilting is a distinct symptom of the disease caused by F. oxysporum, the authors propose the disease be renamed gwilt h to differentiate the two disease. In addition to the gwilt h of taro, there was a report of a soft corm rot of taro caused by F. oxysporum4). But the symptoms induced by the soft corm rot differed from that of the gwilt h, and the report didn't mention to vascular browning. Therefore it is not clear if both diseases are the same or not. We wish to thank Dr. Jerome T. Gaspard, Nematec Inc., for critical reading of the manuscript. Literature cited 1. Armstrong, G.M. and Armstrong, J.K. (1968). Formae speciales and races of Fusarium oxysporum causing a tracheomycosis in the syndrome of disease. Phytopathology 58: Cohen, S.I. (1946). A wilt and root rot of Asparagus officinalis L. var. altilis L. (Abst.) Phytopathology 36: Farr, D.F., Bills, G.F., Chamuris, G.P. and Rossman, A.Y. (1989). In Fungi on Plants and Plant Products in the United States. APS Press, St. Paul, Minnesota. pp , Gollifer, D.E. and Brown, J.F. (1973). Corm rot of Colocasia esculenta caused by Fusarium oxysporum. Plant Dis. Rep. 57: Jarvis, W.R. and Shoemaker, R.A. (1978). Taxonomic status of Fusarium oxysporum causing foot and root rot of tomato. Phytopathology 68: Kendrick, J.B. and Snyder, W.C. (1942). Fusarium wilt of radish. Phytopathology 32: Kurata, T. (1931). A new disease of taro, dry rot. J. Plant Prot. 18: (in Japanese). 8. Matuo, T. (1972). Taxonomic studies of phytopathogenic fusaria in Japan. Rev. Plant Protec. Res. 5: Matuo, T. (1980). In Fusarium Disease of Cultivated Plants: Taro Dry Rot (Matuo, T. et al. eds.). Zenkoku Noson Kyoiku Kyokai Publishing Co. Ltd., Tokyo. pp. 471 (in Japanese). 10. Matuo, T. and Ishigami, K. (1958). On the wilt of Solanum melongena L. and its causal fungus Fusarium oxysporum f. melongenae n.f. Ann. Phytopath. Soc. Japan 23: (in Japanese). 11. Matuo, T., Matsuda, A., Ozaki, K. and Kato, K. (1975). Fusarium oxysporum f. sp. arctii n.f. causing wilt of great burdock. Ann. Phytopath. Soc. Japan 41: Matuo, T., Tooyama, A. and Isaka, M. (1979). Fusarium basal rot of Allium bakeri Regel and its causal fungus, Fusarium oxysporum Schl. f. sp. allii n.f. Ann. Phytopath. Soc. Japan 45: Matuo, T. and Yamamoto, I. (1967). On Fusarium oxysporum f. sp. lagenariae n.f. causing wilt of Lagenaria vulgaris var. hispida. Trans. Mycol. Soc. Japan 8: Nagai, Y. (1983). Occurrence of taro dry rot and effect of seed bulb disinfection. Kongetsu no Noyaku 27: (in Japanese). 15. Nishimura, N. and Kudo, K. (1988). Taro dry rot caused by Fusarium oxysporum. Kyushu Agric. Res. 50: 121 (in Japanese). 16. Nishimura, N. and Kudo, K. (1989). Isolation of Fusarium oxysporum from soil naturally infested with the pathogen of taro dry rot. Kyushu Agric. Res. 51: 106 (in Japanese). 17. Nishimura, N. and Kudo, K. (1992). Population of taro dry rot pathogen (F. oxysporum) in continuous cropping and rotation fields. Ann. Phytopath. Soc. Japan 58: 550 (Abstr. in Japanese).

6 Ann. Phytopath. Soc. Japan 60 (4). August, Nishimura, N. and Kudo, K. (1993). Disease development and chlamydospore formation in taro dry rot caused by Fusarium oxysporum. Bull. Kyushu Natl. Agric. Exp. Stn. 28: (in Japanese). 19. Owen, J.H. (1956). Cucumber wilt, caused by Fusarium oxysporum f. cucumerinum n.f. Phytopathology 46: Snyder, W.C. and Hansen, H.N. (1940). The species concept in Fusarium. Am. J. Bot. 27: Sugahara, Y. and Suzuki, I. (1966). Studies on pathogenicity of Fusarium wilt fungus in vegetable crops. 1. An isolate of Fusarium oxysporum f. lycopersici having different pathogenicity from common race in Japan. Bull. Hort. Res. Stn Japan Ser. B5: 1-28 (in Japanese). 22. Takakuwa, M., Ishizaka, N., Kodama, F. and Saito, I. (1977). Host range of Fusarium oxysporum f. sp. cepae, causal fungus of Fusarium basal rot of onion. Ann. Phytopath. Soc. Japan 43: (in Japanese). 23. The Phytopath. Soc. Japan (1980). In Common Names of Economic Plant Diseases in Japan, Vol.2, Second edition: Taro. The Phytopath. Soc. Japan, Tokyo. pp. 128 (in Japanese). 24. The Phytopath. Soc. Japan (1992). In Supplements for the Common Names of Economic Plant Diseases in Japan (12): Taro Dry Rot. Ann. Phytopath. Soc. Japan 58: 172 (in Japanese). 25. Tooyama, A. (1976). In Diseases and Pests of Vegetables: Taro Dry Rot (Kishi, K. eds.). Zenkoku Noson Kyoiku Kyokai Publishing Co. Ltd., Tokyo. pp (in Japanese). 26. Winks, B.L. and Williams, Y.N. (1965). A wilt of strawberry caused by a new form of Fusarium oxysporum. Queensland J. Agric. Anim. Sci. 22: Yu, T.F. and Fang, C.T. (1948). Fusarium diseases of broad bean. III. Root-rot and wilt of broad beans caused by two new forms of Fusarium. Phytopathology 38: