Diagnostic Protocol
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Diagnostic Protocol Kernel smut Anamorph: Tilletia horrida Synonym: Tilletia barclayana (Bref.) Sacc. & Syd.; Neovossia barclayana Bref.; N. horrida (Tak.) Padw. & Kahn. Vincent Lanoiselet Quarantine Plant Pathologist Department of Agriculture and Food, Western Australia Phone: 08 9368 3263 Email: [email protected] Eric Cother Private Consultant Phone: 02 6361 4753 Email: [email protected] April 2005 (reviewed 2008) Photo: M. A. Marchetti The scientific and technical content of this document is current to the date published and all efforts were made to obtain relevant and published information on the pest. New information will be included as it becomes available, or when the document is reviewed. The material contained in this publication is produced for general information only. It is not intended as professional advice on any particular matter. No person should act or fail to act on the basis of any material contained in this publication without first obtaining specific, independent professional advice. Plant Health Australia and all persons acting for Plant Health Australia in preparing this publication, expressly disclaim all and any liability to any persons in respect of anything done by any such person in reliance, whether in whole or in part, on this publication. The views expressed in this publication are not necessarily those of Plant Health Australia. Want more info? If you would like more information, or to download a copy of the Industry Biosecurity Plan, visit www.planthealthaustralia.com.au, email [email protected] or phone (02) 6260 4322. PLANT HEALTH AUSTRALIA | Rice Industry Biosecurity Plan 2009 Part of plant or commodity affected Rice florets and grains (only noticeable at crop maturity). Primary host range The taxonomy of the fungus causing kernel has been subject to controversy. If T. barclayana found on weeds is the same organism as T. horrida found on rice, the host range could include species of Brachiara, Digitaria, Erichloa, Panicum and Pennisetum. The latest research suggests that the fungus occurring on rice (N. horrida, syn. T. horrida) is different from N. barclayana (T. barclayana) which infects weeds (Whitney 1989). Current distribution The pathogen is present in most rice producing countries. In the USA, kernel smut occurs in southern USA and also infrequently in California. In Australia, the disease has been reported on rice in Queensland at Mareeba, Ayr and Brendon (Tonello 1980; R. Shivas, personal communication). Diagnostic Principle Formal identification is based on both direct observation of the symptom on rice plants and direct observation of the characteristic teliospores (chlamydospores) produced by the fungus (T. barclayana). Symptoms recognition of kernel smut The disease is noticeable at crop maturity. Most of the infected plants display only a few smutted grains (Figure 1a). Black pustules of teliospores (Figure 1b) push through the glumes, especially if they swell from moisture provided by overnight dew. Diagnostic protocol: Kernel smut | PAGE 2 PLANT HEALTH AUSTRALIA | Rice Industry Biosecurity Plan 2009 a b Figure 1. Rice panicles infected with kernel smut; plants display only a few smutted grains (a), black pustules of teliospores oozing through the glumes following dew period (b) (photo a: M. A. Marchetti; photo b: G. Templeton) a b Figure 2. Rice seeds infected with kernel smut on whole seed (a) and on glumes (b) (photos: IRRI) Detection of T. horrida on rice plants Collecting, preparing and dispatching the suspected samples Samples presenting symptoms should be placed into paper bags and taken/sent to the Plant Pathologist as soon as possible. Plastic bags are not recommended, especially during the hot Australian summer as they tend to trigger favourable conditions for the growth of saprophytes. After collection, the samples should be placed into a closed container such as a cardbox to protect the samples from physical damage and dusty conditions. Placing the container into a cooler box is then highly recommended especially if long distance driving is required to take the samples to the Plant Pathologists. If sending the samples by mail, express mail should be used and (they) should not be sent at the end of the week to prevent samples staying in hot conditions over the week end. Recording sample collection details: Collection details should be recorded and send with the plant samples to the Plant Pathologist. The sample sender should also keep a copy of the collection details for his own record. 1. Collector’s details (name, address, e-mail address, phone and fax numbers) Diagnostic protocol: Kernel smut | PAGE 3 PLANT HEALTH AUSTRALIA | Rice Industry Biosecurity Plan 2009 2. Disease symptoms 3. Host (common name and scientific names and cultivar/variety name) 4. Locality (with as much details as possible and with GPS position if possible) 5. Date of collection 6. Disease distribution and severity Identification of the causal agent: T. horrida Formal identification is based on both direct observation of the symptom on rice plants and direct observation of the characteristic spores (teliospores or chlamydospores) produced by the fungus on the rice grains (Figure 3). Figure 3. Teliospores of Tilletia horrida (photo: IRRI) a b Figure 4. Teliospores of Tilletia horrida showing surface ornamentation pattern on both surface (a) and median view (b) (photos: G. M. Murray) The teliospores of T. horrida are brown to opaque black and their diameter ranges from 17 to 36 µm. Under microscopic median view, the spines present a characteristic curved shape (Figure 4), which is not the case for T. indica and T. walkeri. Samples with all teliospores < 36 µm, curved spines and brown to opaque black most likely are T. horrida (NAPPO 1999). Diagnostic protocol: Kernel smut | PAGE 4 PLANT HEALTH AUSTRALIA | Rice Industry Biosecurity Plan 2009 Possible source of confusion Symptoms “similar” to kernel smut: FALSE SMUT False smut is caused by Ustilaginoidea virens (Cooke) Takah. Infected rice kernels are replaced by velvety “galls” (1 to 5 cm diameter). At maturity the “galls” are olive green to black which may be confused with kernel smut symptoms by an unexperienced identifier (Figure 5). a b c Figure 5. Panicle infected with false smut; (a) early symptoms showing kernels covered by a silver membrane, (b) turning orange and dusty, (c) turning olive green to black at maturity (photos: R. D. Cartwright) Possible confusion with other Tilletia species: T. indica and T. walkeri T. indica (causing Karnal bunt on wheat, triticale and possibly rye) and T. walkeri (causing ryegrass bunt) are morphologically very similar to T. horrida (causing kernel smut on rice). Neither T. indica nor T. walkeri infect rice but teliospores of these two species can be present within the seed lot sample presented to the Plant Pathologist for examination (Figure 6, Figure 7). While Karnal bunt is not present in Australia, ryegrass bunt is commonly found across the country. Important Note: While it is relatively easy to differentiate T. horrida from T. indica and T. walkeri teliospores, it is very difficult to differentiate T. indica from T. walkeri teliospores due to their very similar morphology. Some teliospores of T. walkeri appear much darker than the ones shown on Figure 7. Teliospores of all three species can be translucent and appear brown. a b Diagnostic protocol: Kernel smut | PAGE 5 PLANT HEALTH AUSTRALIA | Rice Industry Biosecurity Plan 2009 Figure 6. Teliospores of Tilletia indica showing surface ornamentation pattern on both surface (a) and median view (b) (photos: G. M. Murray) a b Figure 7. Teliospores of Tilletia walkeri showing surface ornamentation pattern on both surface (a) and median view (b) (photos: G. M. Murray) Detection of T. horrida on rice bulk grain Direct examination of the grain – dry seed inspection Rice seeds from the submitted grain sample are visually inspected using a stereo binocular microscope. The grains are inspected for the presence of black globose teliospores on the glumes and/or cotyledon (Figure 2). Any suspicious blackening should be scraped, mounted on slides and observed under a compound microscope equipped with 10, 20 and 40X objectives for presence of typical teliospores of T. horrida (Figure 7). This method is used by the IRRI’s Rice Seed Health Unit (Mew 1991, 2002). Sieve Wash Technique See PHA Diagnostic protocol for Karnal bunt (caused by T. indica). References Mew TW (1991) Rice seed health evaluation at the International Rice Research Institute. In 'Proceedings of the Inter-Centre Meting on Germplasm Health and Movement'. (Ed. EA Frison) (IBPGR: Rome, Italy) Mew TW, Gonzales P (2002) 'A Handbook of Rice Seedborne Fungi.' (Science Publishers, Inc.: Enfield, NH, USA) NAPPO (1999) NAPPO Standards for Phytosanitary Measures: a harmonised procedure for morphologically distinguishing teliospores of Karnal bunt, ryegrass bunt and rice bunt. www.nappo.org Tonello PEA (1980) Rice-a new crop at Mareeba. Queensland Agricultural Journal 106, 241-246. Diagnostic protocol: Kernel smut | PAGE 6 PLANT HEALTH AUSTRALIA | Rice Industry Biosecurity Plan 2009 Whitney NG (1989) Taxonomy of the fungus causing kernel smut of rice. Mycologia 81, 468-471. Whitney NG (1992) Kernel smut. In 'Compendium of Rice Diseases'. (Eds RK Webster and PS Gunnell) pp. 28-29. (APS Press: St Paul, MN, USA). Diagnostic protocol: Kernel smut | PAGE 7 .