Pestic. Sci. 1997, 51, 383È386

Resistance Management in Diseases of Top Fruitin Japan*

Hideo IshiiNational Institute of Agro-Environmental Sciences, MAFF, Tsukuba, Ibaraki 305, Japan

(Received 30 June 1997 ; accepted 22 July 1997)

Abstract : Top fruit is grown widely in Japan but the wet climate favours thedevelopment of diseases ; this, and the demand for blemish-free fruit, necessitatesthe use of chemical disease control. Many applications of fungicides during thegrowing season could lead to the development of resistance and this paper pre-sents a brief history, current status and future management of resistance to fungi-cides with special reference to diseases of apples and Japanese pears.

Pestic Sci., 51, 383È386, 1997No. of Figures : 0. No. of Tables 0. No. of Refs : 19

Key words : top fruit, apples, Japanese pears, fungicides, Alternaria alternata,V enturia inaequalis, Pyrus pyrifolia, V enturia nashicola

1 INTRODUCTION

In Japan, top fruit is grown almost all over the countryand many diseases occur due to the wet climate whichfavours plant pathogens. In addition, Japanese con-sumers require blemish-free fruit, and consequentlythere is a high dependence of growers on chemicaldisease control with fungicides applied many timesduring the growing season. This paper covers a briefhistory, the current status, and future management offungicide resistance, mainly in apple and Japanese peardiseases.

2 APPLES

2.1 Alternaria blotch (Alternaria alternata (Fries)Keissler apple pathotype = A. mali Roberts)1

This is a major disease in Eastern Asia, including Japanand Korea, but it has recently become a serious summer

* Based on a presentation at the Conference “Resistance Ï97ÈIntegrated Approach to Combating ResistanceÏ organised bythe Institute of Arable Crops Research in collaboration withthe SCI Pesticides Group and the British Crop ProtectionCouncil and held at Harpenden, Herts, UK on 14È16 April1997.

disease also in the USA. In Japan, the antibiotic polyox-in had been widely used for Alternaria blotch controlsince 1968, but in 1972, strains of the pathogen resistantto polyoxin were Ðrst recorded.2 In the orchards wherehighly resistant strains were present in 1978, applica-tions of polyoxin were stopped for Ðve years from 1979.As a result, the proportion of resistant strains rapidlydeclined to below 20%. However, when polyoxin wasapplied again, twice a year during the 1985 growingseason, in combination with captan, the proportion ofresistant strains tended to increase. Polyoxin remains amajor component for the control of this disease butresistant strains are increasing again in fungal popu-lations. In some cases, the Ðeld performance of polyoxinhas also decreased. This antibiotic interferes with chitinbiosynthesis and causes swelling of conidial germ-tubes,3whereas resistant conidia germinate normally. Sporegermination tests can, therefore, be used to distinguishresistant from sensitive isolates.

In orchards where iprodione had been applied morethan 20 times in total, control efficacy of this fungicidestarted to decline, and in 1978 iprodione-resistantstrains were detected (Hiraragi & Nakano,unpublished). Monitoring results indicated that theresistant strains were probably less Ðt than sensitiveones in the absence of iprodione (Asari & Takahashi,unpublished). To avoid any increase of resistant strains,an anti-resistance strategy is recommended as follows :

3831997 SCI. Pestic. Sci. 0031-613X/97/$17.50. Printed in Great Britain(

384 Hideo Ishii

polyoxin or iprodione in a mixture with oxine copper-captan ; fosetyl-captan mixture and the alternate spray-ing of captan and other fungicides containing polyoxinor iprodione. The multi-site-inhibiting fungicide captanis particularly important for the control of summer dis-eases and as a partner fungicide in mixed applications.It is generally accepted that risk of resistance to captan,if any is low. A few cases of captan resistance have beenreported so far, but only in Botrytis cinerea Pers. exFr.,4 where the impact of resistant strains on controlefficacy has remained unclear. A decrease of Ðeld per-formance of captan against Alternaria blotch has beenreported frequently in Japan, although an involvementof resistance has not been well demonstrated in labor-atory tests.5

Registration of two broad-spectrum strobilurin fungi-cides, azoxystrobin and kresoxim-methyl is under wayin Japan, and we are trying to establish in-vitro testingmethods for strobilurin sensitivity. However, inhibitorye†ects of azoxystrobin on conidial germination of A.alternata are not so strong as those of conventional res-piration inhibitors (Ishii et al., unpublished).

2.2 Scab (Venturia inaequalis (Cooke) Wint.)6

Experiments for the simultaneous control of scab,powdery mildew, etc. were conducted in an experimen-tal orchard in Hokkaido. However, in 1972, the thirdyear of the use of benzimidazoles, failure of scab controlwas recognized,7 and this was conÐrmed to be causedby the development of benzimidazole-resistant strains ofV . inaequalis.8 Subsequently, benzimidazole fungicideshave been withdrawn as scab control agents, althoughthiophanate-methyl is often used for the control ofcanker caused by V alsa ceratosperma (Tode ex Fries)Maire.

Sterol demethylation inhibitors (DMIs) are mostimportant for scab control. Immediately after regis-tration of DMIs, application of these fungicides wasstrictly limited to two or three times per year to avoidthe practical problem of resistance. In the spray calen-dar for apple diseases and pests published by AomoriPrefecture, 1997, application times of DMIs are limitedto three out of 13 total applications. They recommendthe use of DMIs as a single application just beforeblossom, followed by two mixed applications. Manyreports are now available on DMI resistance in applescab worldwide,9 but in Japan, neither loss of Ðeld effi-cacy of DMIs nor detection of resistant strains has beenexperienced so far. Monitoring DMI sensitivity of V .inaequalis is being carried out regularly.

The anilinopyrimidine fungicide mepanipyrim hasbeen registered for the control of scab, Alternariablotch, fruit spot, etc. and a tank mixture of mepanipy-rim with ziram and thiram is recommended. Strobilu-rins have been submitted for registration and areplanned to be used as a single application for the simul-

taneous control of main diseases in summer (Fujita,pers. commun.). Methods for assessing sensitivity tothese new fungicides and establishing the baseline sensi-tivity are being developed. In V . inaequalis, MIC valuesin vitro of azoxystrobin based on mycelial growth onpotato dextrose agar plates varied widely, but EC50values were below 1 mg litre~1 for most isolates tested(Ishii et al., unpublished).

3 JAPANESE PEAR (Pyrus pyrifolia Nakai var.culta Nakai)

3.1 Black spot (Alternaria alternata Japanese pearpathotype = A. kikuchiana Tanaka)

In 1971, the sudden appearance of black spot, regard-less of successive treatments with polyoxin, was report-ed in Tottori Prefecture.10 The results from extensivework clearly indicated the emergence of polyoxin-resistant strains in these natural populations. In theorchards where control efficacy of polyoxin declined,other unrelated fungicides were used instead and annualchanges of the frequency distribution of resistant strainswere subsequently monitored in the Ðeld.11 When theuse of polyoxin was discontinued, highly resistantstrains dominated the fungal populations, and no inter-mediately resistant strains were detected at that time inany of the orchards examined. The highly resistantstrains declined year by year, and by the tenth year afterthe withdrawal of polyoxin, such strains resistant to thismaterial were undetectable. In contrast, the rates ofoccurrence of intermediately resistant strains, and, to alesser extent, of sensitive ones increased slowly.

Some years later, it was suspected that emergence ofiprodione-resistant strains played a role in poor diseasecontrol with this fungicide.12 Recently, the black-spot-tolerant pear cultivar “Gold NijisseikiÏ was obtained byirradiation breeding and has been released to peargrowers.13 It is expected that introduction of this newpear cultivar will halve both the frequency of fungicideapplications and the risk of fungicide resistance, as theintegrated use of disease-resistant cultivars togetherwith fungicides will reduce selection for fungicide-resistant strains of the pathogen.

3.2 Scab (Venturia nashicola Tanaka & Yamamoto)

Benzimidazole fungicides had been used extensivelysince 1971 for the control of scab, and efficient controlwas achieved initially. In 1975, however, spray applica-tions of benzimidazoles failed to control scab.14Benzimidazole-resistant strains still constitute a highproportion of the population despite the fact that theuse of benzimidazoles was stopped in many orchards.15

DMIs have become a major group of fungicides sincethe mid-1980s, when benzimidazoles lost their efficacy

Resistance Management in Diseases of T op Fruit in Japan 385

in most areas. Public sector organisations like Prefec-tural Research Stations or the extension service, gener-ally advise growers to use DMIs around three times ayear in mixture with other fungicides with a di†erentmode of action. Nevertheless, growers tend to applyDMIs more frequently than recommended in the spraycalendar. These days, growers occasionally complainabout the Ðeld performance of DMIs, although no signsof resistance have been detected from oridinary moni-toring studies.

At Ðrst, baseline sensitivity data on fenarimol wereobtained using fungal isolates which were collectedfrom non-DMI-treated trees in Japan and even inChina.16 In these two cases, mean values ofEC50fenarimol for mycelial growth were 0É199 and 0É12 mglitre~1, respectively. Monitoring of fenarimol sensitivityin 1992 through 1994 revealed a shift to lower fenarimolsensitivity in many strains isolated from DMI-treatedpear orchards. To examine whether efficacy of fenarimolhad also declined, spore samples were collected fromcommercial orchards from which less fenarimol-sensitive strains had been found in tests in vitro.However, in inoculation tests on pear seedlings, fenari-mol still showed adequate control, indicating that theperformance of fenarimol was still maintained in theÐeld.

I will brieÑy mention some of the difficulties experi-enced in monitoring DMI sensitivity in V enturiaspecies. Mycelial growth in V enturia spp. is very slowand we often experienced a decrease in the level of DMIresistance when we stored and/or subcultured pure iso-lates of V . inaequalis and V . nashicola.17 Therefore, it isdoubtful whether our monitoring studies reÑected thereal situation of the DMI sensitivity of the fungal popu-lations from the Ðeld. The long time required for conÐr-mation of fungicide resistance interferes with quick andaccurate decision-making to minimize the controlfailure with the fungicides. It is deÐnitely necessary todevelop more rapid and simpler methods for testingDMI sensitivity with this pathogen. In DMI-sensitiveconidia of V . nashicola, thickening and swelling ofgerm-tubes are frequently observed under the micro-scope in the presence of DMIs,18 but, this method hasnot yet been used in practice.

4 ACTIVITIES OF THE RESEARCHCOMMITTEE FOR FUNGICIDE RESISTANCE

AND OTHERS

This Research Committee was founded in 1991 and it isnow one of the committees authorized and sponsoredby the Phytopathological Society of Japan. A sympo-sium is held regularly on the day following the annualmeeting of the Society. Manuals for testing fungicideand bactericide sensitivity of major pathogens havebeen compiled and will be published in 1997. They will

also cover almost all bibliographies on fungicide andbactericide resistance which have been reported withinJapan since 1971.

Based on the agreement between British and Japa-nese governments, an International Workshop on Fun-gicide Resistance was held in Tsukuba, Japan in 1995,and several references in this paper are from the Pro-ceedings of that workshop.19

REFERENCES

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2. Ohnuma, Y., Sanada, T. & Eguchi, J., Fungicide resistancein Alternaria blotch of apples. Ann. Rept Plant Prot.North Japan, 24 (1973) 70.

3. Eguchi, J., Sasaki, S., Ota, N., Akashiba, T., Tuchiyama, T.& Suzuki, S., Studies on polyoxins, antifungal antibioticsIX. Mechanism of action on the diseases caused by Alter-naria spp. Ann. Phytopath. Soc. Japan, 34 (1968) 280È8.

4. Pepin, H. S., Strains of Botyris cinerea resistant tobenomyl and captan in the Ðeld. Plant Disease, 66 (1982)404È5.

5. Osanai, M., Fukushima, C. & Tanaka, Y., Testingmethods for captan sensitivity in Alternaria alternataapple pathotype. Ann. Rept Plant Prot. North Japan, 39(1988) 259.

6. Koike, H., Resistance of V enturia inaequalis to benzimid-azoles and sensitivity distribution of V enturia inaequalis tothe sterol demethylation inhibitor fenarimol in Nagano.Abstr. Int. W orkshop Fungic. Resistance : a Factor L imit-ing Quality Food Production, Tsukuba, Japan 1995, p. 20.

7. Nishida, H. & Baba, T., Appearance of fungicide-resistantV enturia inaequalis. Ann. Phytopath. Soc. Japan, 41 (1975)127.

8. Sawamura, K., Fujita, T. & Goto, H., Tolerance of applescab fungus V enturia inaequalis to thiophanate methyland benomyl fungicides in Japan. Bull. Fac. Agric. Hiro-saki Univ., 26 (1976) 1È9.

9. Hermann, M., Szith, R. & Zinkernagel, V., Reduced sensi-tivity of some scab isolates (V enturia inaequalis) fromSteiermark (Austria) to EBI-fungicides. Gartenbauwissens-chaft, 54 (1989) 160È5.

10. Nishimura, S., Kohmoto, K. & Udagawa, H., Field emer-gence of fungicide-tolerant strains in Alternaria kikuchianaTanaka. Rept T ottori Mycol. Inst. (Japan), 10 (1973)677È86.

11. Udagawa, H., Kohguchi, T., Otani, H., Kohmoto, K. &Nishimura, S., A decade of transition of polyoxin-tolerantstrains of Alternaria alternata Japanese pear pathotype inthe Ðeld ecosystem. J. Fac. Agric. T ottori Univ., 18 (1983)9È17.

12. Watanabe, H., Occurrence of Alternaria alternata tolerantto iprodione in Japanese pear orchards. Ann. Phytopath.Soc. Japan, 58 (1992) 609.

13. Sanada, T., Induced mutation breeding in fruit trees :Resistant mutant to black spot disease of Japanese pear.Gamma Field Symposia, 25 (1986) 87È108.

14. Ishii, H. & Yamaguchi, A., Tolerance of V enturia nashi-cola to thiophanate-methyl and benomyl in Japan. Ann.Phytopath. Soc. Japan, 43 (1977) 557È61.

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386 Hideo Ishii

phenylformamidoximes to control benzimidazole-resistantV enturia nashicola on Japanese pear. Plant Pathol., 41(1992) 543È53.

16. Ishii, H. & Yamazaki, H., Monitoring for fenarimol sensi-tivity in Ðeld strains of V enturia nashicola. Abstr. 11th Int.Symp. Modern Fungic. Antifungal Compounds, 1995.

17. Ishii, H., Homma, F., Miura, T., Suzaki, H. & Van Raak,M., Resistance of V enturia inaequalis to DMIsÈ

Phenotypic instability and genetic control. Abstr. 6thInter. Congr. Plant Pathol., 1993, p. 92.

18. Ishii, H., Yamazaki, H., Miura, T., Haga, K. & Tomita, Y.,Monitoring for DMI sensitivity in V enturia nashicola andthe methods. Ann. Phytopath. Soc. Japan, 61 (1995) 607È8.

19. Anon, Abstr. Int. W orkshop Fungic. Resistance : a FactorL imiting Quality Food Production, Tsukuba, Japan 1995,pp. 66.