Higa, S.Y.

Namba, R.

Proceedings of the Hawaiian Entomological Society

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Vol. XXI, No. 2, December, 1972 235Susceptibility of Varieties of papaya and Cucumberto the Papaya Mosaic Virus When Transmittedby the Green Peach Aphid1R. Namba and S. Y. HigaUNIVERSITY OF HAWAIIHONOLULU, HAWAIIPapaya mosaic is a very destructive disease of papaya, Carica papayaL., in Hawaii. No effective control program has been developed otherthan complete eradication of diseased plants. Attempts to determinewhether there is a genetic basis for resistance among the caricaceous plantsrevealed that several species of Carica were resistant to the papaya mosaicvirus (Malaguti, Jimenez, and Horovitz, 1957; Capoor and Varma,1961; Cook and Milbrath, 1971). However, no infraspecific taxon ofC. papaya has been found to be resistant to this disease according to Cookand Zettler (1970) who tested 90 "accessions" of C. papaya from variouspapaya growing areas of the world. All these tests were conducted bymeans of sap inoculation and they possibly may not reflect the infectivityof the papaya mosaic virus when transmitted by means of aphid vectors.Reviews by Swenson (1963, 1969) cite many examples of the variationin susceptibility of plant species and varieties to inoculation of plant virusesby aphids. With papaya mosaic virus, Namba and Kawanishi (1966)found that papaya, cucumber {Cucumis sativus L.), and watermelon (Citrullusvulgaris Schrader), which are important hosts of this virus in Hawaii, wereequally susceptible to the virus when transmitted by the green peachaphid, Myzus persicae (Sulzer). In line with this finding, the present studywas carried out to test the susceptibility of some infraspecific taxa of papayaand cucumber to the papaya mosaic virus when transmitted by aphids.Materials and MethodsThe plants tested were papaya cultivars, Line 8, Kapoho Solo, andWaimanalo Solo; cucumber, variety Colorado Long, and genetical linesLehua 64A1 and 70-A-68 provided by Dr. James C. Gilbert, HorticultureDepartment, Hawaii Agricultural Experiment Station. The geneticallines are resistant to cucumber and watermelon mosaic. The papayaplants tested were in the 4-6 leaf stage, and the cucumber plants in thetwo-leaf cotyledon stage. All plants were grown in sterilized soil in 3-inch plastic pots.Published with the approval of the Director of the Hawaii Agricultural ExperimentStation as Journal Series No. 1473.236 Proceedings, Hawaiian Entomological SocietyThe virus sources were leaves of young diseased Line 8 papaya plants.The leaves were excised and the petioles kept in water during the tests.The vectors used were late-instar apterae of the green peach aphid.They were the progeny of a single female. Colonies of this female havebeen maintained in our greenhouse for many years on mustard cabbage,Brassica jumea (L.) Cosson. The aphids were kept without food in glassvials for at least 1/2 hour before the tests. Each aphid was watched witha hand lens and the feeding was timed with a metronome so that eachaphid had an acquisition probe of 10-30 seconds on the virus source. Oneaphid was then placed on each of the test plants.It was not feasible to test all the varieties of a plant species in onetest period. Consequently one variety was selected as the standard to whichthe others were compared. Line 8 was the standard for papaya andColorado Long for cucumber. One hundred plants per variety or a totalof 200 plants were used per trial. Five trials were conducted per comparison. The trials were conducted on separate days using differentvirus sources.Results and DiscussionCucumber (Table 1). In all trials comparing Colorado Long andLehua 64A1, there were more Lehua test plants infected than those ofColorado Long. In 4 trials the chi-square values indicated that thedifferences were significant. In the remaining trial the chi-square was3.58 which almost reaches the 5% level. The sum of the chi-squares ofthe 5 trials and the pooled chi-square were large which further indicate thattable 1. Comparisons of the susceptibility of cucumber varieties to thepapaya mosaic virus when transmitted by the green peach aphid.Trial Variety Chi-square12345SumPooled12345Colorado Long19l32485425178chi-square = 54.56**Colorado Long412936858Lehua 64A12854626750261Heterogeneity chi-square70-A-6848374118503.5818.61**7.74**7.06**24.02**61.01**= 6.451.692.410.846.13*2.26Sum 172 194 13.33*Pooled chi-square = 3.89* Heterogeneity chi-square = 9.44l — number of test plants infected out of 100 tested.Vol. XXI, No. 2, December, 1972 237differences do occur between the varieties and that there is a predominanttendency for the difference to be one way. The heterogeneity chi-squarewas not significant indicating the consistency of the results.The tests provide strong evidence that differences do occur betweenColorado Long and Lehua 64A1 in their susceptibility to inoculation ofthe papaya mosaic virus by the green peach aphid and that Lehua is moresusceptible than Colorado Long.In 4 trials comparing Colorado Long and 70-A-68 there were more70-A-68 test plants infected than those of Colorado Long but only in onetrial the chi-square test indicated that the difference was significant.This evidence for the predominant tendency of the difference to be one way,i.e., 70-A-68 more susceptible than Colorado Long was supported by thepooled chi-square which was significant. However, the heterogeneitychi-square almost reaches the 5 % level of significance indicating that thesedifferences may not be consistent. Thus it would be desirable to conductadditional tests to obtain more conclusive evidence to show that differencesbetween these two varieties do exist.Papaya (Table 2). In 3 trials comparing Line 8 and WaimanaloSolo there were more Line 8 test plants infected than those of WaimanaloSolo. In one of the trials the chi-square value indicated that the difference was significant. The pooled chi-square, however, was not significantindicating that there is no predominant tendency of Line 8 being moresusceptible than Waimanalo Solo. In one of the 2 trials in which moreWaimanalo Solo test plants were infected than those of Line 8 the chi-squarewas significant. It appears that the variations in susceptibility betweentable 2. Comparisons of the susceptibility ofpapaya varieties to thepapaya mosaic virus when transmitted by the green peach aphid.Trial12345SumPooled chi-square12345Line 833l25305015153= 2.75Line 83452382329VarietyKapoho Solo4127173714136Heterogeneity chi-squareWaimanalo Solo4649281930Chi-square2.320.1211.07**6.70**0.0220.23**= 17.48**5.33*0.254.48*0.800.01Sum 176 172 10.87Pooled chi-square = 0.11 Heterogeneity chi-square = 10.76*1 = number of test plants infect out of 100 tested.238 Proceedings, Hawaiian Entomological Societythe two cultivars are inconsistent and the large heterogeneity chi-squaresubstantiates this. These trials provide no clear evidence that one cultivaris more suceptible to the papaya mosaic virus than the other when transmitted by the green peach aphid.In 3 trials comparing Line 8 and Kapoho Solo there were more Line8 test plants infected than those of Kapoho Solo. In two of these trialsthe chi-square values were significant. In the two trials where moreKapoho Solo test plants were infected the evidence for the differences werenot significant. It seems that Line 8 is more susceptible than KapohoSolo, but the pooled chi-square is not significant suggesting that there isno predominant tendency for one cultivar to be more susceptible than theother. Furthermore the heterogeneity chi-square was significant, indicating that the evidence for the differences were inconsistent. Here againthe trials provide no clear evidence that one cultivar is more susceptiblethan the other to the aphid transmission of the papaya mosaic virus.In interpreting the results of this study it must be remembered thatthe degree of susceptibility was based on the transmissibility of the virusto one test plant by one aphid. If more than one aphid were used pertest plant the difference in the degree of susceptibility between the twocompared varieties would be different. The probability of transmissionincreases exponentially with the increase in the number of aphids used pertest plant as per formula p = 1 — qx were x is the number of aphids usedin a group per test plant and q equals the probability of not obtaining aninfection when one aphid is used (Sylvester, 1954). As the probability oftransmission approaches 1.0 the difference in the susceptibility of the twovarieties becomes smaller and eventually if enough aphids are used pertest plant both varieties would have 100% transmission and there wouldbe no evidence for a difference in susceptibility.LITERATURE CITEDCapoor, S. P. and P. M. Varma. 1961. Immunity to papaya mosaic virus in the genusCarica. Ind. Phytop. 14: 96-97. (Rev. Appl. Mycol. 41: 322, 1962).Cook, A. A. and G. M. Milbrath. 1971. Virus diseases of papaya on Oahu (Hawaii)and identification of additional diagnostic host plants. Plant Disease Reptr. 55(9):785-788.Cook, A. A. and F. W. Zettler. 1970. Susceptibility of papaya cultivars to papayaringspot and papaya mosaic viruses. Plant Disease Reptr. 54(10): 893-895.Malaguti, G., H. Jimenez and S. Horovitz. 1957. Pruebas de transmision del mosaicode la lechosa a otras especies de Carica. Agronomia Trop. (Maracay, Venezuela)7: 23-31. (Rev. Appl. Mycol. 38: 28, 1959).Namba, R. and C. Y. Kawanishi. 1966. Transmission of papaya mosaic virus by thegreen peach aphid. J. Econ. Entomol. 59(3): 669-671.Swenson, K. G. 1963. Effect of insect and virus host plants on transmission of virusesby insects. Ann. New York Acad. Sci. 105(13): 730-740.Swenson, K. G. 1969. Plant susceptibility to infection by insect transmission, p. 143-157. In: Viruses, Vectors, and Vegetation. Edited by K. Maramorosch. Inter-science Publishers, New York, London, Sydney, Toronto.Sylvester, E. S. 1954. Aphid transmission of nonpersistent plant viruses with specialreference to the Brassica nigra virus. Hilgardia 23(3): 53-98.

Susceptibility of Varieties of papaya and Cucumber to the Papaya Mosaic Virus When Transmitted by the Green Peach Aphid

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Susceptibility of Varieties of papaya and Cucumber to the Papaya Mosaic Virus When Transmitted by the Green Peach Aphid

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