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Papers in Plant Pathology Plant Pathology Department

1987

Concentration of Chlorotic Mottle Increased in Mixed Infections with Maize Dwarf Mosaic Virus, Strain B

Karen-Beth Goldberg University of Nebraska-Lincoln

Myron K. Brakke University of Nebraska-Lincoln

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Goldberg, Karen-Beth and Brakke, Myron K., "Concentration of Maize Chlorotic Mottle Virus Increased in Mixed Infections with Maize Dwarf Mosaic Virus, Strain B" (1987). Papers in Plant Pathology. 177. https://digitalcommons.unl.edu/plantpathpapers/177

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Concentration of Maize Chlorotic Mottle Virus Increased in Mixed Infections with Maize Dwarf Mosaic Virus, Strain B

Karen-Beth Goldberg and Myron K. Brakke

Agricultural Research Service. U.S. Department of Agriculture and Department of Plant Pathology, Nebraska Agricultural Experiment Stat ion. University of Nebraska, Lincoln, 68583. Present address of first author: Plant Pathology Department, University oTKentucky, Lexington, 40546. Cooperative investigation of the ARS, USDA, and the Nebraska Agricultural Experiment Station. Research conducted under project 21-3, and published as Journal Series Paper 7939 of the Nebraska Agricultural Experiment Station. Mention of a trademark or proprietary product does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply approval to the exclusion of other products that may also be suitable. The assistance and advice concerning the ELISA technique provided by Drs. Ellen Ball and Manuel Palomar are gratefully acknowledged. Accepted for publication 18 June 1986 (submitted for electronic processing).

ABSTRACT

Coldberg, K.-B.. and Brakke, M. K. 1987. Concentration of maize chlerotic mottie virus increased in mixed infections with maize dwarf mosaic virus, strain B. Phytopathology 77: 162-167.

The concentration of rnaize chlorotic mottle virus (MCMV) was up to chlorophyIl and a lower than normal ratio of chloroplast to cytoplasmic 5.4 times higher in plants infected with both MCMV and maize dwarf rRNA. Purified MCMVhadanextinctioncoefficient of6.7cm2mg'at260 mosaic virus. strain R (MDMV-BE. rhan in plants infected with MCMY nm, an absorption maximum at 258 nm, minimum at 240 nm. and 25% only. The concentration of MDMV-B was the same in doubly and singly RNA. infected plants. Plants infected with both had a reduced level of Additional kqy words: corn lethal necrosis, %ea may.r.

In the United States. a combination of maize chlorotic mottle the disease on chloroplast and cytoplasmic rRNAs. a biochemical virus (MCMY) and wheat streak mosaic virus (WSMV) or maize index of cell damage, is also reported because this information was dwarf mosaic virus (MDMV), strain A or B, causes a disease available as a byproduct of the MCMV assay. known as corn lethal necrosis (CLN) (30). In fields with CLN,yield A preliminary report of these resutts has appeared (14). losses of up to 90% (24) have been reported in north central Kansas and south central Nebraska, where the disease is currently confined MATERIALS AND METHODS despite the fact that all three viruses have different aerial vectors (h,23.26). Viruses. To eliminate possible contaminants, the MCMV stock MCMV is a spherical ssRMA (1 -47 M Da) virus with a capsid isolate was obtained from pIants inoculated with a solution protein of 24.6 kDa (22). It is readily transmitted mechanically removed from the virus zone of a sucrose gradient and diluted to and, experimentally, by six species of chrysomelid beetles (18,23). near the dilution end point. M DMV-B (type strain) was obtained MDMV-B (26). an -transmitted poryvirus. has a capsid from S. Jensen of this department. The viruses were propagated on protein of 35 kDa. an inclusion body protein of 66 kDa (1 9), and a maize inbred B73Ht plants grown in a greenhouse. B73Ht is ssRNA of 3 M Da (17). WSMV is a mite-transmitted, 700-nm moderately susceptible to CLN and has ranked as more susceptible flexuous rod with a capsid protein of 45 kDa, an inclusion body than 60% of inbreds tested in the NCR2 regional uniform inbred protein of 66 kDa (9). and a ssR NA of 2.8 MDa (6). nursery over the last 5 yr (Rrakke, unpublished dare). To reduce Plants with CCN show severe yellowing of leaves, which the chances of contamination, virus cultures were started anew at develops into necrosis beginning at the tips and edges of the leaves monthly intervals from dried leaves kept over CaCll at 4 C. Five and progressing inward. Plants of many corn lines, particularly plants (B73Ht) were grown per 10-cm pot, inoculated by leaf in breds. develop a severe, often fatat disease if they become doubly rubbing 10- 12 days after planting when they were in the early infected when young. three-leaf stage, and harvested 6. 10, and t 5 days after inoculatian. 'The primary goal of this study was to determine if the concen- Sap from virus-infected leaves was used for inoculum after dilution trations of MCMV and MDMV-B were higher in plants with in water to about 1:s for MDMV-B and 1:15 for MCMV and mixed infections than in those with singte infections. The effect of addition of a few percent Celite. MCMV purification. Infected Ieaves were harvested 12- 3 5 days The pubhcmtlon costs of this srtlele were defrayed In part by page charge payment. This after inoculation and ground in 0.1 M K>HPO4(1 g per 3 ml) in a artlcle must therefore be hereby marked "adv%rt~sement"in accordance w~th18 U S.C. % 1734 solely to nnd~cate~hls (act blender for 2 min. The extract was filtered through cheesecloth and centrifuged in a Beckman R centrifuge with for 5 This article is in the public domain and not copyrightable. It may be freely J21 a JA20 rotor reprinted with customary crediting of the source. The American min at 8,000 rpm at 5 C. One-motar CaClz (1/5Q volume) was Phytopathological Society387. added to the supernatant, which was stirred for 20 min. and then

162 PHYTOPATHOLOGY

Published in PHYTOPATHOLOGY 77:2 (1987), pp. 162-167. Published in PHYTOPATHOLOGY 77:2 (1987), pp. 162-167. tions. The MCMV concentration was, therefore, determined by ELlSA (Fig. 4). The concentrations of MDMY-B were 1.5-2.5 measuring viral RNA present in density gradients of whofe cell times higher 10 days after inoculation than 6 days after inoculation RN A preparations. MCMV RN A sedimented faster t han the cyto- and fell slightly by 15 days after inoculation. plasmic and chloroplast rRNAs and was easily detected in whole Extracts from MDMV-B-infected plants were partly purified cell nucleic acid preparations (Fig. 3). and assayed by SDS-PAGE for inclusion body protein (66 kDa) The concentration of MCMV genomic RNA resulting from (19). There was no apparent change in concentration of the 66 kDa single and mixed infections differed dramatically (Table 1, Fig. 4). protein in singly versus doubly infected plants (data not shown). The MCMV-infected third and terminal leaves had concentrations Effect of virus infection on rRNA and chlorophyll. There was much lower than the corresponding CLN-infected leaves. Except little change in amounts of 23 Sand 28 S rRNAs or In the 23 S/28 S For the third leaf 6 days after inoculation, the CLN-infected leaves rRNA ratio of the MCMV and MDMV-R plants compared with had 1.7-5.4 times as much MCMY-RNA as did singly infected healthy plants (Table 1). However, the amount of 23 S rRNA and leaves. All MCM V- and CLN-infected leaves, except the MCMY- the 23 St28 S rRNA ratio was usually reduced slightly in CLN- infected third leaf. had less MCMV RNA 15 daysafter inoculation than at 10 days. There was no consistent difference between singly and doubly infected plants in the concentration of MDMY-B determined by

WAVELENGTH, nm Flg. 2, Absorbance spectra of purifred maize chlorotic mottle virus A, in buffer 8,after disruption in SDS. and C, difference spectra.

Fig. 1. Absorbance (254 nm) scans of centrifuged gradfcnt columns with extracts orcorn leaves or of purified maizc chlorotic mottle virus (MCMV). The peaks are at depths expected ror rrec ribosomal or viral RNA. Peak I rcpresentsrhe 16Sand I8 S rRNAs, pcak 2 thc23 S rRNA,peak 3 the28 S rRNA,and peak4 the MFMV RNA. A. Anextractofadouhly (corn lcthal necrosis) rnfected prant in buffer C (0.I M IN HJ)~COI. 0.3 M NHoCI. I mM EDTA, 1% SDSpH 9.4). %. Anextract ofdoubIy infected plant in NaGPS buffer (0.3 M NaCI, 0.05 NalH Pod. 0. I M glycine. 1 mM EDTA. pH 9.4). C C. An extract or healthy corn leaves with purified MCMV added during OEPTH grinding in buffer C. D, An cxtract of healthy corn leaves with purihed Ag. 3. Absorbance (254 nm) scans of centrifuged gradicnt columns MCMV addcd during grinding In NaGPS. E. Purlfled MCMV diluted in containing nucleic acids from maire leaves that were A. Uninfected. B, buffer C. F. Purificd MCMV diluted in NaGPS. The same amount of Infected with maize chlorotic mottlc v~rus(MCMV), or C, With MCM V purified virus was present in ssmples placed on gradients C. D, E, and F. and maize dwatF mosaic virus, strain B, (MDMV-B) rcpresenring corn representing purified virus from five times the amount of leaf tissue placed lethal necrosis. Each pattern has nuclcic acid from 9.5 mg of leaves. an gradtents A and B (demonstrating the loar of virions during Ccntriiuged for 3 hr at 54,000 rprn at 15 C in a 7530% (wlv) sucrose purification). The arca of the MCMV RNA pcaks corresponds to 20, 19, gradient in thc Beckmun SWhO rotor. Peak 1 reprcscnts thc 16 5 and 18 5 20, and 19 pgvirus Tor patterns C,D, E,and F, respectively. Centrifuged for rRNAs, peak 2 the23 S~chloroplast)rRNA. peak 3the 28 S{cytoplasmic) 3 hr a1 54.000 rpm at 15 C in a 7.5-30s (wlv) sucrosc gradicnt in a SW6O rRNA, and peak 4 the MCMV RNA. The concentration of MDMV-R FkckrnanPublished rotor. in PHYTOPATHOLOGY 77:2 (1987), pp. 162-167. RNA was too low to be detected on the gradient column in pattern C. 164 PHYTOPATHOLOGY infected plants. The occasional increase in concentration of 28 S degradation or loss or virions during purification or rRNA on a wet weight basis does not mean that each cell had more unencapsidated MCMV-RNA in infected cells, In an attempt to 28 S rRNA because cells in infected leaves might have beensmaller distinguish between these possibilities. extracts of infected kcavcs in than in healthy leaves. The standard errors in the rRNA NaGPS buffer were assayed for viral RNA. Many viruses. e.g.. concentrations reflect primarily the variation from oneexperiment tobacco mosaic virus. sauthcrn bean mosaic virus. and barley to another rather than variation in the duplicate dcnsity gradient stripe mosaic virus, are stable in NerGPS (7.12.25). whereas other centrifugation assays (Table I). A ratio of about 4: 1 of chlorophyl1 a: chlorophyll b (milSigrarn per gram of leaf tissue) was observed in all infected and uninfected leaves6. l0,and I5 days after inoculation (data not shown). There was no reduction in chlorophyll in infected leaves h days aftcr inoculation, but reductions were consistently found in Infected leaves tO and 15 days after inoculation, with the largest reduction (50%) in CLN-infected terminal leaves 15 days aftcr inocuIation 5). (Fig. CLN-1 MCMV-RN A inrectivity. MCM V RN A recovered from sucrose gradients was infectious and plants infected with it had typical symptoms. Partial purification followed by SDS-PAGE of proteins confirmed the presence of MCMV virions in the RNA MCMV -3 inoculated plants. The 10-rnin thermal inactivation point OF MCMV RNA was 85 C in buffer D. 'NICMV-T Stability of MCMY. Tissue contained more viral RNA than could be accounted for in terms of virion yield. This could reflect

TARLE 1. Cnnccntrationsmoi ribosomal and rnai~echlorotic mot tlc virus 0.3 . (MCMV) RNAs in plants with MCMV onty, with make dwarf mosaic MDMV-I virus, strain R( M DMV-R). orwith both viruscs(corn lethal necrosis. CI+N) CLN-I MDMV-T Post-inoculation ea 0.1 CLN-T 23SrRNA MCMV day, Peaf location, and treatment 23 S rRNA 28 S rRNA 28 S rRNA RNA Day b Third leaf DAYS AFTER INOCULATION Healthy Fig. 4. Concentration of virtonr in tarn plants with single IMCMV or MCMV MLIMV) or mixed inrection (CLN). A. Uonccntration (mg'g wct wcipht) MDMV-R" of matre chtorotic motlle (MCMV)calculated as five time5 ~hcomount or CLN viral genomic RNA measurcd by density gradient centrifugation analysis of Terminal leaf total nucleic acid. B. Concentration of maipc dwarf rnosalc virus, strain R. Healthy (MCMV-R) measured by EtlSA. Concentrations are given for leiif three MCMV' (-3) and the youngest or terminal (-*TI tcaC at h, IU, and 15 dayr after CLN inoculation. Day I0 Third lcai Healthy MCMV M DMV-B' C1.N Tcrminal leaf Hcalthy MCMV M DMV-B" C1.N Day 15 Third lcai Healthy MCMV MDMV-B' CLN' Terminal leaf HcaIthy MCMV M DMV-R" CLN 'Concentrationr given in milligrams of RNA per gram (wet weight) of leaf tissue. '~eanand standard error of the mean for measurements madc on threc gradicnt columns Cnr each of zhrcc cxpcriments, except where specified otherwise. 'Mean and standard error of the mean of ratios calculated From areas of 23 S and 28 S rRNA peaks of each gradient. DAYS AFTER INOCULATlON 'values = mcan and standard error of thrcc mcasurcments for one experiment. Fig. 5. Concentrations ofchlorophyll a in milIigrams per gram of leaitissue 'Values = mean and ~tandardcrsor or seven measurements for three for thethird leaf A and tcrrnrnal IcalB of corn plants 6. 10. and 15 daysafter experimenrs. inoculation w~th(1) buffer (healthy), (2) maizc dwarf mosaic vlrur, (3) 'values = mean and standard error of eight measurements for three maizechloroticmottlevirus, or(4)a mixture of the two viruses(corn Icthal experiments.Published in PHYTOPATHOLOGY 77:2 (1987), pp. 162-167. necrosis).

Vol. 77, No. 2, 1987 165 nucleoproteins. such as ribosomes. arc not. The RNAs released Effect of mixed infection on virion concentration and rRNA. from these nucleoproteins are stablc in NaG PS buffer, which is an Thc effects of dual infection with MCMV and MDMV resembIed erfectivc inhibitor of leaf RNase. MCMV virions were unstable in the results of Rochow and Ross (27) with PVX and PVY. In both NaGPS; as much MCMV RNA was found in extracts of infected systems. there were large Increases of one virus (PVX or MCMV) leaves ground in NaGPS buffer as in those ground in SDS- and no corresponding increase of the (PVY or MDMV- containing buffer (buffer C) (Fig. I). Similar results were obtained B). The higher concentration of MCMV in the mixed infection when purified virus was added to healthy leaves before they were could be due either to a higher concentration of virus in each ground, and when purified virus wasdiluted into NaGPS buffer or infected cell or to infection of n higher proportion of cells than in a buffer C (Fig. I). singly infected leaf. If the first hypothesis is correct, then Additional indications that at least a portion of the MCMV MDMV-B may suppress the regulatory system that normally virions arc unstable were given by the difficulty of scaling up would limit the concentration of virus in a cell. as suggested by purification procedures and by the repeated observation of two Ross (28). If the second hypothesis is true, then MDMV-B would zones in 30-60s (wlv) sucrose gradients. Only the lower zone facilitate movement of MCMV from cell ta cell. fluoresced in the presence of ethidium bromide, suggesting tha~it Ir is likely that the increased MCMV concentration accounts For containcd swollen, permeable particles. In addition, particles of the synergism seen in mixed infections and for the severe symptoms the lower zone in the sucrose gradient were more thermally labile of CLN as compared with the relatively mild symptoms and low than those of the upper zone. concentration (less than 0.4 mg of virus per gram of leaf tissue) of MCMV and M DMY. There was little change in MDMV-R concen- DISCUSSION tration in plants with mixed infection. Even though Ieaves of doubly infected plants were yellow I0 Difference of MCMY from sobemoviruses. MCMV has been days after inocutation and were yellow with necrotic edges 15 days assigned to the sohcrnoviruses on the basis of its morphology, after inoculation (and by then contained about one-third as much molecular weights of RNA and capsid protein, and beetle viral RNA as total rRNA) there was still relatively little change in transmission. in which it resembles the type member. southern their rRNAs. The 23 S/28 S ratio of rRNA provides information bean mosaicvirus(SBMV) (15.29). MCMVdiffers from SBMV in on the differential effects of the disease on chloroplast and its lack of seed transmission (3). cytoplasmic ribosomes and is independent of grinding errors and Our observations indicate additional differences between the number of plant cells per gram of tissue. The primary effect of MCMV and SRMV. The apparent instability of MCMV during MCMV infection, eitheralone or with MDMV-B,does not seem to purif~cationis unusual for a virus with a high dilution end point be on either chloroplast or cytoplasmic ribosomes. ( 10-"10-~) and thermal inactivitation point (85 C) (1 5). In contrast The RNA extractian procedure in a high salt, high pH buffer to the erratic instability of MCMV, SBMV has been the most allows for rapid and simuttaneous analysis of the concentration of consistently stable virus worked with in this laboratory. A second, MCMV and rRNA on gradients. This assay measures intact but possibly related difference, is the lower concentration of genomic RN A (Fig. 3), whereas an assay based on partly purified aromatic amino acids in MCMV than in SBMV.The difference virions (Fig. 1) measures encapsidated (degraded as well as intact) spectrum of SBMV (not shown) indicates that it has more than RNA, and ELISA measures capsid protein whether part of a virion twice as many aromatic amino acids as MCMV (Fig. 2) or brome or not. Agreement between these assays may conceivably vary with mosaic virus (spectrum not shown). For SBMV and brome mosaic the stage of infection and environmental conditions which might virus, this conclusion is supported by published data (I, 13). The influence either efficiency of encapsidation'or in vivo degradation low concentration of aromatic amino acids in MCMV, shown by of the viral RNA. inflections at 286 and 295 nrn in the difference spectrum, may contribute to the instability of the virions because aromaticamino LiTElRATURE CITED acids have been implicated in protein-nucleic acid interactions 1. Agrawal, H. 0,and Tremainc, J. H. 1972. Proteins ofcowpea chlorotic (10.16). This difference in stability is directly illustrated by the mottle, broad bean mottle, and brome mosaic viruses. Virology disintegration or MCMV in NaGPS buffer in which SRMV 473-20. remains intact (7). Finally MCMV differs from SBMY in 2. Amon. D. 1. 1949. Copper enzymes in isolated chloroplasts. extinction coefficient (5.8 for SBMY, 6.7 lor MCMY). and in Polyphenolaxidase in Beta vulgaris. Plant Physiol. 24: 1-1 5. percentage of RNA (21% for SBMV,25% for MCMY). 3. Bockelman. D. L,Clafl in, L. E.. and Uyen-1010,J. K. 1982. Host range Estimation of RNA content from Az60,zson, ratios is unreliable and seed-transmission studies of rnaizechlorotic mottle virus in grasses and corn. Plant Dis. 66:2 16-218. for viruses with low content of aromatic amino acids such as 4. Brakke, M. K. 1963. Photomctric scanning of centrifuged density MCMV and brorne mosaic virus. The cstimatien of RNA content gradient columns. Anal. Biochem. 5:271-283. of MCMV reported here is based on determinations of virus 5. Brakke, M. K. 1964. Nonideal sedimentation and the capacity of concentration by dry weight and of separated. hydralyzed RNA by sucrose gradient columns for virus in density-gradient centrifugation. absorbance. Arch. Biochem. Biophys. 107:388-403. Effect of disease on chlorophyll. The chlorophyll results (Fig. 5) 6. Brakke, M. K. 1971. Wheat streak mosaic virus. No. 48 in: indicated less reduction than cxpected from the appearance of the Descriptions of Plant Viruses. Commonw. Mycol. lnst., Assoc. Appl. leaves. The lerminal leaves of doubly infected plants were bright Riologists, Kew, Surrey, England. yellow at 10 and 15 days after inoculations and appeared to have na 7. Brakke, M.K. 1972. Slowly sedimenting infectiousentities of southern much bean mosaic virus. Virology 50:669-680. chlorophyll, but assay showed they had 50% as chlorophyll 8. Rmkke. M. K. 1979. 1Jltravloiet absorption spcctra and difference as healthy leaves. Although doubly infected third leaves had mild spcctra of barley stripe mosaic and tobacco mosaic viruses in buffer symptoms they had relatively high concentrations of MCMV and sodium dodecyl sulfate. Virology 98:76-87. (0.60.7 mg per gram of tissue) (Figs. 4 and 5) and about a 25% 9. Brakke, M. K.. Ball, E., Hsu, Y.-H., and Joshi, J. 1984. Non-capsid reduction in chlorophyll. protein associated with wheat steak mosa~cvirus infection. (Abstr.) In cont cast, MCMV singly infected terminal leaves developed Phytopatholmgy 74:860. anly mild symptoms late in infection. The chlorophyll assay 10. Burley, S. K.,and Petski, G. A. 1985. Aromatic-aromatic interaction: showed only a 5% reduction from healthy leaf tissue accompanied A mechanism of protein structure stabilization. Science 22923-28. by a tow virion concentration of less than0.25 rng pergram(Figs. 4 I I. Clark, M. F.,and Adams. A. N. 1977. Characteristics of the microplate method of enzyme-linked immunosorbent assay for the detection of 5). and plant viruses. J. Gen. Virol. 343475483. At 20 days after inoculation, mild symptoms continue on 12. Dicner, T. 0. 1962. lsolatian of an infectious. ribonucleasc-sensitive MCM V-infected leaves, but the daubly infected leaves become fract~onfrom tobacca leaves recently inoculated with tobacco mosaic necrotic at the tips and the edges and the plants were stunted. virus. Virology 16: I40-146. MDMV-R maintained a noticeable mosaic through 20 days 13. Ghabrial. S. A,. Shepherd, R. J., and Grogan. R. G. 1967. Chemical post-inoculation,Published in PHYTOPATHOLOGY 77:2 (1987), pp. 162-167. properties of three strains of southern bean mosaic virus. Virology 166 PHYT OPATHOLOGY 33: 17-25. 22. Lomrnel. S. A. 1985. Partial characteri7ation oCmaisc chloratic mottle t4. Goldberg. K.-8.. and Brakke. M. K. 1985. Concentration of make virus. (hbstr.1 Phytopathology 75:1292. chlorotic mottle virus (MCMV) and mai7.e dwarf mosaic virus 23. Nault, L. R.. Stycr. W. E.. Coffey. M. E., Gordon, D. T.. Ncgi. 1,. S.. (MDMV-8) in corn lethal necrosis. (Abstr.) Phytopathology 75: 1357. and Niblett. C. L. 197H. Transmission of mai7c chlorotic rnottlc virus 15. Gordon, D. T.,RradTute. O E.. Cingcry. R. E., Nault, I-. R.. and by chrysomelid beetles. Phytopathology 68:I07 1-1074. Ilyemoto. J. K. 1984. Maize chlorotic mottle virus. No. 284 in: 24. Kib1ett.C. I,.. and Clatlin, L. E. 197H. Corn lethal necrosish ncw vtrus Dcscriptionr of Plant Viruses. Commonw. Mycol. Inst. Assoc. Appl, disease of corn in Kansas. Plant Dis. Rep. 62: 15- 19. B~ologists,KCW. Surrey, England. 25. Palomar, M, K.. and Arakkc, M. K. 1976. Concentration and 16. Helene. C., and Mauri~ot,.I. C. 1981. Interactions of oligopeptider infectivity of barley stripe mosaic virus in barley. Phytopathology wrth nucleic acids. CRC Cri~.Rev. In Biochem. IO:2t3-258. 66: 1422-1426. I?'.Hill, J. H.. and Benner. H. 1. 1976. Properties of potyvirus RNAs: 26. Pirone. T. P. 1972. . No. a8 in: Descriptions of Turnip mosaic. tobacco etch. and maizc dwarf mosaic viruses. Plant Viruses. Commonw. Mycol. Inst. Assoc, hppl. Riologists, Kew. Virology 75:419-432. Surrey, England. 18. Jenscn. S. G. 1985. Laboratory transmission of maize chlorotic mottle 27. Rochow, W. F., and Ross. A. F. 1955. Virus rnul~iplicationin plants virus by three species of corn rootworms. Plant Dis. 69:Xb4868. doubly infected by potato viruscs X and Y. Virology 1: 10-27. 19. Jensen, S. G., Long-Davidson, B., and Seip, L. 1986. Size variation 28. Ross, A. F. 1974. Interactions of viruscs in thc host. Pages 247-2h0 in: among proteins induced by sugarcane mosaic viruses in plant tissue. Virus Diseases of Ornamental Plants. R. H. tawson and M. K. Phytoparhology 76528-532. Corbett, eds. Society for Horticultural Science, The Hague. Thc 20. Jensen. S. G.. Palomar. M. K., Rall, E. M., and Samson, R, 1985. Netherlands. Factors influencing virus titer in maize dwarf mosaic virus-infected 29. Tremaine, J. H., and Hamilton. R. 3. 1983. Southcrn bcan mosaic sorghum. Phytopathology 75:1 132-1 136. virus. No. 274 in: Descriptions of Plant Viruses. Commonw. Mycol. 21. Laemmli. IJ. K. 1970. Cleavage of structural proteins during the Inst. Assoc. Appl. BioIogists, Kew, Surrey, England. assembly of the head of bacteriophage T4. Nature (London) 30. Ilyernoto, J. K. 1983. Biology and control or maize chlorotic mottlc 2273680-685. virus. Plant Dls. 67:7-10.

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