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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 6, Num. 3, 1998, pp. 293-301
African Crop Science Journal, Vol

African Crop Science Journal, Vol. 6. No. 3, pp. 293-301, 1998

SEROLOGICAL AND BIOLOGICAL PROPERTIES OF A MAIZE-INFECTING POTYVIRUS FROM ZIMBABWE

J. SIBIYA, C. NYANHETE and E. CHINHEMA1

University of Zimbabwe, Crop Science Department, P. O. Box MP 167, Mt. Pleasant, Harare, Zimbabwe
1ICRISAT/SADC Matopos Research Station, P. O. Box 776, Bulawayo, Zimbabwe

(Received 2 December, 1997; accepted 5 August, 1998)

Code Number:CS98031
Sizes of Files:
      Text: 48K
      Graphics: Photograph (jpg) - 71K

ABSTRACT

A maize-infecting potyvirus isolate was collected from field maize (Zea mays L.) in Mutoko, northeast of Harare, Zimbabwe. The virus isolate was sap-transmissible from maize to maize, sorghum (Sorghum bicolor L. Moench) and pearl millet (Pennisetum americanum L. Leeke), but failed to infect oats (Avena sativa L.), barley (Hordeum vulgare L.) and wheat (Triticum sativum L.). A wide range of annual and perennial grass species were hosts to the virus with the exception of johnsongrass (Sorghum halepense (L.) Pers.), Setaria homonyma and Phalaris minor Retz. which were immune. Symptoms on Rio and Atlas sorghum cultivars were similar to those induced by sugarcane mosaic virus, strain MDB (SCMV-MDB). Serologically, in enzyme-linked immunosorbent assay and electroblot immunoassay, the virus appeared more closely related to SCMV-MDB than to maize dwarf mosaic virus, strain A (MDMV-A). The apparent molecular mass of the local virus isolate capsid protein was 37.1 kDa. Based on these tests, it was concluded that the isolate belongs to the species, SCMV-MDB.

Key Words: Host range, johnsongrass, maize dwarf mosaic virus strain A, molecular mass, serology, sugarcane mosaic virus strain MDB

RÉSUMÉ

Un isolat de potyvirus infectant du maïs a été collectionné dans un champ de maïs (Zea mays L. ) à Mutoko au nord est de Harare, Zimbabwe. L'isolat du virus a été transmise par la sève du maïs au maïs, sorgho (Sorghum bicolor L. Moench) et au millet (Pennisetum americanum L. Leeke), mais il n'a pas réussi à infecter les avoines (Avena sativa L.), orge (Hordeum vulgare L.) et le blé (Triticum sativum L.). Une gamme d'espèces herbeuses annuelles et pérennes étaient hôtes au virus à l'exception de Johnsongrasss (Sorghum halepense (L.) Pers), Setaria homonyma et Phalaris minor Retz. qui étaient immunes. Les symptômes sur certains cultivars de sorgho notamment Rio et Atlas étaient similaires à ceux induits par la souche MDB (SCMV-MDB) du virus de la mosaïque de la canne à sucre. Sérologiquement , en essai immunoabsorbant d'enzyme lié et de l'immunoessai d'electroblot, le virus apparaissait plus proche au SCMV-MDB qu' à la souche A (MDMV-A) du virus de la mosaïque du rabougrissement La masse moléculaire apparente de la protéine capsule de l'isolat du virus local était de 37.1 kDa. En se basant sur ces tests, il a été conclu que l'isolat appartient à l'espèce SCMV-MDB.

Mots Clés: Gamme de hôtes, johnsongrass, souche A du virus de la mosaïque de rabougrissement du maïs, masse moléculaire, sérologie, souche MDB du virus de la mosaïque de canne à sucre

INTRODUCTION

Maize virus surveys conducted between 1992 and 1996 in Zimbabwe revealed the presence of viruses closely related to maize dwarf mosaic potyvirus, strain A (MDMV-A), maize mosaic rhabdovirus (MMV), maize streak geminivirus (MSV), maize stripe tenuivirus (MStV) and sugarcane mosaic potyvirus, strain MDB (SCMV-MDB) (Bonga and Cole, 1997). These viruses were detected by symptomatology, transmission, particle morphology and serological reactions in enzyme-linked immunosorbent assays (ELISA) (Nyanhete, 1996; Bonga and Cole, 1997).

The two potyviruses, MDMV-A and SCMV-MDB were encountered in mixed infections in almost all the samples tested by ELISA (Bonga and Cole, 1997). Host range responses indicated that the two viruses were not typical of the characterised United States (US) isolates of MDMV and SCMV in terms of symptoms and transmission to johnsongrass (D.T. Gordon, unpublished). Strains of MDMV and SCMV from the US have been differentiated mainly by symptoms incited in maize inbreds and rate of transmission by Acyrthosiphon pisum (Harris) and Myzus persicae (Sulzer) (Louie and Knoke, 1975). However, the recognition of the distinguishing symptoms can be problematic since symptom expression depends on the environ-mental conditions (Jensen et al., 1986).

Studies involving the serology of capsid proteins of MDMV and SCMV to identify and differentiate viruses and strains have been conducted (McDaniel and Gordon, 1985; Langham and Toler, 1986; Shukla et al., 1989). Electroblot immunoassays (EBIA) were used to differentiate strains of MDMV-A, D, E and F from isolates formerly designated as MDMV-B and MDMV-O now SCMV-MDB and johnson-grass mosaic potyvirus (JGMV-O), respectively (Lernadon et al., 1993). These strains were reported to have common epitopes in the core capsid protein and specific epitopes on the N-terminal end (Lernadon et al., 1993).

The virus strains can also be identified by use of differentiating hosts. Sorghum bicolor (L.) Moench. cultivars (cv) such as Rio, Atlas, Sart and Tracy were found suitable for differentiating MDMV and SCMV strains (Tosic and Ford, 1983). MDMV-A induced mosaic and no necrotic symptoms on cv. Atlas, while SCMV-MDB induced only local necrosis on inoculated leaves (Tosic and Ford, 1972, 1983). On cv. Rio, MDMV-A induced mosaic symptoms on new leaves whereas SCMV-MDB caused the development of chlorotic streaks on new leaves. Transmission to johnsongrass (Sorghum halepense (L.) Pers.) also differentiates MDMV strains from SCMV-MDB, since it is immune to SCMV-MDB (Gordon and Williams, 1970). Other differential hosts present in Zimbabwe include annual and perennial wild grass species like Phalaris spp., Eragrostis spp., Imperata spp. Panicum spp., Paspalum spp., Saccharum spp., and Sporobolus spp. (Rosekranz, 1981; Johnston, 1983; Drummond, 1984;).

This paper reports on the biological and serological properties of the maize potyvirus isolates found in Zimbabwe with the aim of establishing their identity in relation to the characterised maize potyviruses. Some of the properties will contribute towards an understanding of the virus epidemiology.

MATERIALS AND METHODS

Virus isolates. The virus isolate was obtained from naturally infected maize plants cv. Pioneer 3435 from Mutoko resettlement scheme, 111 km northeast of Harare (Bonga and Nyanhete). The virus was propagated in the greenhouse by sap inoculation to maize plants cv. Pioneer 3435.

Inoculum preparation and mechanical transmission. Inoculum was prepared by triturating young infected maize leaves in a mortar with a pestle in 0.1 M potassium phosphate buffer, pH 7.4 at a ratio of 1:4 (w/v). The sap was filtered through two layers of cheesecloth. Celite powder (545 mesh) was dusted onto the leaves of test-plants at the two- to three-leaf stage. The dusted leaves were then rub-inoculated with the prepared inoculum. Plants similarly inoculated with buffer or extract from symptomless plants served as controls. The plants were left in insect-proof cages in the greenhouse and observations made at 3, 7, 14 and 21 days after inoculation.

Serology. Polyclonal antisera to maize dwarf mosaic potyvirus, strain A (MDMV-A), sugarcane mosaic potyvirus, strain MDB (SCMV-MDB), maize streak geminivirus (MSV), maize stripe tenuivirus (MStV), johnsongrass mosaic potyvirus (JGMV-O), maize mosaic rhabdovirus (MMV) and maize chlorotic mottle sobemovirus (MCMV) were obtained from D.T. Gordon, Ohio State University/OARDC, Wooster, USA., and used in enzyme-linked immunosorbent assay (ELISA) and electroblot immunoassay (EBIA).

Indirect antigen coat enzyme-linked immunosorbent assay. Virus was detected in an indirect antigen coat ELISA as described by Converse and Martin (1990). Samples were prepared as described by Bonga and Cole (1997), with lyophilised infected and fresh non-infected tissue being used as positive and negative controls, respectively. Each test sample was replicated in five wells of the same microtitre plate and tested against all the antisera available. Absorbance values were recorded using an EL311S microplate autoreader (Bio-Tek Instruments, Highland Park, Winooski, Vermont). Values greater than the mean value for healthy controls plus three times their standard deviation (x + 3SD) were considered above the threshold and thus positive for virus presence in the extracts.

Electroblot immunoassay. Viral proteins were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) using the Laemmli system (Laemmli, 1970; Bonga, 1992). Polyvinylidene difluoride (PVDF) membranes (Millipore Corporation, Bedford, MA) were used for EBIA. Transfer was done using the Mini Trans-Blot electrophoretic transfer cell (Bio-Rad Laboratories) at 30V and 40 mA overnight at 4°C.

The PVDF membranes to which the proteins had been transferred were treated with blocker solution, Tris-buffered saline (TBS), pH 7.4 containing 5% (w/v) non-fat dry milk (NFDM) and 0.02% (w/v) sodium azide (NaN3) for 1 hr or overnight at room temperature before adding the specific antiserum. The antibody-enzyme used was goat anti-rabbit IgG (GAR-IgG) conjugated to alkaline phosphatase (Sigma Chemical Company, St Louis, MO). Membranes were washed with TBS, pH 7.4 containing 0.1% NFDM and 0.02% NaN3, three times between steps. Substrate solution, a mixture of p-nitro blue tetrazolium (NBT) (Sigma Chemical Co.) and 5-bromo-4-chloro-3-indolyl phosphate (BCIP) (Sigma Chemical Co.) in AP buffer (100 mM Tris, pH 9.5, 100 mM NaCl, 5 mM MgCl2) was used. The membranes were then washed with distilled water to stop the reaction.

To test for cross-reactivity of antisera, the local virus isolate, MDMV-A and SCMV-MDB were all probed with MDMV-A and SCMV-MDB antiserum separately.

Host range. To identify the natural host range of the Zimbabwean isolate, 10 annual grasses, 5 perennial grasses and 29 cultivated crop species of the Gramineae family were tested. The annual grasses were Dactyloctenium aegyptium (L.) Beauv., Echinochloa colonum (L.) Link., Eleusine indica subsp. africana (L.) Gaertn., Eragrostis aspera, Phalaris minor Retz., Rottboellia conchinchinensis L., Setaria homonyma, S. pumila, S. verticillata (L.) Beauv. and Urochloa panicoides; the perennial grasses were Cynodon dactylon (L.) Pers., Panicum maximum Jacq., Paspalum scrobicultaum L., Rhynchelytrum repens (Willd.) S.E. Hubb and Sorghum halepense (L.) Pers. (provided by D.T. Gordon); Gramineae crop species were oats (Avena sativa L.), barley (Hordeum vulgare), pearl millet (Pennisetum americanum) cv. GAS, wheat (Triticum sativum L.), and 14 sorghum (Sorghum bicolor (L.) Moench) cvs. Rio, Sart, Atlas (provided by D.T. Gordon, OSU), cvs. 90152, SAR33 (dwarf), SAR19, SAR33 (tall), SV-1 (dwarf), SAR16, SAR29, 8959 (tall), FA10 (India) and Framida (provided by C. Mutengwa and P. Kambidzi, University of Zimbabwe) and 11 maize (Zea mays L.) cvs. Cargill 4141, Cargill 4585, Pannar 473, Pannar 6549, Pioneer 3435, R201, R215, SC501, SR52, ZS225 and OH 28 which was provided by D.T. Gordon.

Inoculum was prepared as described for mechanical transmission. Ten plants per host species were inoculated and another 10 plants served as controls and these were inoculated with buffer. The plants were kept in the greenhouse in insect proof cages and symptom development checked at 3, 5, 7, 14 and 21 days after inoculation. ELISA tests were performed on leaves harvested from both symptomless and symptomatic plants. Back assays to maize were also performed on the symptomless plants.

RESULTS

Mechanical transmission. Pioneer 3435 maize plants inoculated with field-collected virus sap developed symptoms within 6-8 days after inoculation. Symptoms were characterised by an irregular light and dark green mosaic or mottle patterns resembling those observed on field-infected maize. None of the test maize seedlings mock-inoculated with extract from symptomless maize plants or with buffer developed similar symptoms or any other symptom following mechanical transmissions.

Serology. The field-collected maize virus isolate and the experimentally infected maize plants reacted positively with antisera to MDMV-A and SCMV-MDB in ELISA and EBIA (Table 1 and Fig. 1). The antisera to the other viruses reacted positively only with the associated homologous antigens used as positive controls. Antiserum to MDMV-A reacted with its homologous capsid protein (CP) as well as SCMV-MDB and the local virus isolate, whereas SCMV-MDB antiserum reacted with its homologous CP and the local virus isolate only, in EBIA (Fig. 1A and B). The molecular mass of the CP of the local virus isolate was 37.1 kDa and the known SCMV-MDB and MDMV-A virus strains were 39.6 and 33.3 kDa, respectively (Fig. 1A and B).

Figure 1: Reactivity of MDMV-A (Fig. 1A) antisera diluted 1:10,000 in TBS, pH 7.4, 1% non-fat dry milk, 0.05% Tween 20, 0.02% sodium azide (NaN3) with MDMV-A, local virus isolate and SCMV-MDB in laanes 2, 3 and 4 respectively. Lane 1 contains Bio-Rad prestained low Mr staandard proteins.

TABLE 1. Mean absorbance (OD 405nm) values for virus-infected maize leaf tissue collected from the field and tested in indirect antigert coat ELISA

Antiserum

Healthy (X + 3SD)1

Test sample

Positive control (Homologous antigen)

MDMV-A

0.84

1.70 ± 0.0142

+3

1.80 ± 0.037

+

MSV

0.71

0.55 ± 0.007

-4

1.67 ± 0.041

+

JGMV-O

0.57

0.53 ± 0.014

-

1.55 ± 0,015

+

SCMV-MDB

0.32

1.75 ± 0.022

+

1.70 ± 0.008

+

MCMV

0.58

0.55 ± 0.002

-

1.37 ± 0.026

+

MStV

0.83

0.55 ± 0.038

-

1.75 ± 0.030

+

MMV

0.45

0.39 ± 0.012

-

1.22 ± 0.034

+

1X + 3SD = mean absorbance of the healthy controls plus three times their standard deviation,
2 Values are means :t: standard deviation, 3+ positive reaction, 4-. negative reaction

Host range. All the maize cultivars inoculated developed systemic infections of the mosaic/mottle type. Pearl millet (Pennisetum americanum L.) cv. GAS, also developed systemic mosaic symptoms. The symptoms started as mild and fine chlorotic speckles at the base of the leaf progressing to the tip. The virus isolated from these plants tested positive with both MDMV-A and SCMV-MDB antisera.

No symptoms were observed on barley, oats and wheat and back assays to maize failed to induce symptoms. Indirect antigen coat ELISA could not detect virus in these plants.

Symptoms on sorghum cultivar Sart, started as local lesions and a light mottle at the base of new leaves. The lesions turned necrotic as the disease progressed. Long and broad interveinal chlorotic specks/streaks were also observed and these coalesced to give a chlorotic background interjected by green areas. In later stages, a distinct red reaction appeared on older leaves.

In cultivar Rio, broad and long interveinal chlorotic specks (bigger than those observed in cv. Sart) were observed on leaves from the base and progressed to the tip. Red spots developed at the leaf tips, margins and mid ribs. In Atlas sorghum, local lesions and necrotic areas developed on new leaves and no mosaic or mottling was observed. Leaf reddening was observed mainly along mid-ribs of lower leaves. Back assays on maize produced a systemic mosaic symptom.

Symptoms observed in Sorghum cultivars 90152, SAR33 (dwarf), SAR19, SAR16, SAR33 (tall), Marcia, and SAR29 were similar. The symptoms were characterised by local lesions on new leaves, 7 days after inoculation. Fourteen days after inoculation, there was a light mottle at the basal end of the new leaves. Chlorotic streaks and islands of dark green mosaics appeared on blades of new leaves. The chlorotic streaks later coalesced to form chlorotic lines.

In sorghum cultivars, SV-1 (dwarf), necrotic lines developed on new leaves and chlorotic areas were observed along leaf margins. In FA10 sorghum, only local lesions and necrosis on new leaves were observed and back assays on maize produced systemic mosaic symptoms. There were no symptoms on sorghum cultivars Framida and 8959 (tall) 21 days after inoculation. No virus was recovered even after back assays onto susceptible maize plants. Results of the indirect ELISA are presented in Table 2. All the sorghum cultivars except Framida and 8959 (tall) tested positive with antiserum to MDMV-A and SCMV-MDB in ELISA tests.

TABLE 2. Reactivity of sorghum cultivars in ELlSA (OD 405nm) with antisera to MDMV-A and SCMV-MDB, after inoculation with the local virus isolate

Sorghum cultivar

MDMV-A

SCMV-ME)B

y1

X + 3SD2 (Healthy)

Status

Y

X + 3SD (Healthy)

Status

Rio

1.54 ± 0.017

0.35

+3

1.23 ± 0.018

0.47

+

Marcia

1.15 ± 0.007

0.44

+

1.17 ± 0.076

0.07

+

SAR33 (dwarf)

1.46 ± 0.014

0.57

+

1.86 ± 0.059

0.45

+

SAR33 (tall)

1.28 ± 0.044

0.48

+

1.44 ± 0.013

0.59

+

SV-1 (tail)

1.78 ± 0.100

0.51

+

1.62 ± 0.063

0.98

+

8959 (tail)

0.29 ± 0.021

0:36

-4

0.37 ± 0.035

0.73

-

FA10

1.70 ± 0.120

0.88

+

1.60 ± 0.120

0.93

+

Sart

1.77 ± 0.040

0.35

+

1.67 ± 0.110

0.45

+

Atlas

1.24 ± 0.034

0.50

+

1.08 ± 0.191

0.29

+

90152

1.87 ± 0.090

0.42

+

1.66 ± 0.096

0.78

+

SAR19

1.97 ± 0.006

0.34

+

1.98 ± 0.002

0.72

+

Framida

0.06 ± 0.006

0.54

-

0.09 ± 0,025

0.23

-

SAR29

1.88 ± 0.016

0.32

+

1.93 ± 0.014

0.47

+

SAR16

1.73 ± 0.070

0.37

+

1.61 ± 0.020

0.49

+

1Y = mean absorbance of the test sample ± standard deviation,
2X + 3SD = mean absorbance of the healthy controls plus three times their standard deviation,
3+ positive reaction,
4. negative reaction

TABLE 3. Reactivity of grass species in ELISA with antisera to MDMV-A and SCMV-MDB after inoculation of the plants with the local virus isolate

Grass species

Growth habit

MDMV-A

SCMV-MDB

Cynodon dactylon (L.) Pers.

P1

+3

+

Dactyloctenium aegyptium (L.) Beauv.

A2

+

+

Echinochloa colonum (L.) Link.

A

+

+

Eleusine indica subsp. Africana (L.) Gaertn.

A

+

+

Eragrostis aspera (Jacq.) Nees

A

+

+

Panicum maximum Jacq.

P

+

+

Paspalum scrobiculatum L.

P

+

+

Phalaris minor Retz.

A

-4

-

Rottboellia cochinchinensis (Lour.) Clayton

A

+

+

Rhynchelytrum repens (Willd.) C.E. Hubb.

P

+

+

Setaria homonyma (Steud.) Chiov

A

-

-

S. pumila (Poir.) Roem & Schult.

A

+

+

S. verticillata (L.) Beauv

A

+

+

Sorghum halepense (L.) Pers.

P

-

-

Urochloa panicoides Beauv.

A

+

+

1P - perennial species
2A - annual species
3+ positive reaction
4- negative reaction

Fourteen grasses tested positive with both MDMV-A and SCMV-MDB in the indirect antigen coat ELISA (Table 3). Of particular interest is Sorghum halepense which was symptomless and did not react with antisera to both MDMV-A and SCMV-MDB. No symptoms developed on Setaria homonyma and Phalaris minor. Back assays to maize failed to induce symptoms. No virus was detected in indirect antigen coat ELISA in these plants.

DISCUSSION

On the basis of mechanical transmission, symptomatology and serology, the relationship of the local virus isolate with the MDMV-A and SCMV-MDB viruses was confirmed.

Serologically, the isolate appeared related to both MDMV-A and SCMV-MDB in ELISA and EBIA. However, antiserum to MDMV-A reacted with its homologous capsid protein as well as SCMV-MDB and the local virus isolate, whereas SCMV-MDB antiserum only reacted with its homologous antigen and the local virus isolate in EBIA. It appears, therefore, that antiserum to MDMV-A contains antibodies reactive with common epitopes on the SCMV-MDB capsid protein, whereas the SCMV-MDB antiserum is quite specific, containing antibodies reactive only with the homologous capsid protein. Cross reactivity has been reported for potyviruses (Gordon et al., 1981; Lernadon et al., 1993). Therefore, based on these observations, the local virus isolate seems to be more closely related to SCMV-MDB and its reactivity with MDMV-A could be because of the cross-reactive antibodies present in the antiserum.

The local virus isolate CP had a different molecular mass from that of the SCMV-MDB and MDMV-A strains from the USA. The molecular mass of MDMV-A CP was higher than that reported by Von Baumgarten and Ford (1976) (30.7 kDa), McDaniel and Gordon (1989) (27.7 kDa) and Lernadon et al. (1993) (29.8 ± 0.8 kDa). However, the value was considerably smaller than the (36.7 ± 1.7 kDa) reported by Langham and Toler (1986). The SCMV-MDB CP migrated as one band only, unlike in other reports where it migrated as several bands (Hill et al., 1973; Lernadon et al., 1993). However, Langham and Toler (1986) reported a single band with a molecular mass of 37.2 kDa. The molecular mass of SCMV-MDB CP in this study was 39.6 kDa. Differences in apparent molecular masses for the same virus capsid proteins are quite common especially among isolates of MDMV and SCMV and therefore provide no means for definitively establishing relationships (D.T. Gordon, personal communication). The differences can be attributed to varying gel concentration, and the technique used to calculate the molecular mass (Hill et al., 1973; Shukla and Ward, 1989) or heterogeneity of the capsid protein (number of migrated bands) due to protein degradation in the plant (Moghal and Francki, 1976; McDaniel and Gordon, 1989).

Failure to infect oats, barley and wheat confirms that the isolate is not JGMV-O or wheat streak mosaic virus (WSMV) both of which induce similar symptoms on maize as MDMV-A and SCMV-MDB (Gordon et al., 1981).

On cultivar Rio, MDMV-A was reported to induce mosaic symptoms on new leaves whereas SCMV-MDB caused the development of chlorotic streaks (Tosic and Ford 1972, 1983). From the results obtained, symptoms on Rio sorghum matched those incited by SCMV-MDB on this cultivar. In Atlas sorghum, local lesions and necrotic areas developed on new leaves but no mottling was observed. MDMV-A was reported to induce mosaic and no necrotic symptoms on Atlas sorghum while SCMV-MDB induced only local necrosis on inoculated leaves (Tosic and Ford, 1972, 1983). This further confirms that the local virus isolate is more closely related to the characterised SCMV-MDB. FA10 sorghum gave similar reactions as Atlas sorghum, that is, development of local lesions and necrosis on new leaves and systemic mosaic symptoms on maize after back assays. This sorghum cultivar could therefore be used as a local lesion host for the local maize potyvirus isolate.

The local potyvirus isolate failed to infect johnsongrass and its host range did not differ substantially from that of SCMV-MDB (Rosekranz, 1981). This observation suggests that the virus is more closely related to SCMV-MDB than MDMV-A, since the characterised MDMV strains infect johnsongrass and the SCMV-MDB does not (Lastra, 1976; Martinez-Lopez, 1977; Rosekranz, 1981; Thottappilly et al., 1993).

Based on all these observations, it can be concluded, therefore, that the local maize potyvirus isolate belongs to the species, SCMV-MDB. All the grasses that were infected by the local virus isolate could serve as reservoirs of the virus and also encourage vector multiplication in the absence of cultivated crops. This is very important in the epidemiology of the virus and control of these grasses could reduce transmission.

ACKNOWLEDGEMENTS

The authors would like to thank D.T. Gordon for supply of lyophilised infected leaf tissue and antisera to the viruses as well as seed, C. Mutengwa, P. Kambidzi for supply of seed. Financial support was provided by The Rockefeller Foundation, grant No. GA AS 9409 and GA AS 9240.

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Copyright 1998, African Crop Science Society


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