Sugarcane Mosaic Virus in Kenya

RAYMOND LOUIE, Research Plant Pathologist, USAID/USDA-SEA, Plant Pathology and Nematology Division,Kenya Agriculture Research Institute, Nairobi, Kenya

ABSTRACTLOUIE, R. 1980. Sugarcane mosaic virus in Kenya. Plant Disease 64:944-947.

Surveys for sugarcane mosaic virus (SCMV) in maize (Zea mays L. subsp. mays) were made in 34 of41 districts in Kenya. SCMV was found in 20 districts and only in the western plateaus, CentralHighlands, and Rift Valley. Provinces with high incidence of SCMV included Nyanza (15.2%), RiftValley (15.8%), and Western (19.6%). SCMV was not found in Coast or Nairobi provinces. Theincidence of SCMV was higher in late-planted maize planted for the April-May rains or theOctober-November rains than in early plantings. This is the first report of natural infection withSCMV in Cynodon dactylon, C. nlemfunsis, Digitaria nuda, D. abyssinica, Eragrostisexasperata,Paspalum notatum, P. scrobiculatum, Rhynchelytrym repens, and an unknown Tripsacumfasciculatum cross. The distribution of SCMV in maize appeared to be related to the distribution ofsources of inoculum, but the periodicity of disease development appeared to be related to vectorpopulations. Maize streak and maize mosaic viruses were most often found in Central (4.5%) andCoast (13.8%) provinces.

Additional key words: corn, maize dwarf mosaic virus, weed hosts

In 1973, Kulkarni (14) reportedsugarcane mosaic virus (SCMV) insugarcane (interspecific hybrids ofSaccharum) and maize (Zea mays L.subsp. mays) in six of 41 districts of Kenyaand in Tanzania and Uganda. BecauseSCMV causes serious losses in maize (14,16), which is grown in most parts of Kenya(1), more information was needed todelineate the distribution of SCMV anddetermine its possible overseasoninghosts.

We report on the distribution ofSCMV in maize in Kenya, the seasonaldevelopment of the disease at Mugugaand Kitale, and the possible sources ofinocula. SCMV includes strains speciallyadapted to sugarcane, maize, or sorghum(Sorghum spp.) that are difficult totransmit from one grass host to another;for example, maize dwarf mosaic virusstrain A is easily transmitted tojohnsongrass (S. halepense (L.) Pers.) butnot to sugarcane (23). No attempt wasmade in this paper to identify strains. Apreliminary report on weed hosts hasbeen published (18).

MATERIALS AND METHODSOne-hundred maize plants along an

edge row of each field and 100 plants

Present address of the author: AR, SEA, USDA,Department of Plant Pathology, Ohio AgriculturalResearch and Development Center, Wooster, OH44691.

Published with the approval of the Director,Agricultural Research Department, K.A.R.I.,Muguga, Kenya.

This article is in the public domain and not copy-rightable. It may be freely reprinted with cus-tomary crediting of the source. The AmericanPhytopathological Society, 1980.

944 Plant Disease/Vol. 64 No.10

from the 10th row within each field wereexamined for SCMV symptoms. In fieldswhere maize was intercropped, 100-200maize plants were examined at random.Information on growth stage, plantheight, leaf number, and cultivar wascollected whenever possible and used tomodify judgment on disease incidencewhen conflicting data were taken within adistrict.

Weed hosts in fields and gardens wereexamined for symptoms of viral infection.Suspect weeds were collected, bioassayedfor SCMV infection, planted in theglasshouse, and later identified at theKenya Herbarium.

At least two sites were surveyed in eachdistrict. The site location was identifiedby the name of the nearest town orvillage. Because the best time to surveySCMV in a district was not known,surveys were made during an entiremaize-growing season. Although SCMVin maize was the main virus investigated,the occurrence of other viral diseases,including those caused by maize streakvirus (MSV) and maize mosaic virus(MMV), was also recorded. Becausespecific vector transmission is often usedto identify MSV and MMV and becausewe did no insect transmission studies,plants with symptoms resembling eitherdisease were recorded as infected witheither virus.

One percent of the field diagnoses ofSCMV infections in maize were confirmedby rub-inoculations to H512 maize testplants. Pathogenicity and symptoma-tology of virus isolates selected fromsamples of maize and weed hosts werestudied by rub-inoculations on 'Atlas'sorghum (S. bicolor (L.) Moench),Columbus grass (S. X almum Parodi),and johnsongrass. (We destroyed the

johnsongrass test plants 21-35 days afterinoculation by heating them in an oven at185 C for 6-8 hr.) A total of 109 samplesof maize (field plants and trap plants[17]), sugarcane, and weed hosts werepreserved in liquid nitrogen for futurestudies.

Trap plants of H512 maize were used todetermine the disease intensity (17)caused by SCMV, MSV, and MMV.Twenty-five pots of 14-day-old maizeplants were exposed for 7 days in the fieldas trap plants at the Muguga and Kitalelocations.

Yellow water pans were used to trapaphids. Aphids were collected weekly,counted, and stored in 70% alcohol forlater identification.

RESULTS AND DISCUSSIONDistribution of SCMV in Kenya.

SCMV, MSV, or MMV was found inmaize in 28 districts of eight Kenyanprovinces surveyed (Table 1). During thesurvey period (1977-1978), SCMV wasfound most often in maize in Nyanza(15.2%), Rift Valley (15.8%), andWestern (19.6%) provinces, and MSV orMMV was found most often in Central(4.5%) and Coast (13.8%) provinces. TheNorth Eastern province was not surveyed.SCMV-infected maize, sorghum, sugar-cane, or weed hosts were found only inthe western plateaus, the CentralHighlands, and the Rift Valley. Elevationin the areas where SCMV-infected plantswere most common ranged from 900 to2,700 m, but some were found at 300-900m(l).

The distribution and incidence ofSCMV suggest that resistance in maize isneeded primarily in the major maizeproduction areas of Kenya. The incidenceof SCMV within these areas, however, isnot uniform; the virus seems to be moreprevalent in the Kiambu district inCentral Province, the Kisii and HomaBay districts in Nyanza Province, theKericho and Trans Nzoia districts in theRift Valley Province, and perhaps theKakamega district in Western Provincethan in other districts in the sameprovinces. Despite obvious trends invirus distribution, our conclusions arelimited by the small fraction of maizesurveyed in any district.

Lack of data on disease development inmaize at all surveyed sites also limits theinterpretation of the survey results.Because the incidence data indicateSCMV distribution only at the time ofsurvey, the incidence of SCMV in maize

in some districts could be higher thanindicated in Table 1. For example, when10% of the maize at the Maize GeneticsDivision overhead-irrigated nursery atKitale was surveyed in August 1977, a4.2% incidence of SCMV was found. InOctober and November, the incidenceswere 41.3 and 97.7%, respectively. In1978, SCMV incidences at the samenursery were 0.6 and 47.5% in June andNovember, respectively.

Similarly, MSV or MMV in maize inCoast Province varied from season toseason and year to year; in 1977,incidence of MSV or MMV averaged7.3% (nine fields) and 3.6% (six fields) inJuly and December, respectively; inSeptember 1978, incidence averaged28.8% (12 fields) and ranged from 1.7% atMatuga to 74.2% at Mtwapa. Finally,disease may be overlooked. For example,we could not confirm the presence,previously reported (14), of SCMV inmaize at Kinoo and Machakos.

Disease incidence in trap plants. Theepiphytotic of sugarcane mosaic in theMaize Genetics Division nursery atKitale closely followed the incidence ofSCMV in trap plants and the number ofaphids trapped (Fig. 1). In 1977, theaverage incidence in trap plants washighest (18%) in October. This findingagrees with observations of more severedisease problems in late-planted nurseriesthan in early-planted nurseries (L. L.Darrah, personal communication). In1978, peak disease incidence in thenursery was also reached by the first weekin November but was lower than in 1977(48 vs. 98%). However, since theincidence of SCMV in trap plants atKitale was also less than 1% after August1978, the lower incidence in the nurseryplants was expected. The Maize GeneticsDivision nursery is considered suitablefor screening for SCMV resistance undernatural conditions provided that maizeentries are planted in late August or earlySeptember and aphid vectors arenumerous. To ensure sufficient inoculumin the test nursery, spreader rows ofinoculated susceptible maize such asH5 12 should be planted between every 10rows of test plants.

Disease incidence in maize appears todepend on the species as well as thenumber of aphids. In 1977, the incidenceof SCMV in trap plants at Muguga washighest in June, with another, smallerpeak in November (Fig. 1 A). The numberof aphids caught in the yellow water-pantrap followed a similar pattern (Fig. 1 B).Eastop (6) reported similar data for aphidpopulations at Muguga in 1953-1954. In1978, however, the aphid catch was lower

during June-July and was nil by the endof the year. In contrast with theoccurrence of high disease incidence ataphid population peaks in 1977, thehighest incidence of SCMV in trap plantsin 1978 occurred from January to March,when aphid catches were lowest. These

Table 1. Distribution of sugarcane mosaic virus (SCMV) and maize streak virus (MSV) or maizemosaic virus (MMV) in maize in Kenya, 1977-1978

No. of fields

Province District

Central 1. Nyandarua2. Nyeri3. Kirinyaga4. Murang'a5. Kiambu

Nyanza 6. Kisii7. Kisumu8. Siaya9. Homa Bay

Western 10. Bungoma11. Busia12. Kakamega

Coast 13. Taita14. Kilifi15. Kwale

Mombasa 16. MombasaRift Valley 17. Kericho

18. Nandi19. Kara Pokot20. Baringo21. Elgeyo-Marakwet22. Trans Nzoia23. Uasin Gishu24. Nakuru25. Liakipia26. Narok27. Kajiado28. Samburu

Eastern 29. Embu30. Meru31. Kitui32. Machakos33. Isiolo

Nairobi 34. Nairobi

No. of fieldsexamined

1977 1978

4 22 22 6

a 310 103 20 3

53 30 0

42 12 13 5

10 6

1 22

7 '4 2

2

10 93 37 41 24 21 1

0 02 22 3

310 30 11 2

19772.80.00.0

11.730.2

0

25.70

78.30.00.00.0

42.0

2.04.5

48.50.06.6

49.00.50.000.01.7

0.000.0

Infected plants (%)SCMV MSV/MMV

196.0.0.0.4121

1.1.

25c

18

100004639

233235243

00

4

CC

cc

78 1977.3 0.0.0 11.6.7 1.0.0 ....7 1.6.7 0.0.5 0.3 ....8 1.0

0.2 ....3 0.9.0 1.9.0 11.1.0 3.5.0 ....0 0.0!.8 ..

9.5.0 0.0.3 ....0 0.0.6 0.0.8 0.13.5 0.0).0 0.0).0 0.00 01.0 2.04.9 0.0

).0 ...).0 1.9).0 0).0 0.0

... 4.5

19780.08.27.31.39.60.00.51.00.00

5.40.07.2

37.022.211.00.03.8

2.00.00.11.00.60.00.00.00

12.40.00.00.00.00.0

Tanzania

Total, KenyaMean, Kenya

37.9

94 9711.2 9.2 1.7 4.0

a.. = not surveyed; 0= no maize grown at time of survey.

201MqqI ' .. Muguga

/0 "\ a/ . A

1977 1978

Fig. 1. Average monthly (A) incidence of sugarcane mosaic virus (SCMV) in 14-day-old H512

maize seedlings and (B) aphid catches in yellow water-pan traps at Muguga and Kitale, Kenya,1977-1978.

Plant Disease/October 1980 945

data suggest that some aphid speciestransmit the virus more efficiently thanothers. Species identification of aphidsremains to be completed.

Overseasoning hosts of SCMV. Manygraminaceous hosts of SCMV are known(2,4,8,11,15,20-23,25). Most of these areincluded as hosts on the basis of rub- oraphid-inoculation results or field obser-vations; some are included based on therecovery of virus from field-infectedplants. Because we wanted to identifypossible sources of inoculum in the field,we were interested in naturally infectedplants.

Species found to be naturally infectedwith SCMV are listed in Table 2. This isthe first report of such infection inCynodon nlemfuensis Vanderyst,Eragrostis exasperata Peter, Digitarianuda Schumach., D. abyssinica (A. Rich)Stapf, a cross of TripsacumfasciculatumTrin. ex Aschers., Cynodon dactylon (L.)Pers., Paspalum notatum (L.) Fluegge, P.scrobiculatum L., and Rhynchelytrumrepens (Willd.) C. E. Hubb. The last fivehosts named have also been reported assusceptible to SCMV infection by rub-inoculation (15,20-23).

All isolates except that from C.dactylon were typical of SCMV inpathogenicity and symptomatology(Table 2). However, the isolate from C.dactylon, collected from Kakamega

district, closely resembled the johnson-grass strain of SCMV (SCMV-Jg ormaize dwarf mosaic virus strain A) of theUnited States. Kulkarni's studies (13,14)indicate that SCMV isolates in Kenya aregenerally related to SCMV-A, -B, and-D, reported in the United States.

Reactions on S. X almum of isolatesfrom Muguga (Kulkarni's originalisolate) and Kitale were also unexpected.Transmission of the Kitale isolate to S.X almum was greater than that of theMuguga isolate (15 vs. 2%) but still wascomparatively low. Tosic and Ford (23)reported that only maize dwarf mosaicvirus strain A, SCMV-I, and SCMV-Jgwere transmissible to S. X almum. Ourdata suggest that variations in SCMVpopulations in Kenya may be moreinvolved than previously reported.

Perennials listed in Table 2 should beprime candidates for overseasoninghosts. Field observations, however, didnot always support this conclusion. Forexample, sugarcane is grown perenniallyalongside maize in many small gardensand is suspected of being a source ofinoculum for disease development inmaize. Symptomless infection withSCMV also occurs in sugarcane (9,14).Our surveys and assays of sugarcaneplants from 10 small gardens in theKiambu district, where the incidence ofSCMV in maize was high, failed to reveal

Table 2. Reaction of maize (Zea mays L. subsp. mays H512), sorghum (Sorghum bicolor (L.)Moench. 'Atlas', and johnsongrass (S. halepense (L.) Pers.) to bioassays of grasses with viralsymptoms collected near maize fields in Kenya

Infection in assay hosta

Tribe H512 'Atlas'Species maize sorghum Johnsongrass

FestuceaeEragrostis exasperata Peter Sy LL 0E. superba Peyr. 0 ......

ChlorideaeChloris gayana Kunth 0 ......Cynodon dactylon (L.) Pers. Sy Sy SyC. nlemfutnsis Vanderyst Sy LL 0

PaniceaeAcroceras macrum Stapf 0 ......Brachiaria rugulosa Stapf 0 ......Digitaria abyssinica (A. Rich.) Stapf Sy LL 0D. nuda Schumach. Sy LL 0D. velutina (Forsk.) Beauv. Sy LL 0Panicum maximum Jacq. 0 ......Paspalum notatum (L.) Fluegge Sy LL 0P. scrobiculatum L. Sy LL 0Pennisetum purpureum Schumach. Sy LL 0Rhynchelytrum repens (Willd.) C. E. Hubb. Sy LL 0Setaria verticillata (L.) Beauv. Sy LL 0

AndropogoneaeSaccharum officinarum L. Sy LL 0Sorghum X almum Parodib Sy LL 0S. bicolor (L.) Moench Sy LL 0

TripsaceaeA cross of Tripsacumfasciculatum Sy LL 0

Trin. ex Aschers.Zea mays L. subsp. mays Sy LL 0

or isolate SCMV from symptomlessplants. Bioassays of apparently healthyplants of sugarcane cultivars B51415,C0281, CP31-294, M423/51, andNC03 10 (reported as symptomless hosts[14]) at the Kenya Agriculture ResearchInstitute's plant quarantine plots werealso negative. One sugarcane plant withSCMV symptoms in the field yieldedSCMV on bioassay.

Pennisetum purpureum Schumach.,another perennial, is also widely grown inmaize-growing areas. However, of sixplants with symptoms resembling viralinfections, only one yielded SCMV onbioassay. SCMV was not transmitted toor recovered from apparently healthy P.purpureum by rub- or airbrush-inocula-tions. Several grass hosts; eg, Digitariavelutina (Forsk.) Beauv. and Paspalumnotatum, were infected with other virusesbesides SCMV. Melinis minutifloraBeauv., a reported host of SCMV in Africa(14), was not encountered in this survey.

Epiphytology. The distribution ofSCMV in maize in Kenya may be affectedby the availability of inoculum sources,maize genotypes, aphid vectors, andenvironmental conditions. The avail-ability of inoculum most likely limitsSCMV in maize despite the prevalence ofnaturally infected weed hosts andsecondary sources such as infected maize,sorghum, and sugarcane. SCMV is foundin the southwest highlands of Kenya butnot in areas southeast of the highlands.However, SCMV was found in maize inTanzania around Arusha, Moshi, andKahe, southeast of the Kenyan highlands.Further east, along the Coast Province inKenya and south along Tanzania'scoastal districts, SCMV was not found. IfSCMV were introduced into areas likethe Coast Province, it probably wouldbecome established and important inmaize and sugarcane.

The availability of susceptible maizedoes not appear to limit SCMV,especially at higher elevations (above2,000 m), where a normal season formaize may extend from late March tomid-November. At present, no maizegrown in Kenya is resistant to SCMV(14,16). Incidence of SCMV commonlywas higher in late-planted (August-October) maize nurseries than in thenormal season crop planted in April orMay, but maize planted in early April wasstill susceptible to SCMV by rub-inoculation as late as 63 days afterplanting (16).

The occurrence of SCMV in maizeprimarily in the southwestern highlandareas of Kenya also suggests that diseasedevelopment, as opposed to distribution,is limited by elevation, temperature, andrainfall. These factors determine the

species and amount of natural vegetation(10,24), which in turn influence vectordevelopment and migration (12). Datafrom the SCMV and weed host surveys(Tables 1 and 2) and preliminary evidence

- Sy = systemic infection; LL = local lesions; 0 = no infection; . not tested.bNot found naturally infected.

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