As there are no products which can be applied to plants in the field to control virus diseases in a manner equivalent to fungicides to control fungal diseases, control of virus diseases generally is achieved by the use of resistant plants or by controlling the spread of the virus between plants. In order to do this the epidemiology of the disease must be understood and control interventions applied at strategic points in the disease cycle. The research program of the Plant Virology Group focuses on grapevine leafroll disease of wine grapes, and the Citrus disease caused by citrus tristeza virus (CTV). The group also studies citrus greening disease, associated with a fastidious bacterium “Candidatus” Liberibacter africanus (Laf), which has an epidemiology not unlike that of plant viruses. These three diseases are amongst the most important diseases of Wine Grapes and Citrus, two high value crops in South Africa

Grapevine leafroll disease is has been controlled for decades within the South African Vine Certification Scheme, where virus-free nuclear stocks of vine cultivars are produced following heat-therapy and meristem tip culture. However such virus-free material remains susceptible to viral infection and often progeny from the nuclear plants, planted in the field become infected again. Surveys by the members of the plant virology group in recent years have shown that grapevine leafroll-associated virus type 3 (GLRaV-3) is the primary cause of these infections. A number of sources and means of spread have been identified by members of the Plant Virology Group in South Africa through spatio-temporal analyses of grapevine leafroll disease in foundation- and mother-blocks. Following these studies a successful control strategy based on planting certified planting material, rouging of infected vines, control of mealybug numbers and dispersal has been developed by us. Assessments of the efficacy of these control strategies have been conducted at a model wine estate, Vergelegen Wine Estate in Somerset West, where the success of the control strategy has been unequivocally demonstrated. Current research by the research group focuses on refinements of the control strategies, for example 1) the development of large scale detection of GLRaV-3 in white cultivars, as disease symptoms are not obvious in these and rouging cannot be based on visual symptom identification, as in red cultivars, but must be on tests for the virus and 2) trials to confirm the spread of leafroll between two successive vineyard blocks and assessment of control methods such as fallow periods, and systemic insecticide use with or without herbicide to prevent the perpetuation of grapevine leafroll infection from a preceding vineyard. Research is also conducted on developing a next generation sequencing-based diagnostic protocol for the viruses of grapevine, in support of the South African Wine Grape Certification Scheme.

As with the wine grapes virus-free Citrus planting material is produced within the South African Citrus Improvement Programme (CIP). However because of the extremely rapid rate of re-infection of such material in the field by CTV, all citrus planting is pre-immunized with mild strain populations of CTV prior to release from the CIP. While this protects the plant from infection by severe forms of CTV in the majority of instances, on some occasions severe symptoms are still found, especially in the case of Grapefruit. It is unknown whether this is due to 1) super-infection of the plant with wild-type severe forms of the virus, 2) mutations within the pre-immunizing mild population to severe forms or 3) selection, under specific environmental conditions for severe forms of CTV, inherently present in the pre-immunizing population. The plant Virology group are busy with studies where the strains present in greenhouse maintained pre-immunizing CTV populations and field planted, pre-immunized Citrus trees with various symptoms are being characterised, pure sources of CTV strains are isolated, and factors affecting strain dynamics are assessed.

Citrus greening disease has been reduced to manageable levels through stringent vector control strategies and rouging, but remains a problem in the cooler citrus production areas of South Africa. The perpetuation of the disease in spite of stringent rouging may be due to the presence of hosts other than citrus, which may serve as reservoirs of the disease. The Plant Virology group are investigating the possibility that other hosts of the bacteria exist with the initial emphasis is being placed on evaluating indigenous Rutaceous species

New Publications

Welgemoed T, Pierneef R, Read DA, Schulze SE, Pietersen G, Berger DK. (2020) Next generation sequencing reveals past and current widespread occurrence of maize yellow mosaic virus in South Africa. European Journal of Plant Pathology 158(1):237-249. 10.1007/s10658-020-02070-1
Roberts R, Pietersen G. (2016) A novel subspecies of ‘Candidatus Liberibacter africanus’ found on native Teclea gerrardii (Family: Rutaceae) from South Africa. Antonie van Leeuwenhoek :1-8. 10.1007/s10482-016-0799-x
Kleynhans J, Pietersen G. (2016) Comparison of multiple viral population characterization methods on a candidate cross-protection Citrus tristeza virus (CTV) source. Journal of Virological Methods 237:92-100. 10.1016/j.jviromet.2016.09.003
Read DA, Pietersen G. (2016) PCR bias associated with conserved primer binding sites, used to determine genotype diversity within Citrus tristeza virus populations. Journal of Virological Methods 237:107-113. 10.1016/j.jviromet.2016.09.004
New SA, van Heerden SW, Pietersen G, Esterhuizen LL. (2016) First report of a Turnip Yellows virus in association with the Brassica Stunting Disorder in South Africa. Plant Disease 100(11):2341. 10.1094/PDIS-12-15-1443-PDN
Lin H, Pietersen G, Han C, Read DA, Lou B, Gupta G, Civerolo EL. (2015) Complete genome sequence of “Candidatus Liberibacter africanus,” a bacterium associated with Citrus Huanglongbing. Genome Announcements 3(4):e00733-15. 10.1128/genomeA.00733-15 PDF
Read DA, Pietersen G. (2015) Genotypic diversity of Citrus tristeza virus within red grapefruit, in a field trial site in South Africa. European Journal of Plant Pathology 142(3):531-545. 10.1007/s10658-015-0631-x
Roberts R, Steenkamp ET, Pietersen G. (2015) Three novel lineages of ‘Candidatus Liberibacter africanus’ associated with native rutaceous hosts of Trioza erytreae in South Africa. International Journal of Systematic and Evolutionary Microbiology 65:723-731. 10.1099/ijs.0.069864-0
Zablocki O, Pietersen G. (2014) Characterization of a novel citrus tristeza virus genotype within three cross-protecting source GFMS12 sub-isolates in South Africa by means of Illumina sequencing. Archives of Virology 159(8):2133-2139. 10.1007/s00705-014-2041-3
Almeida RPP, Daane KM, Bell VA, Blaisdell GK, Cooper ML, Herrbach E, Pietersen G. (2013) Ecology and management of grapevine leafroll disease.. Frontiers of Microbiology 4(94):1-13. 3389/fmicb.2013.00094 PDF
Pietersen G, Spreeth N, Oosthuizen T, Van Rensburg A, Van Rensburg M, Lottering D, Rossouw N, Tooth D. (2013) A case study of control of Grapevine Leafroll Disease spread at a commercial wine estate in South Africa. American Journal of Enology and Viticulture 64:296-305. 10.5344/ajev.2013.12089 PDF
Scott KA, Hlela Q, Zablocki ODJ, Read DA, Van Vuuren SP, Pietersen G. (2013) Genotype composition of populations of Citrus tristeza virus grapefruit cross-protecting GFMS12 in different host plants and aphid-transmitted sub-isolates. Archives of Virology 158(1):2-37. 10.1007/s00705-012-1450-4 PDF
Walsh HA, Pietersen Gerhard. (2013) Rapid detection of Grapevine leafroll-associated virus type 3 using a reverse transcription loop-mediated amplification method. Journal of Virological Methods 194:308-316. PDF
Phaladira MNB, Viljoen R, Pietersen G. (2012) Widespread occurrence of “Candidatus Liberibacter africanus subspecies capensis” in Calodendrum capense in South Africa. European Journal of Plant Pathology 124:39-47. PDF
Rey MEC, Ndunguru J, Berrie LC, Paximadis A, Berry S, Cossa N, Nuaila VN, Mabasa KG, Abraham N, Rybicki EP, Martin DP, Pietersen G, Esterhuizen L. (2012) Diversity of Dicotyledenous-Infecting Geminiviruses and Their Associated DNA Molecules in Southern Africa, Including the South-West Indian Ocean Islands.. Viruses 4(9):1753-1791. 10.3390/v4091753 PDF
Jooste AEC, Pietersen G, Burger JT. (2011) Distribution of grapevine leafroll associated virus-3 variants in South African vineyards. European Journal of Plant Pathology 131(3):371-381. PDF
Kruger K, De Klerk A, Douglas-Smit N, Joubert J, Pietersen G, Stiller M. (2011) Aster yellows phytoplasma in grapevines: identification of vectors in South Africa. Bulletin of Insectology 64:S137-S138. PDF
Van Vuuren SP, Cook G, Pietersen G. (2011) Lack of Evidence for Seed Transmission of . Plant Disease 95(8):1026-1026. 10.1094/pdis-02-11-0104 PDF
Jooste A, Maree H, Bellstedt D, Goszczynski D, Pietersen G, Burger J. (2010) Three genetic grapevine leafroll-associated virus 3 variants identified from South African vineyards show high variability in their 5'UTR. Archives of Virology :1-10. PDF
Lamprecht RL, Kasdorf GGF, Stiller M, Staples SM, Nel LH, Pietersen G. (2010) Soybean blotchy mosaic virus, a New Cytorhabdovirus Found in South Africa. Plant Disease 94(11):1348-1354. 10.1094/pdis-09-09-0598 PDF
Pietersen G, Arrebola E, Breytenbach JHJ, Korsten L, Le Roux HF, La Grange H, Lopes SA, Meyer JB, Pretorius MC, Schwerdtfeger M, Van Vuuren SP, Yamamoto P. (2010) A Survey for . Plant Disease 94(2):244-249. 10.1094/pdis-94-2-0244
De Graca JV, Pietersen G, Van Vuuren SP. (2009) Research on HLB in South Africa. Phytopathology 99:S157.
Doddapaneni H, Liao H, Lin H, Bai X, Zhao X, Civerolo EL, Irey M, Coletta-Filho H, Pietersen G. (2008) Comparative phylogenomics and multi-gene cluster analyses of the Citrus Huanglongbing (HLB)-associated bacterium 'Candidatus Liberibacter'. BMC Research Notes 1(75) PDF
Lamprecht R, Pietersen G, Kasdorf G, Nel L. (2008) Characterisation of a proposed Nucleorhabdovirus new to South Africa. European Journal of Plant Pathology 123(1):105-110. 10.1007/s10658-008-9339-5 PDF
Meyer JB, Kasdorf GGF, Nel LH, Pietersen G. (2008) Transmission of activated-episomal Banana streak OL (badna)virus (BSOLV) to cv. Williams banana (Musa sp.) by three mealybug species. Plant Disease 92(8):1158-1163.
Saccaggi DL, Ger K, Pietersen G. (2007) A multiplex PCR assay for the simultaneous identification of three mealybug species (Hemiptera: Pseudococcidae). Bulletin of Entomological Research 98:27-33. PDF
Stewart K, Pietersen G. (2007) Dynamics and molecular characterization of citrus tristeza virus (CTV) strains within the South African GFMS 12 cross-protecting population. South African Journal of Science 103(1-2):II-II.
Pietersen G. (2004) Spread of grapevine leafroll disease in South Africa-a difficult but not insurmountable problem. Wineland (South Africa)
Pietersen G, Morris J. (2002) Natural occurrence of groundnut ringspot virus on soybean in South Africa. Plant Disease 86(11):1271.
Pietersen G, Smith MF. (2002) Tomato yellow leaf curl virus resistant tomatoes show resistance to Tomato curly stunt virus. Plant Disease 86(5):528-534.