TOMATO YELLOW LEAF CURL DISEASE IN TANZANIA: CHARACTERIZATION, ALTERNATIVE HOSTS AND TRANSMISSION

TOMATO YELLOW LEAF CURL DISEASE IN TANZANIA: CHARACTERIZATION, ALTERNATIVE HOSTS AND TRANSMISSION

Abstract:

Tomato farms in Arusha, Morogoro, Dodoma, Iringa, Kilimanjaro and Coast regions of Tanzania were surveyed to assess the incidence of the yellow leaf eurl disease, and to collect infected tomato leaf and weed samples for diagnosis and identification. Results indicated that disease incidence and severity were significantly higher (P < 0.05) in Dodoma region, but low and least severe in Iringa region. The disease-causing agent was identified to be a geminivirus by electron microscopic examination of the geminate particles in diseased samples. Squash and dot blots of extracts from the samples hybridized to DIG-labelled viral DNA probes under high strigency conditions. Similarly, Southern hybridization of genomic DNA extracted from the samples by phenolchloroform procedures, revealed viral bands in their open circular, closed circular and single-stranded forms. The putative viral DNA was amplified by polymerase chain reaction using primer pairs designed to yield up to 1.4 kb amplicons. The amplified viral DNA was digested with Alu\ and clectrophorcsed in polyacrylamidc gel. Restriction fragment length polymorphisms analysis showed similar banding patterns for all the Tomato yellow leaf curl Tanzania virus isolates. Furthermore, the amplified viral DNA was ligated to pBluescript II KS’ and transformed into Escherichia coli strain JM 83 cells by electroporation or Calcium chloride-mediated transformation. The viral DNA was extracted from colonies containing the insert and sequenced using a Li-Cor DNA Automatic Sequencer. The BLAST programme was used to search for viruses with similar sequences, and phylogenetic relationship was established using the CLUSTAL function of the Vector NTI.5 software. Sequence comparison and phylogeny showed that Tomato yellow leaf curl Tanzania virus was closely related to African tomato leaf curl virus and to Tomato yellow leaf curl Sardinia virus. However, the isolates of Tomato yellow leaf curl Tanzania virus were biologically and genetically similar. More than 90 % homology in the Rep and/or coat protein was found between Tomato yellow leaf curl Tanzania virus and some crop- and weed-infecting geminiviruses. Representative samples of the Tomato yellow leaf curl Tanzania v/rw.s-infected tomato plants collected from the regions were maintained in the screenhouse by grafting and through whitefly-mediated inoculations. The biological properties of the virus relating to acquisition and inoculation time, persistence, graft transmission, mechanical transmission and host range were studied. Results obtained indicated that the virus was transmitted persistently by Bemisia tabaci Genn., and by grafting, but it was not mechanically transmissible. Minimum acquisition and inoculation times were 30 min for all the virus isolates, except the Kilimanjaro and Iringa isolates, which had minimum acquisition feeding time of 1 h. For the first time, the present study demonstrated that the following weed species are non-cultivated hosts of TYLCTZV: Commelina erecta, Amaranthus spinosus, Erigeron floribundus, Ageratum conyzoides, Bidens pilosa, Sida acuta, Ipomea batatas, Amaranthus viridis, Portulaca oleracea, Cassia oblusifolia, Euphorbia hirta, Calopogonium mucunoides, Crotalaria retusa, Trianthema portulacastrum, Alternanthera sessilis, Celosia trigyna, Commelina diffusa, Chromolaena odor at a, Kclipta pros (rata, Syne dr el la nodi flora, Cassia occidental is, Spigelia anthelmia, Boerhavia diffusa, Physalis angulata, and Acanthospermum hispidium. Kxperimental studies using screenhouse cultures of the Tomato yellow leaf curl Tanzania virus representative isolates resulted in the infection of five plant species (Capsicum annuum, Datura stramonium, Lycopersicon esculcntum, Nicoliana glutinosa and N. tabacum). Phaseolus vulgaris, Gossypium hirsutum, Solanum melongena, Solanum tuberosum. Glycine max, and Arachis hypogea were not infected. Field and sereenhouse experiments were conducted to determine the resistance of tomato genotypes to Tomato yellow leaf curl Tanzania virus by comparing the yield and yield components between inoculated and un-inoculated plants of each genotype. With the exception of the resistant genotype TY172, both sereenhouse and field inoculated plants of all the genotypes showed statistically significant (P<0.001) reductions in plant fresh weight, fruit number, fruit weight and relative total yield when compared with the uninoculated plants. On the overall, tomato genotype TY172 was the most resistant followed by Tengeru 97, Cal-J and Marglobc. It is recommended that the resistance in TY172 be introgressed into the Tengeru 97, Cal-J, Marglobc and Moneymaker, which are commonly cultivated by farmers in Tanzania, to achieve high resistance to the virus. Meanwhile, the cultivation of the tolerant tomato cultivar, Tengeru 97 should be popularised among farmers in Tanzania while the search for sources of resistance among wild Lycopersicon relatives continues.

TOMATO YELLOW LEAF CURL DISEASE IN TANZANIA: CHARACTERIZATION, ALTERNATIVE HOSTS AND TRANSMISSION

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