Abstract:
Tamarillo mosaic virus (TamMV) describes isolates of Potato virus A (PVA) infecting tamarillo. TamMV results in economic losses in the commercial production of tamarillo within New Zealand due to virus-induced blemishes on the exocarp of infected fruit. Identification of a naturally occurring mild isolate of TamMV, LL6, raised the possibility of implementing cross protection as a control method within orchards. The discovery of an extremely severe symptom-inducing isolate of TamMV, B14, and a traditional severe symptom-inducing TamMV isolate, 007, provided the basis for cross protection analysis with LL6. Molecular characterisation, transmission, symptomatology and cross protection of TamMV infections on indicator and tamarillo plants was examined. Direct sequencing of the open reading frames of the three isolates revealed previously unidentified diversity within TamMV, with B14 most closely related to potato-derived isolates of PVA. In addition, a split into two clades within worldwide PVA isolates was observed. Recombination was detected within the coat protein of 15 PVA isolates, with 007 detected as the minor parent. Acquisition and transmission of LL6, 007, and B14 by the green peach aphid (Myzus persicae) was confirmed. Whitefly acquisition of PVA in New Zealand tamarillo orchards was excluded; whitefly are not involved in TamMV spread within orchards. Cross protection was successful in the glasshouse using indicator plant species Nicotiana benthamiana and N. clevelandii with LL6 as a protectant against 007 and B14. Not all LL6 inoculated tamarillo planted in an infected orchard were protected. Although encouraging cross protection by LL6 was demonstrated in this research, further research is required to increase the LL6 infection rate of tamarillo and to address cross protection within tamarillo, including when challenge virus is delivered via the aphid vector. Understanding the complete diversity of PVA isolates within New Zealand and worldwide will reveal their origins, evolution and impacts on host plants. This may also identify novel severe and mild isolates on tamarillo, for cross protection studies, and use in New Zealand tamarillo orchards to mitigate their economic impacts.