Characterisation of a mycoplasma-like organism infecting Northern Territory sweet potato

  • Marian Barbara Chisholm

    Student thesis: Other thesis - CDU

    Abstract

    Since MLOs cannot be cultured in vitro little is known about them and the plant diseases they are associated with. A number of criteria such as disease response to tetracycline treatment, transmission by grafting and insect vectors, and observation of the organism in phloem cells of diseased plants are used to study these associations. These criteria form the basis of this study which investigates an association between sweet potato 'little-leaf (SPLL) and a MLO in the Northern Territory . Characteristics of this disease-plant interaction were also determined by plant growth and cultivar susceptibility trials.

    Mycoplasma-like bodies were observed in the phloem sieve cells of both graft inoculated plants and field infected plants using electron microscopy. The phloem tissue of diseased plants that was treated with a fluorescent DNA stain exhibited a distinct intense fluorescence that indicated the presence of MLOs. Using this method it was shown that the phloem tissue of diseased plants was not uniformly infected although adjacent cells of a continuous strand of phloem were infected.

    Disease symptoms were alleviated following treatment with tetracycline at concentrations of 10 and 25 ug/ml. At these concentrations the maximum mean number of symptomless leaves as a percentage of the total numbers of leaves increased until week 3 and 4 respectively, after which the mean percentage of symptomless leaves declined indicating that this disease response was temporary. In this study, the increase in the number of symptomless leaves is defined as symptom remission in leaves that were present at the time of treatment, and the suppression of symptoms in new leaves formed after tetracycline was applied.

    The MLO vector Orosius argentatus was collected from Trianthema portualcastrum (pigweed) that invaded SPLL infected local sweet potato crops. Specimens of Orosius argentatus collected from this weed, which also showed 'little-leaf' symptoms, transmitted a MLO to Catharanthus roseus healthy test plants during transmission trials. The greening of flowers observed in these test plants was similar to symptoms observed in Catharanthus roseus that had been infected with SPLL using the parasitic vine dodder.

    Sweet potato plants (cv. LO323) graft inoculated with SPLL infected scion material produced less total dry weight than control plants. At the final harvest six months after inoculation, the leaf area of diseased plants was reduced by 95%. A 75% reduction in the mean number of tubers per plant was also recorded in diseased plants. In cultivar susceptibility trials symptoms of the disease were observed in all seven sweet potato varieties that were graft inoculated with the disease.

    These results characterise the response of sweet potato to this disease in the Northern Territory and provide evidence for an association between 'little-leaf' symptoms in sweet potato and a MLO. This work also reports the first transmission of a MLO by field collected Orosius argentatus in the Northern Territory.
    Date of Award1993
    Original languageEnglish
    SupervisorKaren Gibb (Supervisor)

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