Pathogenic associations with yellows disease of Dodonaea viscosa in Hawaiʻi

Abstract

Dodonaea viscosa (L.) Jacq. in Hawai'i is afflicted with a severe yellowing disorder with symptoms which include the production of pendulous, chlorotic witches' brooms, a decrease in leaf size combined with the distortion of leaf lamina, the suppression of flowering, and progressive defoliation leading to the eventual death of afflicted plants. The disease was first reported in 1984 occurring in Hawai'i Volcanoes National Park on the island of Hawai'i and has since been observed on the islands of Kauai, Maui, and Oahu. Field studies initiated in 1988 and continued over a three year period indicate a slow rate of spread of the disease based upon visual symptom expression. Viruslike particles 16 nm in diameter and 700 nm in length and double-stranded RNA of molecular weight 3 X 106 daltons were isolated from diseased plants but were absent from healthy plants. DNA complementary to dsRNA was cloned in E. coli and shown to be of non-host origin. Leaves from symptomatic and symptomless plants collected from field sites on Hawai'i, Maui, and Kauai were tested with a probe prepared from the highly conserved 165 ribosomal gene of Western X MLO, which has been shown to reliably detect MLOs from a wide range of hosts. On all the islands sampled, 80% of the symptomatic plants and 33% of the symptomless plants growing near diseased plants tested positively with this probe. Leaves and roots of healthy plants grown from seed collected from symptomless D. viscosa did not react with this probe. Pleiomorphic bodies resembling MLOs were observed in phloem tissues of diseased plants using DAPI staining and transmission electron microscopy. Such structures were not observed in healthy plants grown from seed. Partial alleviation of symptoms on diseased plants was noted following stem injections of oxytetracycline at 100 µg/ml. Attempts to transmit MLOs between D. viscosa and Catharanthus roseus by Cuscuta sandwichiana and C. campestris were not successful. The evidence suggests a complex etiology for the yellows disease of Dodonaea viscosa in Hawai'i which includes both viral and mycoplasmal agents.