Publication: Studies on the basal rot of the golden shower orchid (Oncidium Goldiana) caused by the basidiomycetous pathogen Marasmiellus inoderma
Abstract
M. inoderma is a recently identified basidiomycetous fungal infection on Oncidium orchids. It causes lethal dry basal rot and was found to be prevalent in a number of orchid nurseries surveyed in Singapore. Disease incidence as high as 71% was recorded. This pathogen was initially found infecting Oncidium Goldiana. This study, for the first time, also reports two other hosts that this pathogen infects, Oncidium Gower Ramsey and Oncidium Taka. The disease and its spread are described and studied. As far as we know, this is also the first and only detailed study of M. inoderma as a serious pathogen of orchids. Basidiospores are likely the source of primary infection. Basidiospore liberation was optimum at 24<sup>o</sup>C and germination was at 24-28<sup>o</sup>C. Temperature does affect growth of the fungus and temperatures conducive for radial growth varied from 24-28<sup>o</sup>C. In host range tests, members of the Maxillarieae tribe were found to be highly susceptible to isolates of the pathogen. Intra-specific differences in aggressiveness were observed in eight single basidiospore isolates screened. However, PCR-RFLP analysis of the ITS region of the rDNA of 8 single basidiospore and 7 somatic hyphal isolates established from three different Oncidium hosts from six nurseries did not reveal detectable intra-specific polymorphisms. The ITS regions were found to be of identical size and identical DNA profiles were also obtained after restriction enzyme digest. A total of 7 enzymes were used. In RAPD-PCR analysis of another 38 single basidiospore and 13 somatic hyphal isolates, a high level of genetic homogeneity was similarly detected. The results of both RAPD and PCR-RFLP studies indicate that there is a high level of homogeneity in the population of M. inoderma infecting Oncidium in local nurseries. Microscopic and macroscopic observations of mycelial pairings of isolates also indicated that isolates were compatible (a few were weakly incompatible). Six fungicides, mancozeb, triadimefon, maneb, propineb, etridiazole and maneb+carbendazim screened gave 100% radial growth inhibition for 3 isolates from O. Goldiana established from three different nurseries. The results identified that M. inoderma isolates were more sensitive to fungicides from the carbamate group. Triadimefon and mancozeb were determined to be the most effective. Both in vitro and in vivo fungicide screening experiments confirmed that treating Oncidium tissues or plants either once or at weekly intervals with these fungicides slowed down the speed of the disease. These fungicides also successfully controlled the formation of basidiomata on infected tissues as well as germination of the basidiospores. The use of these fungicides, together with other phytosanitary practices, is recommended as an integral part in the management of M. inoderma basal rot.