Cymbidium black rot caused by an aberrant strain of Phytophthora multivesiculata in Taiwan

Abstract

In 1996 and 1997, a severe outbreak of a disease on Cymbidium orchid (cymbidium hybridium) occurred in a garden in I-Lan, Taiwan, with numerous plants showing black rot symptoms on leaves, pseudostems and roots and dying. A phytophthora-like microorganism was constantly isolated from the diseased plants and two of the eleven isolates collected from this graden were used in this study. Inoculation tests were carried out using zoospore suspensions to fulfill Koch’s postulates. Results showed that both isolates (P96013 and P96016) were pathogenic on C. hybridium with the same black rot symptoms as the organism reisolated from the inoculated plants was morphologically identical to the culture of the original inoculum. In addition to C. hybridium, the two Phytophthora-like isolates were also pathogenic on other Asian Cymbidium species, indluding C. ensifolium, C. sinense and C. rubrigemmum, causing black rot symptoms and death of plants. Both Phytophthora-like isolates were similar to Phytophthora multivesiculata in most of the morphological characteristics. However, they were different from P. multivesiculata in some features such as dimensions of sporangia, oogonia and oospores as well as maximum growth temperature. The ITS sequences of these two Phytophthora-like isolates were identical to that of P. multivesiculata. Based on the morphological characteristics and the molecular data, both Phytophthora-like isolates from C. hybridium were identified as an aberrant strain of Phytophthora multivesiculata. This is the first report of the aberrant strain of Phytophthora multivesiculata in Taiwan. Diseased orchid plants in the garden of Yuan-Shan, I-Lan county, were destroyed in 1998 and this aberrant strain of P. multivesiculata was not found in the survey of 1998 and 1999 in I-Lan county or any other orchid production areas of Taiwan. 1996 年與 1997 年宜蘭員山一處虎頭園出現嚴重的黑腐病，許多植株之偽球莖、葉片及根部均褐色水浸狀病斑，並嚴重腐敗。從罹病組織分離到一種未曾在台灣發表過的疫病菌，共獲得11支菌株。將純培養的游走子接種在健康的虎頭蘭植株上，可以造成相同的病徵，且相同的病菌可從發病組織上分離得到。此外，該疫病菌可以引起四季蘭、觀音素心蘭及報歲蘭產生水浸狀斑及黑腐病徵。該菌的特性為：在5% V-8 瓊脂上菌絲可生長溫度為10-29℃，最適生長溫度24℃。在水中，可以產生大量胞囊，胞囊無乳凸、橢圓形、頂生，游走子釋放後，多數形成再生外生胞囊(external proliferate sporangia)。不形成厚膜孢子；但無論固態培養或在水中可產生大量成直角分歧、成長串唸珠狀的菌絲膨脹體(hyphal swellings)，為該菌的特徵。該菌為同絲型(Homothallic)，在V-8瓊脂上口獨培養可形成大量卵孢子，藏卵器表面成波浪狀，不平滑；藏精器單生底著。依據該菌的形態特徵與核醣體內轉彙區間DNA序列(ITS)的相似度，該菌被鑑定為非典型的Phytophthora multivesiculata。該病害並未在其他蘭花與其他地區出現。