柚果面枝孢菌斑点病病原鉴定

Identification of pathogens causing fruit spot disease on Citrus grandis

对近年来在广东、广西、福建等地的柚[Citrus grandis Osbeck, Syn. C. maxima (Burm.) Merr.]果实上普遍发生的一种斑点病观察发现:该病害主要发生在已转色的成熟果实表面,斑点细小,红褐色,近圆形或不规则形,中央略凹陷,渐变灰白色,偶尔可见少量的黑色霉状物;病斑发生仅局限于果皮,仅影响果实的商品性,不影响果实的内在品质。对发病部位进行病菌的组织分离培养,从中获得20株疑似枝孢属真菌菌落,对其进行了菌落特征和形态学特征观察,并结合rDNA的内转录间隔(ITS)区、转录延长因子(TEF)基因区和微丝肌动蛋白(ACT)基因区的序列比对分析,对这些真菌进行了鉴定。结果表明,这些分离物都属于枝孢属真菌(Cladosporium)。根据形态学和分子系统发育学分析,这20株枝孢菌分为2个种,其中18株属于极细枝孢(C. tenuissimum),2株为散黄枝孢霉(C.xantochromaticum),说明极细枝孢(C. tenuissimum)为优势种。而散黄枝孢霉(C. xantochromaticum)是我国的一个新记录种,也是首次从柑橘属植物上分离得到的。接种试验表明,这2种枝孢菌均可在柚果上引起典型的斑点症状,其症状与田间观察到的相一致。本研究为今后进一步探究枝孢斑点病的发生规律和防治技术奠定了基础。

Recently, a fruit spot disease was found on pummelo [Citrus grandis Osbeck, Syn. C. maxima(Burm) Merr.]fruits during our investigation of diseases in pummelo orchards as well as in packing houses in Guangdong, Guangxi,and Fujian provinces and municipality. Symptoms of irregular or rounded small spots on the fruit surface firstly appeared, similar as the size of sesame, which were red brown and sunk. The center of spots gradually turned gray.The occurrence of spots was only limited to the flavedo and never extended into the sac of pummelo fruits, which significantly reduced the marketable quality of fruits, although did not decrease inner quality. Twenty fungal pure isolates were obtained using the tissue-isolation method, and the identification of these fungi was carried out based on morphology and molecular phylogenetic analysis which was performed with sequencing of internal transcribed spacer(ITS) r DNA, transcriptional elongation factor(TEF) and filamentous actin(ACT) genes. The results of morphological and phylogenetic studies demonstrated that the 20 isolates were identified as Cladosporium genus. These Cladosporium isolates were classified into two species, in which eighteen isolates were grouped into C. tenuissimum and two isolates belonged to C. xantochromaticum. It implied that, C. tenuissimum was the predominant species, and C. xantochromaticum was a new record fungus in China isolated from Citrus genus plant for the first time. A pathogenicity test was performed by inoculating a piece of fungal colony on the surface of healthy pummelo fruits.The result showed that the same symptoms appeared after 7 days of inoculation, confirming that the isolates were the pathogens of pummelo fruit spot disease. This research lays a foundation for further study on the way of disease occurrence and prevention.