Zivanovic M, Walcott R R
Department of Plant Pathology, The University of Georgia, Athens 30602.
Phytopathology. 2017 Jan;107(1):29-35. doi: 10.1094/PHYTO-06-16-0245-R. Epub 2016 Nov 1.
Bacterial fruit blotch of cucurbits (BFB) is caused by the gram-negative bacterium Acidovorax citrulli, whose populations can be distinguished into two genetically distinct groups, I and II. Based on visual assessment of BFB severity on cucurbit seedlings and fruit after inoculation under greenhouse conditions, group I A. citrulli strains have been reported to be moderately to highly virulent on several cucurbit hosts, whereas group II strains have exhibited high virulence on watermelon but low virulence on other cucurbits. Additionally, group I strains are recovered from a range of cucurbit hosts, while group II strains are predominantly found on watermelon. The goal of this research was to develop tools to characterize and rapidly distinguish group I and II A. citrulli strains. We first sought to determine whether quantification of A. citrulli colonization of cucurbit seedling tissue reflects the differences between group I and II strains established by visual assessment of BFB symptom severity. Spray inoculation of melon seedlings with cell suspensions containing approximately 1 × 10 CFU/ml resulted in significantly higher (P = 0.01) population growth of M6 (group I; mean area under population growth curve [AUPGC] = 43.73) than that of AAC00-1 (group II; mean AUPGC = 39.33) by 10 days after inoculation. We also investigated the natural spread of bacterial cells and the resulting BFB incidence on watermelon and melon seedlings exposed to three group I and three group II A. citrulli strains under mist chamber conditions. After 5 days of exposure, the mean BFB incidence on melon seedlings exposed to representative group II A. citrulli strains was significantly lower (25 and 3.98% in experiments 1 and 2, respectively) than on melon seedlings exposed to representative group I strains (94.44 and 76.11% in experiments 1 and 2, respectively), and on watermelon seedlings exposed to representative group I and II strains (70 to 93.33%). Finally, we developed a polymerase chain reaction assay based on the putative type III secretion effector gene, Aave_2166, to rapidly distinguish group I and II A. citrulli strains. This assay will be important for future epidemiological studies on BFB.
葫芦科细菌性果斑病（BFB）由革兰氏阴性菌西瓜嗜酸菌引起，其菌群可分为两个基因不同的组，即I组和II组。根据温室条件下接种后对葫芦科幼苗和果实上BFB严重程度的视觉评估，据报道I组西瓜嗜酸菌菌株对几种葫芦科寄主具有中度至高度毒力，而II组菌株对西瓜表现出高毒力，但对其他葫芦科植物表现出低毒力。此外，I组菌株可从多种葫芦科寄主中分离得到，而II组菌株主要存在于西瓜上。本研究的目的是开发工具来鉴定和快速区分I组和II组西瓜嗜酸菌菌株。我们首先试图确定对葫芦科幼苗组织中西瓜嗜酸菌定殖的定量是否反映了通过对BFB症状严重程度的视觉评估所确定的I组和II组菌株之间的差异。用含有约1×10 CFU/ml的细胞悬液喷雾接种甜瓜幼苗，接种后10天，M6（I组；群体生长曲线下平均面积[AUPGC]=43.73）的群体生长显著高于AAC00-1（II组；平均AUPGC=39.33）（P=0.01）。我们还研究了在雾室条件下，暴露于3种I组和3种II组西瓜嗜酸菌菌株的西瓜和甜瓜幼苗上细菌细胞的自然传播以及由此产生的BFB发病率。暴露5天后，暴露于代表性II组西瓜嗜酸菌菌株的甜瓜幼苗上的平均BFB发病率显著低于暴露于代表性I组菌株的甜瓜幼苗（实验1和2中分别为25%和3.98%），以及暴露于代表性I组和II组菌株的西瓜幼苗（70%至93.33%）。最后，我们基于假定的III型分泌效应基因Aave_2166开发了一种聚合酶链反应检测方法，以快速区分I组和II组西瓜嗜酸菌菌株。该检测方法对未来BFB的流行病学研究将具有重要意义。
