哈茨木霉菌株NF9和TC3对辣椒疫霉病菌具有强烈的抑制作用。超微结构与细胞化学研究表明，木霉菌丝能够缠绕并寄生疫霉菌菌丝，且重寄生作用主要发生在培养基内的菌丝上。但在绝大多数情况下，木霉菌可直接水解和破坏疫霉菌菌丝，说明木霉抑菌作用的主要机制是直接向外分泌水解酶分解疫霉的细胞壁及细胞质，而不是重寄生作用。不同培养基对木霉的重寄生作用有重要影响，减少培养基中葡萄糖含量可以增强木霉的重寄生作用和抑菌效果。此外，在菌落生长后期，疫霉的新生菌丝均可侵染自身的老菌丝。

从300条RAPD随机引物中筛选出扩增多态性丰富的13条引物，对寄主来源不同的29个寄生疫霉（Phytophthora parasitica）菌株进行基因组DNA遗传变异分析。用筛选出的13条引物对供试菌株进行RAPD-PCR扩增，共产生139条RAPD条带，其中133条为多态性条带，多态检测率为95.7%。利用Pop Gene Version 1.31软件对供试菌株间的遗传距离进行聚类分析并构建系统树状图，供试29个菌株被划分为5个遗传聚类组，不同菌株间具有丰富的遗传变异。其中寄主为烟草（Nicotiana spp.）、腊梅（Chimonanthus praecox）、凤尾兰（Yucca gloriosa）和西番莲（Passif lora edulis）的菌株的遗传结构与其寄主来源具有明显的相关性，而寄主为刺槐（Sophora chinensis）和番茄（L ycopersicum esculentum）的菌株与其寄主来源的相关性较小，来源于不同寄主植物的菌株之间遗传距离较远。结果提示在寄生疫霉与其不同寄主植物的长期协同进化过程中，寄主对病原菌的遗传结构具有一定的影响。

We examined the role of salicylic acid (SA) accumulation in the tobacco hypersensitive response (HR) and systemic acquired resistance (SAR) induced by PB90, a 90 kDa protein elicitor that is secreted by Phytophthora boehmeriae. The elicitor induced HR of a consistent shape and size on tobacco plants expressing the bacterial gene nahG. Salicylate hydroxylase is encodec by nahG and inactivates SA by converting it into catechol. The mutant NahG does not accumulate SA. In contrast, infiltration with of a wild type tobacco (cv. Xanthi nc) leaves with the elicitor caused the increase of SA levels. The same SA levels were observed in leaves not treated by the elicitor. HR appears to be mediated by a SA-independent signaling pathway. PB90 treatment resulted in enhanced resistance of wild-type plants to infection by black shank fungus, P. nicotianae, and TMV, but not in NahG plants. Moreover, the elicitor-induced expression of an SAR marker gene encoding PR-1a was suppressed in NahG plants. These results indicate that SA mediates SAR but not HR in tobacco treated with PB90. During plant-elicitor interactions, HR and SAR may be regulated by distinct signal pathways, or SA may function as an intermediate signal upstream of SAR but downstream of HR, and HR may not be a direct defense mechanism against pathogen infection.

A 90 kDa protein elicitor, PB90, was purified from the culture filtrate of Phytophthora boehmeriae. Procedures including hydrophobic interaction and ion-exchange chromatography were used to purify the protein. PB90 induced hypersensitive reaction in tobacco leaves at a concentration as low as 200 pM and induced systemic acquired resistance (SAR) to TMV, Alternaria alternata, P. parasitica and Ralstonia solanacearum. In addition, it also caused hypersensitive necrosis in leaves of Chinese cabbage and induced SAR to Collectorium higginsianum in the plant. Tobacco leaves infiltrated with 200 pM of the elicitor exhibited an enhanced phenylalanine ammonia lyase activity and peroxidase activity. The behavior on ion-exchange columns indicated that PB90 is an acidic protein. It is stable to heat, acidic and alkaline conditions, but sensitive to ProteaseK. An antiserum raised against the pure protein allowed localization of PB90 by immunogold labeling on the cell wall of the mycelia and encysting zoospores when the fungus is grown in vitro. It was implied that there is a novel protein elicitor secreted by P. boehmeriae, which has a broad activity spectrum. The characters of PB90 distinguished it clearly from the other elicitors previously identified from Phytophthora spp. The possible implications of such a novel elicitor in plant–microbe interactions are discussed.