page_title: Phytophthora nicotianae
section_title: Disease cycle
hierarchical_section_title: Phytophthora nicotianae / Disease cycle
caption_attribution_description: English: This image shows Black Shank (Phytophthora nicotianae) infections on American Box wood (Pcinnamomi)
context_page_description: Phytophthora nicotianae or black shank is an oomycete belonging to the order Peronosprales and family Peronosporaceae.
context_section_description: Black Shank is a polycyclic soil borne disease, with the possibility of multiple disease cycles per growing season occurring from May to October. There are important structures this pathogen uses in its disease cycle. Chlamydospores are produced asexually and serve as long lived resting structures, surviving from four to six years. Chlamydospores are the primary survival structure, the primary inoculum, and are usually produced in abundance. These spores germinate in warm and moist soil to produce a germ tube that infects plants or produces a sporangium. Another asexual structure and secondary inoculum, appearing ovoid, pear, or spherical in shape are called sporangium. These spores are produced and can either germinate directly or release motile zoospores within 24 hours of inoculation with the right conditions. Zoospores are kidney shaped with an anterior tinsel flagellum and a posterior whip like flagellum that helps to navigate toward root tips were infection occurs. Black Shank needs water for germination and movement because zoospores swim through soil pores and standing water. Splashing water from rain or irrigation can infect healthy plant leaves leading to more repeating secondary cycles. Zoospores move toward nutrient gradients around root tips and host wounds. Once the root surface is contacted, zoospores encyst and a germ tube will emerge penetrating the epidermis. Infection leads to systemic rotting of the root system and wilting and chlorosis in the leaves. Another structure called hyphae is colorless, transparent, and coenocytic, but colonies may yellow with age. Also, there is much morphological variation in colony type with different isolates of P. nicotianae and the growth may differ when grown on different media. The hyphae are heterothallic and require two mating types to produce oospores, the sexual survival structure. Many fields only contain one mating type, so the zoospores rarely germinate and rarely cause epidemics.
caption: Black Shank infection on American Boxwood.
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