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Oomycetes (water molds) • Diploid (2N) • Hyphae are coenocytic (tubes) • Cell wall made of cellulose • Sexual reproduction: oospore • Asexual reproduction: sporangia and zoospores Websites to brush up on Oomycete diseases • Introduction to Oomycetes http://www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/IntroOomycetes.aspx • Why are Phytophthora and other Oomycota not true Fungi? http://www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/Oomycetes.aspx • Oomycetes http://www.apsnet.org/edcenter/intropp/LabExercises/Pages/Oomycetes.aspx • Phytophthora root rot of soybean http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/PhytophthoraSojae.aspx • Late Blight of Potato/Tomato http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/LateBlight.aspx • Downy mildew of grape http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/DownyMildewGrape.aspx 1 Oomycete videos • Pythium root rot of cereals http://www.youtube.com/watch?v=HClhO1Q-ErQ • Phytophthora ramorum sporangia and zoospores http://www.youtube.com/watch?v=rTo6UwnZiQg Apple scab disease cycle video Things to look for: overwintering? how is a new infection cycle initiated? is there a secondary (summer) cycle of reproduction? are spores formed by sexual or asexual processes? which spore type is most important economically? 2 Apple Scab Video Apple scab disease cycle video Things to look for: overwintering? how is a new infection cycle initiated? is there a secondary (summer) cycle of reproduction? are spores formed by sexual or asexual processes? which spore type is most important economically? 3 Lecture 5: Disease Cycles Example 1: Late blight of potato ASEXUAL (summer) REPEATING STAGE OVERWINTERING ASEXUAL CYCLE SEXUAL CYCLE Generic Disease cycle Outside Host OVERWINTERING Deposition Dissemination REPEATING STAGE SEXUAL or ASEXUAL CYCLE Survival ASEXUAL (summer) CYCLE Infection Incubation Disease Development Reproduction Inside Host 4 Disease cycle diagrams provide useful information, BUT what information is difficult to obtain from the drawings? • A weighting of the practical significance of the various steps, cycles, and subcycles • A sense of the length of time required by various steps, cycles, and subcycles. Lecture 5: Disease Cycles Example 1: Late blight of potato ASEXUAL (summer) OVERWINTERING REPEATING STAGE *ASEXUAL CYCLE SEXUAL CYCLE *For late blight, this is the most important method of overwintering 5 Terms associated with disease cycles Primary inoculum – infectious propagule, usually from an overwintering source, that initiates/ perpetuates the primary disease cycle, as opposed to infectious propagules spread disease during the season. For fungal and oomycete pathogens, if a sexual spore is involved in the disease cycle, it typically serves as (e.g., ascospore) or gives rise to (oospore to zoospore) the primary inoculum. disease cycle terms continued Secondary inoculum – infectious propagules that were produced by infections that took place during the same growing season - this type of inoculum is nearly always asexual, and the infectious propagules are relatively short-lived 6 disease cycle terms continued Monocyclic disease – a disease where infections are caused by primary inoculum only - Soilborne diseases are commonly monocyclic - The timing of individual infections can still vary disease cycle terms continued Polycyclic disease – a disease where one to many cycles of infection are initiated by secondary inoculum - Many foliar diseases (e.g., powdery mildews and rusts of grasses) are polycyclic 7 disease cycle terms continued Terms that address the ‘TIME REQUIRED’ issues – they relate to the period after infection but before symptom expression • Incubation period- the time between infection and symptom expression – typically regulated by temperature • Latent period – similar to incubation period, it is time the time from infection to new spore production – essentially, the generation time – the latent periods governs the speed of increase of polycyclic diseases • Quiescent infections- inactive, symptomless, microscopic infections – important in many fruit rots and seed-borne diseases Generic Disease cycle Outside Host OVERWINTERING Deposition Dissemination REPEATING STAGE SEXUAL or *ASEXUAL CYCLE Survival ASEXUAL (summer) CYCLE Infection Incubation period Incubation Disease Development Reproduction Inside Host Latent period = generation time 8