Biology of two common phyto-pathogenic Oomycetes

Phytophthora infestans

Obligate parasite.

Infect potatoes and tomatoes with late blight disease.

   

Life cycle

Mycelium:  Non septate, coenocytic

Asexual reproduction

In the host tissue inter-cellular somatic hyphae colonizes and club shaped haustorias penetrate the tissues, feeding the parasite.

Sporangiophores are produced from the hyphae and emerge through stomata.

Sporangiophores produce lemon-shaped sporangia laterally and at the terminal end/apex of the sporangiphore. A Sporangium possesses a papilla on its tip.

Outcome of the sporangium depends on the temperature and humidity.

Low humidity or high temperature: Sporangium germinates by a germ tube that later results the somatic hapha.  

High humidity (when raining) or low temperature (12⁰C or below: Cytoplasm of the sporangium divides and gives rise to biflagellate Zoospores. Either by dissolution of papilla or through its opening tip, zoospores are released to outside. Zoospores directly penetrate host tissue and undergo encystment in cells. They develop into germ tubes that later result the hyphae.

Sexual reproduction

Plasmogamy occurs as oogonium punches the antheridium, going through that antheridium, oogonium grows above it. The antheridium therefore appears as a funnel at the base around the oogonium. Karyogamy and meiosis occurs respectively. Oospore is spherical and adapted to adverse environmental conditions. It results out a sporangiophore that again develops into a sporangium.

via www.apsnet.org

Plasmopara viticola

Obligate parasite.

Infect grape vines with Downy mildews.

Life Cycle

Mycelium is non septate and coenocytic.

Life cycle is much similar to Phytophthora infestans as both are oomycites.

Asexual reproduction

Somatic hyphae develops in inter cellular space host tissue penetrating with haustoria.

Hyphae give rise to sporangiophores that emerge through stomata, branching and producing sporangia at each apex of the branches.

Sporangia may either germinate directly into mycelium or produce kidney shaped, biflagellate zoospores, depending on humidity and temperature just like the Phytophthora infestans. Zoospores encyst in cells, germinate and result the somatic hyphae.

Sexual reproduction

Sexual phase is taken place at unfavorable conditions. Plasmogamy occurs by gametangial copulation as oogonium and anthredium come close together. Karyogamy results zygote that later gives the spherical oospore. Oospore divides by meiosis and undergoes germination producing a sporangium.

More about downy mildew http://fruit.cfans.umn.edu/grape/IPM/downy.pdf