Spore Type: Oospore Oospores

Oomycetes: Sexual Reproductive Antheridium Oogonium Oospore

Hyphae of Oomycetes Hypha Coenocytic hyphae

Asexual Reproductive Structures of Oomycetes Sporangia

Sporangia Borne on Single or Branched Sporangiophores Lemon to globose shaped structures are sporangia

Zoospores Single cell, but membrane bound Product of mitosis Released from sporangium Motile Not long lived Sporangium Zoospores

Germination of Zoospores

Oomycetes: Pythium, Phytophthora & Aphanomyces

Diseases caused by Oomycetes Seed rots and Seedling Damping-off

Diseases caused by Oomycetes Damping-off

Diseases Caused by Oomycetes Root rots

Bean root rot Pythium ultimum

Pythium Blight of Turfgrasses Diagnosis based on microscopic signs Oospores Coenocytic hyphae

Diagnosis based on symptoms and macroscopic signs

Control: Fungicides - mefenoxam, propamocarb, fosetyl Al

Phytophthora Root and Crown Rot: Symptoms Leaves are small, yellowish, and sparse; fruit are small; often turn red prematurely. Bark and wood at crown is orange to brown and water-soaked; margin between diseased and healthy tissue is usually distinct.

Alfalfa root rot caused by Phytophthora medicaginis

Stunting and yellowing Wet low area of field Disease resistance Black lesions Alfalfa root and crown rot - Phytophthora medicaginis

Diseases Caused by Oomycetes Maple Decline

Diseases Caused by Oomycetes Maple Decline Root and Crown Rot

Phytophthora capsici on watermelon Plant Disease cover 2003

Figure 1. Typical symptoms of Phytophthora blight on cucurbits: (A) post-emergence damping-off of a pumpkin seedling; (B) crown infection of a summer squash plant; (C) leaf spot of pumpkin; (D) fruit rot of processing pumpkin. (Courtesy M. Babadoost)