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Life Cycle of an Oomycete
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Oomycetes Pythium, Phytophthora Aphanomyces
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Diseases caused by OomycetesSeed rots and
Seedling Damping-off
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Diseases caused by OomycetesDamping-off
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Diseases Caused by OomycetesRoot rots
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Diseases Caused by OomycetesConcept of Primary
Secondary Symptoms
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Phytophthora Root andCrown 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.
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Alfalfa root rot caused by Phytophthora
megasperma
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Diseases Caused by Oomycetes

• Maple Decline

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Diseases Caused by OomycetesMaple DeclineRoot
and Crown Rot
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Pea Root RotAphanomyces euteiches
Wet soils followed by dry hot weather severe
disease
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OomycetesAphanomyces

• Sporangium is a modified hypha
• Primary spores exit and aggregate at end of
  sporangium
• Primary spores release zoospores

Spores
Sporangiophore
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OomycetesAphanomyces

• Primary spores
• Two means to resume growth
• Germinate to germ tube
• Release zoospore through pore

Exit pore
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Oomycetes Aphanomyces
Exit pore
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Pea Root RotAphanomyces euteiches
Management Early planting Resistant pea varieties
Management Crop rotation Avoid high risk fields
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Pythium Blight of TurfgrassesDiagnosis based on
microscopic signs
Oospores
Coenocytic hyphae
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Control Fungicides - mefenoxam, propamocarb,
fosetyl Al
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Diagnosis based on symptoms and macroscopic signs
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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)
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Late blight caused by Phytophthora infestans
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Tubers infected with P. infestans
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Late blight of Potatoes (Phytophthora
infestans)
(Image courtesy of The American Phytopathological
Society)
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Late blighted potato tubers
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View of test field - Hancock Research Station
1994
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Downy Mildew of Grape
Downy Mildew Plasmopora viticola
Sporangia

• Source of secondary inoculum
• Inoculum to new leaves
• and fruit

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Downy Mildew Symptoms and Signs
Diseased fruit are off-color, shrivel and become
covered with white, downy fungal growth.
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Dew and rain contributes to duration of leaf
wetness
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Germination of Zoospores Penetration of germ
tube Highly dependent on duration of free water
on leaf surface
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Albugo candida
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Oomycetes Aphanomyces
Spores
Sporangiophore
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Phytophthora compared to Downy Mildew Fungi
A
B
A B
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Phytophthora compared to Downy Mildew Fungi
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B
A Phytophthora B Downy mildew fungus
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Life Cycle of an Oomycete
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Downy Mildew - Grape Plasmopora viticola Obligate
biotrophic pathogen Photosynthetic area
lost Fruit yield reduced Fruit rot
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Downy Mildew Symptoms and Signs
Signs
Early symptoms
Late symptoms
Leaf spots are yellowish on upper surface, white
and downy on lower surface. Late-season infection
results in a mosaic of yellow, red, and brown
angular spots.
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Downy Mildew GrapePlasmopora viticola
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Phytophthora capsici on watermelon
Plant Disease cover 2003