فطريات Oomycete: الخصائص والتصنيف

• These notes cover the topic of oomycete fungi.
• Oomycetes are classified within Kingdom Myceteae and Division Mastigomycota.

Classification of Oomycetes

• Division: Mastigomycota constitutes the lower, non-true fungi, which produce motile zoospores.
• This division is further divided into two sub-divisions based on the number of flagella: Haplomastigomycotina (one flagellum) and Diplomastigomycotina (two flagella).

Characteristics of Diplomastigomycotina

• They exhibit oogamous sexual reproduction.
• Asexual reproduction occurs via motile zoospores with two flagella, one smooth and anterior, and the other plumose and posterior.
• These fungi are aquatic or saprophytic on organic matter, or parasitic on higher plants.
• This sub-division encompasses the class Oomycetes.

Characteristics of Oomycetes (Class Oomycetes)

• Oomycetes lack cross-walls in their hyphae.
• Asexual reproduction involves zoospores with two flagella formed within sporangia.
• Sexual reproduction is oogamous, with distinct male (antheridia) and female (oogonia) sexual organs.

Order Peronosporales

• Occupy a crucial position among flagellated fungi, with most members exhibiting parasitic behavior and causing severe plant diseases in various crops.
• Some members thrive in air and soil.
• The mycelium grows inter cellularly, sending haustoria into plant cells to absorb nutrients.
• Asexual reproduction happens via either sporangia that release zoospores under humid conditions, or conidia and conidiophores are formed under dry conditions.

Family 1: Albuginaceae; Genus: Albugo.sp

• These are obligate parasites causing white rust disease on cruciferous plants, such as radish, turnip, and cabbage.
• The mycelium grows within the plant's intercellular spaces. Small, spherical haustoria penetrate the plant's cells to absorb nutrients.
• Asexual reproduction occurs through the formation of conidia under the host's epidermis.
• At low temperatures and high humidity, the contents of the sporangium divide into up to 12 motile zoospores.
• Zoospores swim, lose their flagella, encyst, and then germinate, forming a germ tube that enters the plant tissue through stomata, leading to new mycelial growth.

Family 2: Pythiaceae; Genus 1: Pythium

• These are saprophytic or parasitic in water on algae, or facultative parasites on higher plants, causing various diseases.
• Seedling blight (damping-off) is a condition caused by infection of seedlings.
• The mycelium consists of hyaline, multinucleate hyphae lacking cross-walls and with copious branching.
• Asexual reproduction occurs via swimming zoospores within spherical, filamentous, or ovoid sporangia.
• Sexual reproduction occurs through the formation of oospores.

Family 2: Pythiaceae; Genus 3: Phytophthora

• Exhibits a saprophytic lifestyle if a host plant is absent, otherwise transforming to an obligate parasite when the host is present.
• These fungi are responsible for illnesses such as potato tuber rot and tomato fruit rot, frequently known as the Late Blight in potatoes, or Downy Mildew on tomato leaves.
• The sexual reproduction for this fungi is the formation of oospores.
• Asexual reprodiction for this fungi is through swimming zoospores

Family 3: Peronosporaceae; Genus 1: Plasmopara

• Causes downy mildew in grapes.
• Conidiophores are slender and branched, with axes emerging at right angles.
• Each branch terminates with sterigmata holding a sporangium.

Family 3: Peronosporaceae; Genus 2: Bremia

• Causes downy mildew on lettuce.
• Conidiophores demonstrate dichotomous branching, with inflated, shallow-dish-shaped tips bearing short denticles, each bearing a sporangium.

Family 3: Peronosporaceae; Genus 3: Peronospora

• Parasitic on plants in the Cruciferae family.
• Conidiophores exhibiting dichotomous branching, with curved, pendulous tips ending in a single conidium, giving a claw-like shape.