Albugo (White Rust) – Exam Notes

Systematic Position

Obligately endoparasitic on higher plants; worldwide.
Common host families: Cruciferae, Convolvulaceae, Compositae, Amaranthaceae.
Typical Indian species: A. candida (white rust of crucifers), A. bliti, A. portulacae, etc.

Early: white, shiny, irregular pustules on abaxial leaf surface → spread to adaxial side & stems.
Pustules coalesce; epidermis ruptures, exposing white powdery mass.
Marked hypertrophy / hyperplasia → thick, fleshy leaves; malformed stems & floral organs; overall stunting.

Mycelium: aseptate, coenocytic, intercellular.
Hyphal wall: chitin; cytoplasm granular with many nuclei, vacuoles, oil drops, glycogen.
Haustoria: knob-like; head ≈ 4\,\mu m diam., stalk ≈ 0.5\,\mu m wide; obtain nutrients via extra-haustorial matrix.

Units: multinucleate conidia / sporangia (zoosporangia).
Conidiophores: short, club-shaped, perpendicular palisade layer under epidermis.
Conidia formation: basipetal chain; youngest at base.
• Each conidium \approx 13\text{–}18\,\mu m, spherical to ellipsoid, 5\text{–}8 nuclei.
• Disjunctor (mucilaginous disc) separates successive conidia.
Conidial wall types (Waterhouse):
• Aequales – uniformly thin.
• Annulati – with internal equatorial thickening.
Germination:
• Moist, cool (≈ 10^{\circ}\text{C}) conditions.
• Protoplast cleaves → ≈ 8 reniform, biflagellate zoospores; released via vesicle.
• Zoospore encysts, produces germ-tube → enters host through stoma/epidermis.
• Some species may germinate directly without zoospores.

Timing: late season, deep in host tissue; indicated externally by hypertrophy.
Oogonium: globose; young (≈ 6\text{–}12 nuclei) → mature with \approx 100\text{–}300 nuclei; differentiates into periplasm & central ooplasm (single functional nucleus).
Antheridium: club-shaped; initially 6\text{–}12 nuclei → only one functional at maturity.
Fertilization: gametangial contact; antheridial fertilization tube delivers male nucleus to egg.
Oospore: thick-walled; layers—exine (warty), intine (thin) ± middle layer.
• Diploid nucleus → meiosis + mitoses → ≈ 32 haploid nuclei (timing debated).
• Resting period: several months; released as host tissues decay.

Spring activation: protoplast cleaves → many biflagellate zoospores.
Exine ruptures; intine protrudes as vesicle; zoospores released, swim, encyst, germ-tube infects new host through stoma.

Dominant mycelial phase likely diploid (per Sansome & Sansome).
Two spore types ensure spread & survival:
• Conidia/zoosporangia → rapid, secondary infection (white pustules).
• Oospores → sexual, overwintering propagule.