• Nature of Plant Phytoalexins:
• Definition: Phytoalexins (coined by Müller and Börger, 1940) are low-molecular-weight antimicrobial secondary metabolites biosynthesized de novo by plant cells in response to pathogen attack or elicitor treatment.
• Chemical Classification: Phytoalexins are primarily phenolic compounds (e.g., isoflavonoids such as pisatin in pea, phaseollin in bean, medicarpin in alfalfa; stilbenes like resveratrol in grapes) and terpenoids/sesquiterpenoids (rishitin in potato, gossypol in cotton).
• Conclusion: Phytoalexins produced in response to infection are predominantly phenolic compounds (Option 1).

Correct option: (1) Phenolic compounds

• Translocation Route of Systemic Fungicides:
• Apoplastic Transport (Xylem Translocation): Over $90\%$ of modern systemic fungicides (such as triazoles, strobilurins, benzimidazoles, and carboxins) move through the plant's apoplast (cell walls, intercellular spaces, and dead xylem tracheary elements).
• Directionality: They are translocated acropetally (upward from roots to leaves) following the transpiration stream.
• Symplastic Transport: Only a few ambimobile fungicides (e.g., fosetyl-Al) move symplastically through the phloem.
• Conclusion: Most systemic fungicides are transported through the apoplast (Option 3).

Correct option: (3) Apoplast

• Nutrition Mode of Powdery Mildew (Erysiphe spp.):
• Ectoparasitic Biotrophic Nature: Erysiphe species (order Erysiphales) are obligate biotrophic plant pathogens that exhibit an ectoparasitic mode of life.
• Infection Pattern: Their vegetative mycelium, conidiophores, and conidia develop entirely externally on the outer cuticular surface of the host leaves and stems, forming a conspicuous white, powdery coating. They absorb nutrients strictly by sending specialized absorbing organs (haustoria) through the cuticle into host epidermal cells without killing them.
• Conclusion: The mode of nutrition of Erysiphe is ectoparasitic (Option 3).

Correct option: (3) Ectoparasitic