Below is a photo of a cucurbit leaf infected with an ascomycete fungus. White mat of hyphae and spores are abundant on the leaf surface, and round brown structures are also present. What are these brown structures and what is their function?

Question

BlackTom AI · Accepted Answer

Chasmothecia are closed, spherical ascomata (fruiting bodies) produced by many powdery mildew fungi; they function as resting/sexual structures that contain asci with ascospores.

1) Relevant knowledge points and concepts
- Powdery mildews (family Erysiphaceae) are ascomycetes that produce an abundant superficial white mycelial mat and asexual conidia on the leaf surface.  
- Their sexual fruiting bodies are chasmothecia (formerly cleistothecia): small, brown, ornamented, globose, closed structures that contain asci and ascospores used for overwintering and sexual recombination.  
- Other fungal structures differ: perithecia are flask-shaped with an ostiole; aecia and telia are from rust fungi (basidiomycetes) with different spore types; pycnidia are asexual, flask-shaped conidiomata.

2) Step-by-step reasoning
- The photo shows a white mat of hyphae and copious surface spores (typical of powdery mildew) plus discrete, round brown, ornamented spheres sitting on the surface.  
- Powdery mildew sexual bodies are chasmothecia: tiny, closed, spherical, brown and often have surface ornamentation or appendages. Their presence alongside the white mycelium matches known powdery mildew morphology (e.g., on cucurbits).  
- Perithecia would appear as flask-shaped structures with an ostiole — not simple round beads on the surface. Aecia and telia are characteristic of rust fungi, which have different symptom patterns and spore stages. Pycnidia are asexual and usually immersed or flask-like, not the distinctive ornamented globose ascomata shown.

3) Why the correct answer is correct
- The brown spheres are classic chasmothecia: closed, resting sexual structures that contain asci/ascospores, enabling survival and sexual recombination.

4) Why other options are incorrect (brief)
- Perithecia: wrong shape and ostiole-bearing, not typical of powdery mildew.  
- Aecium/telia: rust fungus structures (basidiomycetes), different life cycles.  
- Pycnidia: asexual, flask-like conidiomata, not the closed spherical sexual bodies.

Conclusion: The brown structures are chasmothecia, resting structures containing ascospores, which matches the provided correct answer.

Below is a photo of a cucurbit leaf infected with an ascomycete fungus. White mat of hyphae and spores are abundant on the leaf surface, and round brown structures are also present. What are these brown structures and what is their function?单项选择题

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