Ergot is caused by

The correct answer is Claviceps.

Step-by-step Explanation:

1. Understanding Ergot:
Ergot is a plant disease that affects cereal grasses, most notably rye, wheat, and barley. It is also the name of the dark, hard structures (sclerotia) that replace the grain kernels on infected plants.

2. Identifying the Causal Organism:
This disease is caused by a group of fungi belonging to the genus Claviceps, with Claviceps purpurea being the most common and well-known species responsible for infecting cereal crops.

3. Analyzing the Options:
- Penicillium: A genus of fungi known for its role in penicillin production and food spoilage, but it does not cause ergot.
- Claviceps: The correct fungal genus that causes ergot disease in plants.
- Rhizobium: A genus of bacteria that forms a symbiotic relationship with the roots of legumes to fix atmospheric nitrogen; it is not a fungus and does not cause plant disease.
- Aspergillus: A genus of common molds, some of which produce toxins (like aflatoxin) or cause infections, but not ergot.

Therefore, ergot is caused by Claviceps.