**Ergot of Rye:**

Ergot of rye is a fungal disease that affects rye and other cereal crops. It is caused by the fungus Claviceps purpurea. The fungus infects the ovaries of the plants, replacing the grain with a mass of dark purple to black fungal tissue, known as the ergot sclerotium. This sclerotium contains toxic alkaloids that can be harmful to humans and animals if consumed.

**Identification of the Causal Organism:**

The correct answer to the question is option 'C' - Claviceps. Claviceps purpurea is the scientific name for the fungus that causes ergot of rye. The other options mentioned in the question are not related to this fungal disease.

- Option 'A' - Uncinula is a genus of fungi that includes the powdery mildew pathogens.
- Option 'B' - Ustilago is a genus of fungi that causes smut diseases in various plants.
- Option 'D' - Phytophthora is a genus of fungi-like organisms that includes many plant pathogens, but it is not associated with ergot of rye.

Therefore, the correct answer is option 'C' - Claviceps.

**Life Cycle of Claviceps purpurea:**

The life cycle of Claviceps purpurea involves several stages:

1. Overwintering: The fungus survives the winter in the form of dormant sclerotia, which are the hard, black masses of fungal tissue. These sclerotia drop to the ground along with the infected grain.

2. Spring Infection: In spring, the sclerotia germinate and produce fungal structures called stromata. These stromata emerge from the soil and produce spore-bearing structures called ascocarps.

3. Ascospore Dispersal: The ascocarps release ascospores, which are the primary infective spores. These ascospores are carried by wind or insects to the flowers of cereal crops, where they initiate infection.

4. Infection and Colonization: The ascospores infect the ovaries of the plants, replacing the developing grain with the ergot sclerotium. The fungus colonizes the developing sclerotium and produces secondary spores called conidia.

5. Secondary Infection: The conidia are released from the infected sclerotia and can infect neighboring plants, spreading the disease further.

6. Harvest and Spread: The infected grain is harvested along with the healthy grain, and the ergot sclerotia are spread to new fields through contaminated seed or by wind or water.

**Conclusion:**

Ergot of rye is caused by the fungus Claviceps purpurea. It infects the ovaries of cereal crops, replacing the grain with a toxic mass of fungal tissue. The fungus has a complex life cycle involving ascospores and conidia for infection and spread. Claviceps is the correct answer to the given question as it is the specific genus responsible for ergot of rye, while the other options are unrelated to this fungal disease.

Ergot is the fungal disease of cereal grasses. It occurs especially in the rye as a result of infection due to ascomycetes fungi named Claviceps purpurea. This disease can infect the crop and hence decrease the production of grains. Hence, the correct answer is option 'C'.