The staple crop in lreland was completely destroyed in 1844-1846, by a fungal disease resulting in a great famine. The causal organism in this case was

Options

A. Puccinia graminis
B. Ustilago tritici
C. Phytophthora infestans
D. Claviceps purpurea

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question states that a staple crop in Ireland was destroyed between 1844-1846 due to a fungal disease, leading to a great famine. We need to identify the causal organism responsible for this disease.

2. **Analyzing the Options**: The options provided are:
- A) Puccinia graminis
- B) Astilago triticae
- C) Phytophthora infestans
- D) Claviceps purpurea

3. **Identifying the Correct Organism**:
- **Option A: Puccinia graminis**: This fungus is known for causing stem rust in cereal crops, which can lead to significant crop losses.
- **Option B: Astilago triticae**: This is another fungal pathogen, but it primarily affects wheat and is not known for causing the famine in Ireland.
- **Option C: Phytophthora infestans**: This is the causal agent of potato blight, which was indeed responsible for the Irish Potato Famine.
- **Option D: Claviceps purpurea**: This fungus causes ergot disease in cereals but is not related to the Irish famine.

4. **Final Decision**: The correct causal organism responsible for the great famine in Ireland during 1844-1846 is **Phytophthora infestans** (Option C), as it caused the potato blight that devastated the potato crop, which was a staple food in Ireland.

### Final Answer:
The causal organism in this case was **Phytophthora infestans**.

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