Black rust of wheat is caused by

Options

A. Puccinia
B. Albugo
C. Ustilago
D. Cystopus

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the organism responsible for causing black rust in wheat.

2. **Identifying the Options**: The options provided are:
- Paxinia
- Alvebo
- Ustilago
- Cystopas

3. **Analyzing Each Option**:
- **Paxinia**: This is a genus of fungi known for causing diseases in plants. Specifically, *Paxinia* (more accurately *Puccinia*) is known to cause black rust in wheat, particularly *Puccinia triticina*.
- **Alvebo (Cystopas)**: This is an older name for *Albugo*, which causes white rust in plants, not black rust. Therefore, this option is incorrect.
- **Ustilago**: This genus is known to cause smut diseases in various plants, including barley. It does not cause black rust in wheat, making this option incorrect as well.
- **Cystopas**: As mentioned, this is an outdated term for *Albugo*, which is not responsible for black rust.

4. **Conclusion**: The only correct option that causes black rust of wheat is *Paxinia* (specifically *Puccinia*).

5. **Final Answer**: The correct answer is **Paxinia**.

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