Blue vitriol is used as a fungicide for grape plants.

Explanation

**Step-by-Step Solution:**

1. **Identify Blue Vitriol**: Blue vitriol is chemically known as copper sulfate pentahydrate, which has the formula CuSO4·5H2O.

2. **Understand the Function**: A fungicide is a substance that kills or inhibits the growth of fungi. It is used in agriculture to protect plants from fungal infections.

3. **Application in Agriculture**: Blue vitriol (copper sulfate pentahydrate) is commonly used in agriculture, particularly in grape cultivation, to control fungal diseases.

4. **Conclusion**: Since blue vitriol is indeed used as a fungicide for grape plants, the statement "Blue vitriol is used as a fungicide for grape plants" is true.

**Final Answer**: True

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