The correct answer is option D: Downy mildew of grapes.

Explanation:
- Bordeaux mixture is a popular fungicide that is used to control fungal diseases in plants. It was first discovered in the mid-19th century in the Bordeaux region of France, which is known for its wine production.
- The discovery of Bordeaux mixture as a fungicide was linked to the control of downy mildew in grapes. Downy mildew is a fungal disease that affects grapevines and can cause significant damage to the crop if left untreated.
- The Bordeaux mixture was found to be effective in controlling downy mildew and became widely used in vineyards around the world.
- The mixture is made by combining copper sulfate (also known as bluestone) with lime and water. The copper sulfate acts as a fungicide, while the lime helps to reduce the acidity of the mixture and improve its adherence to the plant surfaces.
- The Bordeaux mixture works by inhibiting the growth and spread of fungal spores, preventing them from infecting the plant and causing disease.
- The discovery of Bordeaux mixture revolutionized the control of fungal diseases in agriculture, not only in grapevines but also in other crops. It provided a safe and effective alternative to the use of toxic chemicals for disease control.
- Today, Bordeaux mixture is still used in organic and conventional farming practices to control a wide range of fungal diseases in various crops, including grapes, potatoes, tomatoes, and citrus fruits.
- It is important to note that while Bordeaux mixture is effective against fungal diseases, it can also be harmful to beneficial organisms and the environment if used improperly. Therefore, its use should be regulated and applied according to recommended guidelines and dosage rates.