Hi @ashroj,
What you're looking at is powdery mildew. Powdery mildew is a fungal disease that appears as white, powdery patches on leaves, stems, buds, and flowers. It thrives in humid, warm conditions and affects a wide range of plants, including roses.
It can be treated with an application of Yates 750ml Fungus Gun Systemic Disease Spray. Make sure you read and follow the instructions on the packaging to ensure best results. Also, remember to wear gloves and a mask when applying the fungicide.
If water is allowed to sit on the leaves, the sitting moisture will allow the powdery mildew to grow. Future instances of powdery mildew can be prevented by pruning your roses to allow for more airflow.
You might like to check out How To Prune Roses for some guidance.
Let me know if you have any further questions.
Jacob
