Heavy dust can cause

The correct answer is D. browning of leaves.

Heavy dust can cause browning of leaves by blocking the stomata, which are small pores on the surface of leaves that allow gas exchange. When the stomata are blocked, the plant cannot take in carbon dioxide and release oxygen, which can lead to the death of the leaf.

Option A is incorrect because leaf blights are caused by fungi or bacteria, not by dust. Option B is incorrect because dust does not cause the stomata to open. Option C is incorrect because dust does not cause the stomata to close.