Red rot of sugarcane is a serious disease that affects sugarcane plants. It is caused by the fungus Colletotrichum falcatum.

Option 1: Altemaria altemata is not the causal agent of red rot of sugarcane. It is a species of fungus that causes leaf spots in many plants, but not red rot in sugarcane.

Option 2: Phylophthora infestans is the causal agent of late blight in potatoes and tomatoes, not red rot in sugarcane. Late blight causes dark lesions on the leaves and tubers of these plants.

Option 4: Cercospora personata is a fungus that causes Cercospora leaf spot on soybeans, but it is not associated with red rot in sugarcane.

Therefore, the correct answer is option 3, Colletotrichum falcatum.