Which of the following is not a pest of rapeseed-mustard crop?

The correct answer is (D) Alternaria blight.

Let's break down each option:

(A) White rust: This is a disease caused by a fungal pathogen, Albugo candida, which affects the rapeseed-mustard crops. Although it's a disease and not a pest, it is a common issue for these crops.

(B) Stalk borer: This is a pest that attacks the stems of rapeseed and mustard plants. The larvae feed on the stems and cause significant damage.

(C) Aphids: These are pests that suck sap from the plants, weakening them and making them more susceptible to diseases. Aphids are common pests of rapeseed and mustard crops.

(D) Alternaria blight: This is a fungal disease caused by the Alternaria fungus, which affects rapeseed-mustard crops. However, it is not a pest; rather, it is a disease that affects the plant’s leaves, stems, and pods.

Thus, Alternaria blight (option D) is a disease, not a pest, and is the correct answer.