Indian Institute of Foreign Trade 2008 Solved Paper

Black heart is a non-parasitic disease of the fruit of cultivated plants, such as potatoes, that causes them to rot from the inside. External signs of the disease may or may not be present. The internal blackening develops in plants exposed to environmental pathology such as drought conditions or poor soil.
Red rot is a very serious disease of sugarcane. It was first described by Went in 1893 (56). Since then it has been found to cause epiphytotics in different countries. The surest symptom of the disease is the reddening of the internal inter nodal tissues with crossbars of white patches in the reddened area. This red colour is caused by a dye which is secreted by the host and is antagonistic to the red rot fungus.
Karnal bunt (also known as partial bunt) is a fungal disease of wheat, durum wheat, and triticale. The smut fungus Tilletia indica, a basidiomycete, invades the kernels and obtains nutrients from the endosperm, leaving behind waste products with a disagreeable odor that makes bunted kernels too unpalatable for use in flour or pasta.
Powdery mildew can be a serious disease of field peas. It occurs sporadically when warm humid conditions favour itsgrowth late in the season. Seed from infected pods can be discoloured and less palatable, which reduces its market value.
Hence, option B is the correct answer.