What does PCNB primarily control?

PCNB, or Pentachloronitrobenzene, is primarily known for its effectiveness in controlling seedborne and soilborne diseases, particularly smuts. This fungicide acts by disrupting the cell division and growth of fungi, which are often the culprits behind these diseases in seeds. When seeds are contaminated with harmful fungi, it can lead to poor germination rates and weak plant development, making the control of such pathogens essential for successful agriculture.

This specific function of PCNB emphasizes its role in protecting seed health by targeting pathogens that affect the seeds either from the external environment or from the soil in which they are planted. The other options do not accurately represent the primary action of PCNB, as it does not focus on controlling insects, only surface fungi on leaves, or weeds and their seeds. Thus, option B accurately captures the primary purpose of PCNB in seed treatment.