Which of the following statements is incorrect?

The correct answer is Option (3) → Cyanobacteria are heterotrophic microbes, many of which can fix atmospheric nitrogen.

The statements come from two parts of the chapter: biofertilisers (bacteria, fungi and cyanobacteria that enrich the soil) and biocontrol agents. The one that does not match NCERT is the answer.

Option (1) The main sources of biofertilisers are bacteria, fungi and cyanobacteria. — A correct statement. NCERT names bacteria, fungi and cyanobacteria as the main sources of biofertilisers.

Option (2) Baculoviruses are pathogens that attack insects and other arthropods. — A correct statement. Baculoviruses attack insects and other arthropods, which is why they are used as biocontrol agents.

Option (3) Cyanobacteria are heterotrophic microbes, many of which can fix atmospheric nitrogen. — An incorrect statement. Cyanobacteria are autotrophic microbes; they make their own food by photosynthesis. Many of them, such as Anabaena and Nostoc, do fix atmospheric nitrogen, but calling them heterotrophic is wrong.

NCERT Extract:
Microbes as Biofertilisers – “… Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. … Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc.”
Microbes as Biocontrol Agents – “… Baculoviruses are pathogens that attack insects and other arthropods.”