Which of the following statements is incorrect?

Rhizobium is a symbiotic bacterium that lives on the stem of legumes.

The correct option is 4. Rhizobium is a symbiotic bacterium that lives on the stem of legumes.

(a) Microbes are used to ferment fish, soybean, and bamboo shoots to make foods: This statement is correct. Fermentation is a process that utilizes microorganisms, such as bacteria or yeast, to convert organic substances into different food products. Examples include the fermentation of fish to make fish sauce, soybean to make soy sauce, and bamboo shoots to make pickles.

(b) Different varieties of cheese are distinguished by their characteristic texture, flavor, and taste, which are influenced by the specific microbes used: This statement is also correct. The production of cheese involves the use of specific bacteria and fungi which contribute to the unique flavors, textures, and aromas of different cheese varieties. The specific combination of microbes used in the cheese-making process influences its characteristics.

(c) Trichoderma sp., a type of free-living fungi, can be found in root ecosystems where they act against several plant pathogens: This statement is correct. Trichoderma is a genus of fungi that are known for their biocontrol abilities. They are commonly found in soil and root ecosystems, where they act as antagonists against various plant pathogens, helping to protect plants from diseases.

(d) Rhizobium is a symbiotic bacterium that lives on the stem of legumes: This statement is incorrect. Rhizobium is a symbiotic bacterium that forms nodules on the roots of leguminous plants, not on their stems. These nodules house the bacteria, which have the ability to fix atmospheric nitrogen into a form that can be utilized by the plant.

In conclusion, based on the information provided, statement (d) is incorrect, as Rhizobium forms nodules on the roots of leguminous plants, not on their stems.