Concept:Gummosis refers to a plant disease in which gum oozes from the affected tissue, commonly caused by fungal pathogens.Explanation:In citrus plants, gummosis mainly affects the trunk and branches, leading to gum exudation.The cause is usually a soil-borne fungus belonging to the genus Phytophthora.Aphids, white flies, and bacteria do not produce this gumming symptom.Therefore, the causal agent belongs to the group called fungi.Answer:B. fungi