Which of the following is an example of a symptom of a plant pathogen?

Dying leaf tissue is a clear symptom of a plant pathogen because it indicates that the plant is undergoing stress or damage caused by a disease agent, which may include viruses, fungi, bacteria, or nematodes. Symptoms are observable changes or reactions in the plant that signify it is affected by a pathogen. In this case, the yellowing, wilting, or browning and subsequent dying of leaves often occur due to the disruption of normal physiological processes, such as photosynthesis and nutrient uptake, due to the pathogenic influence.

The other options, while indicative of possible plant problems, represent either signs or physical evidence of the pathogen rather than symptoms caused directly by the disease. For instance, mold growth on leaves is a sign of fungal infection but not a direct symptom experienced by the plant itself. Similarly, nematodes on roots can be a sign of infestation but do not reflect the plant's immediate response to a pathogen. Bacterial ooze on stems may indicate infection as well but is also more indicative of a pathogen’s presence rather than the plant's symptomatic response to that presence.