What does the cross section of an infected stem look like when it is infested with fusarium wilt?

The correct answer describes the appearance of an infected stem affected by fusarium wilt, which is characterized by brown discoloration that is particularly concentrated in the inner layers of tissue. Fusarium wilt is known to cause wilting and yellowing of the plant, and upon examination of the stem cross-section, the infection typically manifests as a brownish discoloration due to the decay of vascular tissues as the fungal pathogen progresses through the plant. This internal browning often points to the effects of the pathogen on the xylem, which is responsible for water transport, leading to symptoms of wilting.

Other options illustrate different types of discoloration that are not characteristic of fusarium wilt. For instance, purple discoloration might suggest other conditions such as environmental stress or different pathogens. Similarly, reddish discoloration, while indicative of certain other diseases, does not align with fusarium wilt's typical presentation. The focus on the outer layers for brown discoloration doesn't reflect the internal vascular damage linked to this specific wilt disease. This understanding of fusarium wilt assists in accurate identification and treatment of affected plants.