Which of the following is a symptom of apple scab?

The identification of symptoms related to plant diseases is crucial for effective management and control. In the case of apple scab, the correct answer is that leaves develop dark lesions. Essentially, apple scab is caused by the fungus Venturia inaequalis, and one of the hallmark symptoms of this disease is the appearance of dark, olive-green to black lesions on the leaves. These lesions may enlarge and cause leaves to eventually yellow and fall off, affecting the tree's overall health and fruit production.

Understanding this key symptom helps in diagnosing apple scab and differentiating it from other diseases or conditions that may affect apple trees. The presence of these dark lesions is a definitive indicator of the disease, which makes it fundamental for anyone involved in plant health to recognize.

The other options represent symptoms that are associated with different plant diseases or issues; for instance, soft rot on fruit is indicative of issues such as bacterial infections rather than fungal ones like apple scab. Curling of leaves often suggests environmental stress or other viral infections, while yellowing of stems is typically related to nutrient deficiencies or root problems rather than the specific symptoms caused by apple scab. Thus, recognizing the specific symptom of dark lesions on leaves is essential in effectively identifying and addressing apple scab.