Which of the following is NOT a symptom of nutrient deficiency due to Grape fanleaf nepovirus?

Dark green foliage is not a symptom of nutrient deficiency caused by Grape fanleaf nepovirus. In fact, many nutrient deficiencies in plants tend to result in pale or yellowing leaves, particularly when there is a lack of essential nutrients like nitrogen. When vines exhibit dark green foliage, it typically indicates that they are receiving adequate nutrients, particularly nitrogen, which is crucial for chlorophyll production.

On the other hand, symptoms like unthrifty vines, leaf wilting, and reduced growth are indeed associated with nutrient deficiencies. Unthrifty vines may show overall poor health and vigor due to inadequate nutrient uptake, while leaf wilting can occur due to the plant's inability to maintain turgor pressure, often resulting from insufficient nutrients that influence water uptake and regulation. Reduced growth is a common manifestation of any nutrient deficiency, as essential nutrients are required for the various physiological processes that support growth.

Thus, recognizing that dark green foliage is a sign of healthy nutrient levels helps distinguish it from the other symptoms associated with deficiencies typical of infections such as with Grape fanleaf nepovirus.