What condition causes infected plants to become stunted and requires all infected plants to be discarded?

Crown gall is the correct answer because it is a specific plant disease caused by the bacterium Agrobacterium tumefaciens. This pathogen induces the formation of galls, which are tumor-like growths on the roots and stems of infected plants. As a result of these growths, the plant's normal growth is severely hampered, leading to stunted or abnormal development. The presence of these galls disrupts nutrient and water uptake, as well as overall plant physiology.

Once a plant is diagnosed with crown gall, it is essential to remove and destroy all infected plants to prevent the spread of the bacterium to healthy plants, as it can survive in the soil and on plant debris. This eradication strategy is crucial to managing the disease and preventing more significant outbreaks, particularly in a horticultural context where healthy plants are of paramount importance.

While phytophthora blight, root rot, and leaf curl are also significant plant diseases, they do not require the immediate removal and disposal of all infected plants in the same manner as crown gall. Each of these conditions can affect plant health and vigor, but the management practices differ based on the nature and behavior of the pathogens involved.