What is a primary benefit of a greenhouse in terms of plant health?

A primary benefit of a greenhouse is its ability to help manage diseases and insects better. In a greenhouse environment, the controlled climate and physical barriers can significantly reduce the exposure of plants to pests and pathogens. This controlled setting allows for the implementation of integrated pest management strategies, making it easier to monitor and respond to outbreaks of diseases or infestations effectively. By maintaining a stable environment, growers can also improve plant health by promoting optimal growth conditions, which can enhance a plant's natural defenses against pests and diseases.

The other options do not accurately reflect the primary benefits a greenhouse offers concerning plant health. While greenhouses can help with nutrient management, they do not guarantee all nutrients, as nutrient levels still require regular monitoring and proper fertility management. Water levels are significant in a greenhouse and must be carefully managed; they are not irrelevant. Lastly, while light can be managed to some extent in a greenhouse, other factors like humidity, temperature, and air circulation are also crucial for optimizing plant health. Thus, the ability to better manage diseases and insects stands out as the most significant advantage.