Which of the following agents is NOT a biotic primary causal agent of plant disease?

The correct answer is nutrients, as they are considered abiotic factors rather than biotic agents. Biotic agents are living organisms that can cause disease in plants, and they encompass a wide range of organisms, including bacteria, fungi, and insects like aphids.

Nutrients, while essential for plant health and growth, do not actively contribute to the pathogenesis of plant diseases as living organisms do. Instead, they serve as necessary elements for metabolic processes and growth regulation in plants. The absence of adequate nutrients can lead to deficiencies that may weaken plants and make them more susceptible to diseases caused by biotic agents. However, nutrients themselves do not have the capability to cause diseases directly like the other options listed do.