Which of the following is a direct method to achieve inoculum reduction?

Applying chemical treatments is a direct method to achieve inoculum reduction because it involves the application of pesticides or fungicides that target specific pathogens in the environment. These treatments can effectively reduce the population of harmful organisms by either killing them or inhibiting their ability to reproduce. This direct intervention can lead to a significant decrease in the presence of pathogens that might cause disease in crops or forests.

While the other methods listed can contribute to overall plant health and disease management, they do not directly eliminate or reduce the existing inoculum of pathogens as efficiently as chemical treatments can. Preventing pathogen entry and using resistant plant varieties focus more on proactive strategies to manage disease risk, while regular crop rotation helps disrupt the life cycles of pests but may not directly reduce the inoculum present at any given time. In contrast, chemical treatments specifically target and mitigate the existing threat of pathogens, making this option the most direct for inoculum reduction.