Macadamia Tree Health Initiative Amendments Act This bill expands the research and extension grant program for the macadamia tree health initiative. Under current law, the Department of Agriculture (USDA) may provide competitive grants under the initiative to support research and extension activities to combat the insect known as the macadamia felted coccid (Eriococcus ironsidei). Specifically, the bill authorizes USDA to provide competitive grants for developing and disseminating science-based tools and treatments to combat plant pests and noxious weeds that impact macadamia trees; establishing an area-wide integrated pest management program in areas affected by, or at risk of being affected by, the invasive plant pests or noxious weeds; surveying and collecting data on macadamia tree production and health; investigating macadamia tree biology, immunology, ecology, genomics, and bioinformatics; and conducting research on various factors that may contribute to or be associated with macadamia tree immune systems and other serious threats to macadamia trees.