When procuring Shiitake spawn, the strain type is the most critical factor; you must choose between high-temperature, medium-temperature, or low-temperature strains based on your local climate. Ensure the substrate composition (usually sawdust, rice bran, and calcium carbonate) is clearly defined. The colonization rate should be high, with the mycelium appearing thick, white, and vigorous, showing no signs of yellowing or thinning. Additionally, verify the purity level, ensuring it is 99% free from competitive molds like Trichoderma.