What evidence does he have for his new position?

Krakauer sends samples of the seeds to be tested, no traces of poisons are found. Krakauer later finds an article about a dangerous mold that can grow on such plants in wet climates, and believes that this, in fact, is what killed McCandless.The moldy seeds make the already weak McCandless incapable of climbing back to civilization or hunting, which leads to further weakness.