More Northwest Christmas trees have fallen victim to disease in the last couple years and scientists are trying to find out why. “There’s a variety of diseases, many of them are caused by fungi,” said Gary Chastagner, a plant pathologist at Washington State University and longtime Christmas tree expert. To Northwest growers, he’s known as “Dr. Christmas Tree.” Two organisms have been causing problems for Northwest Christmas trees, Chastagner said. One is a fungus called Armillaria. …“The other is a fungus-like organism called Phytophthora.” …In addition, high temperatures in the summer have been stressing Northwest fir species leaving them more susceptible to disease. …Among the most affected trees are Pacific Northwest Noble Firs, Douglas Firs and Fraser Firs, the most popular varieties for Christmas trees, Chastagner said. …Eurasian tree species such as Turkish Fir might be more resistant to the pathogens and changing weather conditions in the Northwest, Chastagner said.