The confocal z-axis sequence above illustrates a stained thin section of pine tree stem infected with White Pine Blister Rust, a serious disease of pine trees caused by the fungus Cronartium ribicola. This fungus also is a disease of the Ribes genus (currant and gooseberry bushes), which serves as an alternate host for the fungus. This fungus has a complex life cycle that takes up to four years to be completed, requires two hosts, and involves five different types of spores. Both pine trees and the Ribes berry bushes have to be present in the environment for the fungus species to survive. Pine trees can only be infected by spores that develop on the berry bushes and the berry bushes can only be infected by spores that develop on the pine trees. Pycniospores and aeciospores are produced on the branches and stems of pines; urediniospores, teliospores, and basidiospores on currant and gooseberry foliage.