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There are three major pistachio rootstocks used in California: P. atlantica, a cold-tolerant but Verticillium-sensitive, and slower-growing rootstock; P. integerrima, a Verticillium-resistant and faster-growing rootstock which is cold-sensitive; and a new interspecific hybrid (P. atlantica X P. integerrima UCBI) which is cold-tolerant, Verticillium-resistant, and faster-growing. These three rootstocks are planted in a field trial at the Kearney Agricultural Center in Fresno county in the San Joaquin Valley. In 1994 and 1995, when the trees were four-and five-years-old, respectively, we examined the feeder roots for endomycorrhizae. By the earliest sampling date in 1995 (15 June) and by the second sampling date in 1994 (14 July), the cortical cells of young roots were extensively colonized. At these dates and throughout the remainder of the growing season, approximately one-half or more of the young root biomass contained …