"Lovell and Nemaguard have been the mainstays for many years, but that was in the best soils."
Joe Connell has seen good results with Marianna rootstock in new orchards in Butte County. "We haven't had much problem with Phytophthora in these orchards," he says. "They are planted on Marianna plum root, on mounds, in marginal soils, and they are doing okay."

Intercropping:
Edstrom suggests removing, narrowing or avoiding altogether intercrops where Phyto- phthora infections occur.

http://www.bluediamond.com/growers/techniques/cultural/rottenTime.cfm