Walnut Rootstock Selection for Resistance to Phytophthora spp.

Principal Investigator: G.T. Browne, Research Plant Pathologist, Crops Pathology and Genetics Unit, USDA, ARS, Department of Plant Pathology, UC Davis, Davis, CA 95616.

California provides about 99 percent of the U.S. walnut crop and is a world leader in walnut culture and genetic improvement efforts. Deep alluvial soils in the Sacramento and upper San Joaquin Valleys and the mild temperate climate are nearly ideal for the crop. All California walnut growers and nurserymen, however, must contend with several important soilborne pests that can debilitate and kill walnut trees of all ages. Methyl bromide has played an important role in controlling many of these pests.

Phytophthora spp. are among the most important soilborne pests of walnut. In California, more than 10 species of the fungus have been implicated as root and crown pathogens of the crop, and affected trees typically die within one or a few seasons. Periods of soil saturation with water, especially during cool to moderate temperatures, favor infection by Phytophthora spp. Soil saturation stimulates production and dispersal of zoospores, the principal infective propagules. Two of the most virulent species in walnut are P. cinnamomi, which has a limited but expanding distribution among California walnut orchards, and P. citricola, which is somewhat less virulent but more widely distributed than P. cinnamomi. Some Phytophthora spp. can survive for extended periods (years) in soil without a living host, and evidence suggests that they are spread among orchards in surface irrigation water and by movement of infested soil and plant material.

Fumigation with methyl bromide plays an important pre-plant sanitizing role in reducing Phytophthora and other pest populations at commercial nursery and orchard sites. The need for genetic and cultural Phytophthora disease control strategies will intensify as the fumigant is banned for nursery and orchard sanitation. Although alternative chemicals to methyl bromide appear necessary at nursery sites to maintain adequate sanitation, genetic and cultural approaches offer the most effective and economical means to control many diseases caused by Phytophthora.

The Crops Pathology and Genetics Unit, USDA–ARS at Davis, with the University of California's Walnut Improvement Program (WIP) and commercial nurserymen cooperating, is pursuing improved genetic resistance to Phytophthora spp. in walnut rootstocks. Paradox hybrid seedling rootstock presently represents the available industry standard for maximum vigor, general tolerance to lesion nematodes, and resistance to most Phytophthora spp. Unfortunately, at least some sources of Paradox succumb to attack by P. citricola and P. cinnamomi.

Although Paradox seedlings are often described as Northern California black × English walnut hybrids (J. hindsii × J. regia), morphological and molecular evidence obtained by the WIP suggests that Paradox hybrids are quite diverse genetically. And, several black walnut species, such as Southern California black (J. californica) and Eastern black (J. nigra), may be serving as maternal parents of commercial Paradox rootstock seedlings.

The variation within Paradox presents an opportunity to select and develop improved hybrid rootstocks. Work is under way to determine particular hybrids, families, and clones of Paradox with superior resistance to Phytophthora spp. In 1997, 26 commercial Paradox hybrid seed families and four controlled hybrid crosses (provided by the WIP) were greenhouse tested for resistance to P. citricola. All families of Paradox sustained some root and crown rot among individual seedlings, but average severity of disease differed significantly among families. For example, nine commercial Paradox seed families were relatively susceptible to P. citricola (root and crown rot averages of 85–100 percent), and two were significantly more resistant (root and crown rot averages of 28–51 percent). Among the other commercial Paradox families, there was essentially a continuous range of susceptibility to root and crown rot between the relatively susceptible and resistant families. Among the four controlled-cross seed families, Arizona Paradox hybrid (J. major × J. regia) was significantly more resistant (root and crown rot averages of 43 percent) than a Southern California hybrid family (J. californica × J. regia, root and crown rot averages of 87–96 percent) and two Northern California black hybrid families (J. hindsii × J. regia, root and crown rot averages of 79–99 percent).

Chinese wingnut (Pterocarya stenoptera) has the highest known genetic resistance to Phytophthora spp. among potential walnut rootstocks. Previous ARS research at Davis revealed that at least some sources of Chinese wingnut are very resistant to all Phytophthora spp. that commonly affect walnuts, including P. citricola and P. cinnamomi. In 1974, Lownsbery et. al reported that Chinese wingnut was more tolerant to lesion nematode than Northern California black, English, or Paradox hybrid rootstocks.

The main obstacle in using wingnut as a rootstock is limited graft compatibility with English walnut cultivars. In an 11-year ARS experiment completed in 1997, one seed source of wingnut was graft compatible with four of eleven English walnut cultivars, but wingnut compatibility with recently released walnut cultivars remains unknown. More work has been initiated with commercial nurseries to evaluate graft compatibility of six diverse wingnut seed sources with six prevalent English cultivars. We will also evaluate wingnut-compatible English cultivars as interstock "bridges" between wingnut and wingnut-incompatible English cultivars.

To determine if resistance to Phytophthora is uniformly present among the diverse wingnut sources being tested for graft compatibility, greenhouse screens with P. citricola and P. cinnamomi were completed. In these tests, all wingnut sources were highly resistant to P. citricola (root and crown rot averages of 0–1 percent) and moderately resistant to P. cinnamomi (root and crown rot averages of 8–36 percent), whereas standards of Paradox, Northern California black, and English seedling rootstock were relatively susceptible (root and crown rot averages of 65–100 percent with P. citricola and 69–100 percent with P. cinnamomi). ARS-Davis is cooperating with UC scientists M.V. McKenry and B.B. Westerdahl in evaluating lesion nematode tolerance among the six wingnut sources and standards of Paradox, English, and Northern California black rootstock.

Results of 1997 Phytophthora screens show important variation in genetic resistance to P. citricola among a limited number of Paradox seed families and indicate good resistance to P. cinnamomi and P. citricola among Chinese wingnuts from diverse sources. Repeat experiments are needed to confirm these findings.

Rootstocks with improved genetic resistance to Phytophthora spp. may provide a key link in methyl bromide alternative strategies for walnut production.

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Last Updated: April 22, 1998