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WALNUT IMPROVEMENT PROGRAM
Gale McGranahan, John Hansen, Ben Iwakiri, and Ron Snyder
This report covers the progress made in the USDA-funded Walnut Improvement Program in 1984. Two Juglans regia rootstock trials designed to assess variation in seedling vigor in the first growing season are summarized. The results indicate that seedlings from IAmigo I and 'Eureka' are the most vigorous. 'Waterloo' seedlings are the most uniform. No J. regia family approached J. hindsii in vigor. A comparison of 'Manregian' seedlings obtained from Oregon with 'Manregian' seedlings from California (UCD variety block) indicated that there are no significant differences between the two sources in vigor. A trial of J. hindsil and J. californica seedlings showed that the source of J. hindsii is a significant factor ln vigor. This trial is a long-term study designed to characterize variation in a number of traits including Phytophthora susceptibility in California black walnuts. Other aspects of the Walnut Improvement Program aimed at improving rootstock are outlined.
Seedling selection evaluation was continued. ApproXlmately 200 seedlings derived from controlled pollinations prior to 1978 were held for additional evaluations. These include 150 which have had less than 2 years evaluation, and 56 which are potential selec- tions or are being saved as representative genotypes. The majority of the seedlings leaf out after 'Hartley', are laterally fruitful, and have high quality nuts. Several of these are ready to harvest at, 'Payne' time0 A list of selections (potential releases) will be . made available to farm advisors along with instructions for completing the test agreement later this year. Those wishing to participate in selection evaluation in their counties are encour- aged to do so. A summary of 1983 cultlvar and selection evaluations is included in this report.
The goal of the Walnut Improvement Program is to develop germplasm which will provide the walnut industry with a means to overcome the major problems limiting walnut production in the United States. These problems, as identifled by farm advisors through their grow- ers, industry representatives, and scientists involved in walnut research, are the blackline virus disease caused by the cherry leafroll virus and Phytophthora root and crown rots. Our program is also involved in the development of new sources of germplasm (see Tulecke & McGranahan), micropropagatlon, and in the evaluation of seedlings and selections developed through controlled crossing prior to 1978.
SPECIFIC OBJECTIVES AND PROCEDURES
Several genetic approaches are available for reducing the impact of the blackline virus on the walnut industry. One is the development of superior rootstocks which will tolerate infection by the cherry leafroll virus; another is the incorpor- ation of hypersensitivity into scion varieties. The latter is a long term project whose feasibility will be unknown until the genetics of the host reaction is understood.
Juglans regia L., the Persian walnut, is currently the only species known to tolerate the virus strains present in Cali- fornia. J. hindsii (Northern California black), J. nigra (Eastern black), J. californica (Southern California black), 'Paradox', and pterocarya stenoptera ('Wingnut') all resist virus infection to some extent by a hypersensitive reaction. This reaction causes a lethal girdle between an infected scion and a hypersensitive rootstock. Therefore, the source of tolerant rootstocks appears to be limited to Persian walnuts or, if only a few genes are responsible for the hypersensitive reaction, to a segregating backcross generation (e.g. 'Paradox' x J. regia).
Persian walnut rootstocks are understood to have many drawbacks including poor vigor and sensi tivity to di seases, nematodes, and adverse soil conditions. To determine whether there is variation in seedling vigor, a study designed to compare first year growth in seedlings from six potential J. regia rootstock sources. was undertaken in 1983. At the end of the first growing season a subset from this trial was replanted in a rootknot nematode block under the direction of Mike McKenry. Because 'Manregian' had not been included in the initial study, a separate trial comparing 'Manregian' from Oregon (Harry Lagerstedt) and 'Manregian' from California (UCD variety block) with J. hindsii, J. regia India, and 'Amigo' seedlings was undertaken in 1984.
To determine whether superior tolerant rootstocks can be ob- tained in the first backcross generation, a replicated trial of 70 'Paradox' x J. regia were grafted onto J. hindsii and J. regia rootstock for evaluation of their reaction to the blackline virus. This study also involves seedlings from the 'Paradox' x J. hindsii backcross, 'Paradox', J. regia, and J. hindsii to determine whether hypersensitivity is governed by one, two, or many genes. Additional 'Paradox' x J. ri::!gia seedlings were created this year by pollinating an isolated 'Paradox' tree with J. regia pollen using bags of dehiscing catkins as a pollen source. More than 1000 nuts resulted from this effort. From these studies it is expected that the feasibility of selecting rootstocks from among the 'Pardox' x J. regia population as well as the potential for breeding for hypersensitivity in the scion will be determined. Prelimi- nary results from evaluations will be available in 1985.
A project aimed at assessing the effect of the blackline virus on own-rooted J. regia cuItivars is in the initial stages. Before evaluation is possible, methods for propagating mature J. regia cultivars must be developed. Seven cultivars have been targeted for research in methods of micropropagation
of mature J. regia cultivars.
Three approaches to the development of Phytophthora resistant or tolerant rootstock are being employed. The first is aimed at determining whether individual trees which have survived in orchards that are severely infested with Phytophthora have rootstocks with genetic resistance. The first step has been to identify surviving individuals in the field and propagate them through tissue culture. They will then be evaluated for their response to Phytophthora. The second approach in- volves interspecific hybridization between species with dif- ferent levels of resistance. Controlled crosses were made between J. ailantifolia (Japanese walnut), J. nigra, J. hindsii, and J. regia. The resulting seedlings will also be evaluated for their response to Phythophthora. The final approach is designed to provide additional information regarding the varia- tion within J. hindsii because it is a commonly used rootstock as well as a-parent of 'Paradox'. To assess the within species variation, seed from J. hindsii trees in 17 locations was field planted together -with five sources of J. californica, one of J. regia, and one of a J. nigra which produces 'Royals' (J. nigra x J. hindsii). Data on germination, height, diameter, and number of branches was recorded. A subset of this collec- tion is being tested for its reaction to Phytophthora in artifi- cially infested soil in the greenhouse.
C. Evaluation and selection of seedlinqs for cultivar development.
The specific goal of this part of the Walnut Improvement Program is to evaluate and select promising individuals from among progeny of controlled crosses undertaken by Serr and Forde 1973 to 1977. Data is collected annually on the following traits: dates of leafing, pollen shedding, female fertilrty and harvest, lateral bud fruitfulness, yield, blight incidence, shell characteristics and seal, nut and kernel weight, kernel color, and frequency of blanks. Compiled data is reviewed by a panel which recommends action (select, save, or discard) on each seedling.
RESULTS AND DISCUSSION
In both J. regia rootstock trials, J. hindsii was superior to all the Persian walnut families in height, diameter, and uniformity at the end of the first growing season (Table 1). The most vigorous J. regia families in the 1983. trial were 'Amigo' and 'Eureka'. The most uniform family determined by the coefficient of variation (standard deviation/mean) was 'Waterloo'. 'Paradox' derived from J. hindsii SCN had the greatest mean diameter. In the 1984- trial 'Amigo' was again the most vigorous J. regia source. The two sources of 'MaIiregian'were almost identical in vigor which suggests that growers interested in using 'Manregian' will not improve performance by obtaining seed from Oregon. (It had been sug-
gested that the male parent of Oregon 'Manregian' might contri- bute substantially to the progeny's performance which could account for the recommendation 'Manregian' in Orgeon). Our results confirm earlier studies that in terms of early vigor, 'Manregian' is not superior to other sources of J. regia. Whether the superiority of 'Eureka' and 'Amigo' will be main- tained beyond the first year is unknown. .
Results from the rootknot nematode trials suggest that J. regia is more susceptible to rootknot infection than J. hindsli or 'Paradox'. There was considerable variation, however, within all familities. This variation indicates that selection within vigorous families followed by clonal propagation may be the most promising method of obtaining rooknot nematode resistant rootstock. It should be noted that rootknot nematode is a potential problem only in sandy soils.
Micropropagated plantlets of mature J. regia cultivars ('Chandler', 'Sunland', 'Chico', 67-11, 'Scharsh-Franquette', 'Vina', and 'Eureka') have been established and are in the multiplication phase in culture. These will be used to assess the impact of blackline on own-r