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D.E. Parfitt and A.A. Almehdi

An improved medium for the in-vitro micropropagation of P. vera genotypes was developed. Several basal medium preparations were tested and DKW (Driver-Kuniyuki-Walnut) medium was selected. Macro and micronutrients were adjusted to provide optimum growth and multiplication of shoots, some of which have been grown and multiplied for more than 2 years. The use of thidiazuron as a growth regulator was tested, but was detrimental to the explants at all tested concentrations. Initial experiments were hindered by significant bacterial internal contaminants. This problem was eliminated through the use of plexiglass chambers to provide up to a 20,000ppm CO2 atmosphere, increased light levels to promote photosynthesis, and the elimination of all carbon sources (sugars) from the substrate. An additional benefit was better shoot growth, better survival when rooting and better acclimation. 3/7 plants were rooted using 2.5 ppm IBA. Continuing experiments are focused on the effect of support medium on growth and multiplication as well as media development for other Pistacia species.

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F.A. Bliss and Ali A. Almehdi

Seedlings of Prunus mahaleb are often used as rootstocks for sweet cherry (P. avium) scion cultivars in commercial orchards. While they are desirable based on ease of propagation and economical production of nursery stock, seedlings may be variable resulting in nonuniform compound trees, and they are susceptible to several important diseases. Seedling sources have shown substantial variability for population uniformity of plant growth, and reaction to crown gall, powdery mildew and Phytophthora root rot. Segregating families also vary for pollen fertility, inbreeding response and control of scion growth. Multiple screening for favorable trait combinations is underway to develop improved sources of cherry rootstocks.

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Ali A. Almehdi and F.A. Bliss

Crown gall incited by Agrobacterium tumifaciens is an important problem for nursery and field production of stone fruit and nut crops. Genotypes reportedly differ for crown gall reaction, but there is little information about resistance of Prunus accessions used as rootstocks. From among four wild-type strains of A. tumifaciens-virulent on apricot and almond, K12 was selected for inoculation of 6-month-old seedlings of cherry, plum, peach, almond, apricot, and miscellaneous species. The large majority of seedlings were very susceptible to crown gall, but some had few or no galls. Cherry, especially some lines of P. mahaleb, showed the most resistant or moderately resistant seedlings, while some accessions of plum, especially P. cerasifera, P. angustifolia, and P. insititia had the most resistant seedlings. Plants with different reactions were propagated to determine adult plant resistance and to study the heritability of crown gall reaction.

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F.A. Bliss, P.L. Schuerman, A.A. Almehdi, A.M. Dandekar, and N. Bellaloui

Crown gall is an important disease of many fruit and nut crops, but little is known about sources of resistance. We screened germplasm from Prunus armeniaca L., P. angustifolia Marsh., P. argentia L., P. avium L., P. besseyi Bailey, P. bokhariensis Schneid., P. brigantica L., P. cerasifera Ehrh., P. cerasus L., P. dulcis (Mill.) D.A. Webb, P. fruiticosa Pall., P. hortulana Bailey, P. insititia L., P. japonica Thunb., P. mahaleb L., P. persica (L.) Batsch, P. serotina Ehrh., P. simonii Carr., P. sogdiana L., and P. webbii (Spach) Vieh. When either main stems or lateral branches of seedlings were inoculated with strains K12 and C58 of Agrobacterium tumefaciens (Smith and Townsend) Conn., the incidence of resistance was less than 10% except in some accessions of P. mahaleb L. where up to 30% of the plants were resistant. Some resistant plants were identified in other species, with P. insititia L. being the most promising. Symptoms based on presence and size of galls should be allowed to develop for up to 90 days after inoculation to reduce the likelihood of misclassifying plants as resistant when they are slightly susceptible.

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Dan E. Parfitt, Siov B. Ly, Ali A. Almehdi, Helen Chan, and Sui-Sheng T. Hua

Aspergillus flavus produces aflatoxin, a cancer-causing contaminant of pistachio in many production areas. A superior yeast strain of Pichia anomala has been demonstrated to inhibit the growth of A. flavus in the laboratory. It was selected for further study and potential release based on tests of durability and ability to inhibit A. flavus growth. This strain has been tested in the field for the past two years to evaluate its ability to survive in a field environment and to inhibit A. flavus production. The yeast was evaluated in the field to determine if: 1) the biocontrol yeast can survive in pistachio orchards; 2) the yeast has no phytotoxic effects on the pistachio trees or nuts; and 3) the yeast can be demonstrated to control A. flavus in the field. Studies during 2003 were conducted using a replicated experiment with three yeast concentrations and a water control. Treatments applied later in the season were found to be most effective. Highest yeast concentrations were observed just prior to harvest. Three spray concentrations and a water control were applied to evaluate possible phytotoxic effects on pistachio during 2004. No differences in leaf or nut appearance, in nut percent splits, or dry weight were observed for any of the treatments when compared to the water control. Artificial wounding experiments were also conducted during 2003 and 2004 to simulate the occurrence of early split nuts, the primary repository for A. flavus contamination. A 5× reduction in A. flavus colonization was observed on treated wounded nuts vs. untreated wounded nuts. A 5× reduction in A. flavus sporulation was also observed on treated wounded nuts vs. untreated wounded nuts.