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Ole Billing Hansen and John R. Potter

Dormant stock plants of apple (Malus domestica Borkh.) rootstocks M.26 and Ottawa 3; Rhododendron `Britannia', `Purple Splendour', and `Unknown Warrior'; and mountain laurel (Kalmia latifolia L.) `Ostbo Red' and seedlings were forced to grow at 18 or 28 °C in continuous darkness or 14-h photoperiods. Etiolated shoots were then acclimated to light with or without aluminum foil wrapped around their bases to keep the bases etiolated. Shoots forced in diurnal light were neither etiolated nor wrapped and served as controls for the etiolation treatments. Compared to controls, wrapping etiolated stems improved rooting of M.26 (60% vs. 82%) and `Ottawa 3' (81% vs. 97%) apple and of `Britannia' (76% vs. 90%) and `Unknown Warrior' (80% vs. 91%) rhododendron. Etiolation improved rooting percentage of `Unknown Warrior' regardless of wrapping. Regardless of etiolation, forcing `Ottawa 3' at 18 °C improved rooting percentage (92% vs. 74%) and roots per cutting (12 vs. 7) compared to forcing at 28 °C. Etiolated mountain laurel cuttings generally rooted best at 18 °C; control cuttings rooted best at 28 °C.

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Douglas V. Shaw, John Hansen and Greg T. Browne

One hundred-eighty six strawberry genotypes from the Univ. of California strawberry (Fragaria ×ananassa Duch.) breeding program were evaluated for resistance to Phytophthora cactorum Schroet. in trials conducted over 6 years; 60 of these genotypes were tested in 2 years or more. Mother plants of each genotype were grown in a propagation nursery beginning in June, and runner plants were set into soil infested with inoculum from a mix of four P. cactorum isolates in August or September of the same year. Runner plants of each genotype were harvested from the inoculated nursery, transferred to a fruiting field location, and evaluated for disease symptoms during the winter and spring following inoculation using a disease severity score. Significant variation for the disease severity score was detected due to years, genotypes, and their interaction. Differences among genotypes were responsible for 60.6% of the phenotypic variance, whereas years and year × genotype interactions contributed relatively little to this variance, 8.2% and 9.3%, respectively. A separate analysis conducted using a balanced subset of six cultivars that were present in all trial years detected variance components due to years and year × genotype interaction slightly smaller than those estimated for the complete trial, 5.0% and 3.9%, respectively. These results highlight the utility of the screening system and suggest that stable resistance to P. cactorum is obtainable in California strawberry breeding populations and production systems.

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Douglas V. Shaw, W. Douglas Gubler and John Hansen

Runner plants from strawberry (Fragaria ×ananassa Duch.) genotypes previously identified as relatively susceptible, intermediate, and resistant to wilt caused by Verticillium dahliae Kreb. were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae at 104, 105, and 106 conidia/mL. Disease symptoms were scored as the number of dead or severely stunted plants per plot, or on the basis of a subjective phenotypic resistance score assigned to each plot on eight dates during the spring after planting. Overall disease symptoms increased with inoculum concentration; resistance scores for all genotypes were 4.8, 3.7, and 3.2, and the percentages of plants stunted or dead were 6.8, 32.6, and 44.9 for the three conidial concentrations, respectively. The relative resistance categories were separable at concentrations of 106 and 105, whereas no separation was obtained at 104 conidia/mL. Genotypes originally classified as intermediate in resistance performed more like susceptible types at the highest conidial concentration. Significant resistance category × conidial concentration level interactions were detected for resistance score but not for the number of dead or severely stunted plants per plot. Regression coefficients for resistance score and percentage of stunting on conidial concentrations were statistically significant only for susceptible and intermediate genotypes. Some stunting was detected within all resistance categories at the highest conidial concentration, and the practical value of the resistance developed to date will depend ultimately on realistic field inoculum levels.

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Douglas V. Shaw, W.D. Gubler, Kirk D. Larson and John Hansen

Resistance to wilt caused by Verticillium dahliae Kreb. was evaluated for 41 strawberry genotypes from the Univ. of California breeding program and 1000 offspring from crosses among 23 of these genotypes. Runner plants from these genotypes and seedlings were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae from strawberry. Symptoms were scored as the number of dead or seriously stunted plants per plot, or based on a subjective phenotypic resistance score assigned to each plot on five dates during the spring after planting. Most of the California germplasm is highly susceptible to V. dahliae, with an average resistance score of 2.1 (±0.10) and 84.1% (±2.1) plants stunted or dead compared with a score of 3.2 (±0.24) and 57.4% (±4.9) of plants stunted or dead for a control set of six non-California genotypes identified previously as resistant. However, a broad range of intermediate resistance was detected, and 4 of the 41 California genotypes evaluated had resistance scores superior to the mean score for the non-California resistant checks. Plot-mean heritabilities for resistance and stunting scores estimated using genotypic, full-sib family, and offspring-parent analyses ranged from 0.44 to 0.88. Comparison of different estimates of variance components suggests that half or more of the genotypic variance for resistance traits detected is due to the additive effects of genes. There appears to be sufficient variation within the California population to proceed with an effective selection program, despite the absence of directional selection for resistance during the past 3 decades. However, developing cultivars with adequate resistance will ultimately depend on the recovery of transgressive segregants from superior parents, as even the most resistant genotypes from all sources showed some disease symptoms.

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Douglas V. Shaw, W. Douglas Gubler, John Hansen and Kirk D. Larson

Strawberry genotypes were retained from biparental progenies previously identified as either relatively susceptible or resistant to wilt caused by Verticillium dahliae based on a phenotypic resistance score. Runner plants from these selected genotypes were inoculated with a conidial suspension containing a mixture of five isolates obtained from symptomatic strawberry plants at 106 conidia/mL, then scored for disease symptoms. Genotypes from resistant progenies had significantly higher phenotypic resistance scores (1 = severe symptoms, 5 = no detectable symptoms) than those from susceptible progenies (4.15 vs. 2.23), and there also was a correlated selection response for the percent plants severely stunted or dead (26.4 and 69.1 for resistant and susceptible selections, respectively). Comparisons of the resistance scores for selected groups with those for the original parents (2.76) indicated that selection had changed relative resistance significantly in both directions and that realized response had been 24% to 43% larger than predicted for selection in both directions. Although several of the selections from resistant progenies were highly susceptible, five of the 21 resistant selections had resistance scores outside the range of the original parents, representing possible transgressive segregants. This comparison is limited by the precision with which individual resistance scores are estimated and by the scope of the disease symptoms in this trial. Detecting genotypes with sufficient resistance ultimately will depend on development of screening methods with greater sensitivity than those used here.