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Resistance to fire blight in blackberry (Rubus subgenus Rubus Watson) was studied in both seedling populations and clonally propagated plants. Seedling populations resulted from a partial diallel crossing of seven cultivars (Apache, Arapaho, Chester Thornless, Illini Hardy, Navaho, Triple Crown, and Prime-Jim™). Clonal material evaluated included eleven cultivars (Apache, Arapaho, Chester Thornless, Chickasaw, Kiowa, Illini Hardy, Navaho, Ouachita, Shawnee, Triple Crown, and Prime-Jim) and six breeding selections. Inoculations were made by injection of suspensions of Erwinia amylovora in sterile water. Significant differences in resistance were found among genotypes; `Navaho' was the most susceptible, and `Kiowa' and a breeding selection A-2095, the most resistant. Seedling inoculations showed resistance to be quantitatively inherited and mostly additive, with an overall narrow-sense heritability of h2 = 0.32.
Trials were established in Fayetteville, Ark., in 1993 and in 1994 to evaluate grape germplasm for downy mildew resistance. Accessions were obtained from the national grape repositories in Geneva, N.Y.; Davis, Calif.; and the Univ. of Arkansas fruit substation, Clarksville. The 1993 trial contained 26 cultivars, eight selections, and 24 Vitis species; the 1994 trial contained 37 cultivars. Each trial had four single-plant replications. Plants were rated on a scale of 0 to 5 for sporulation, chlorosis, and necrosis on 23 Aug. and 26 Sept. 1994. Fungicides were not applied to the vineyard to better determine the level of natural downy mildew resistance. The results from both trials indicated that several grape accessions showed little or no symptoms of downy mildew, which suggests these have resistance that would be beneficial to use in a breeding program.
Plasmopara viticola infects and sporulates through stomata of susceptible grape leaves. Sporulation, chlorosis, and necrosis ratings were made in 1994 and 1995 on grape selections and cultivars and Vitis species grown in a fungicide-free vineyard. Cellulose-acetate impressions were made of the abaxial leaf surfaces and stomata were carefully counted within a circle 100 μm in diameter under a light microscope. Leaves were rated as either pubescent or glabrous. There were significant differences among genotypes for sporulation, chlorosis, and necrosis for 1994 and 1995, with highly significant correlations over both years. Stomatal densities were significantly different, but there were no correlations among levels of downy mildew and stomata! densities. Pubescent leaves had significantly higher sporulation, chlorosis, and necrosis ratings for downy mildew than glabrous leaves over both years.
Disks (18 mm in diameter) from young, fully-expanded leaves of susceptible and resistant germplasm were inoculated on the abaxial surface by spray or pipette with 2 × 104, 5 × 104, 1 × 105 sporangia per milliliter of distilled water. Disks were placed on filter paper bridges in plastic containers with tap water to maintain high relative humidity and incubated at 24 C with a 14-hour photoperiod. Disease ratings were made five days after inoculations and disks were cleared in lactophenol (100 C) for two to three hours, stained with lactophenol/cotton blue, and examined by light microscopy. Uncleared disks of resistant material showed no symptoms, and susceptible material showed a range of symptoms and sporulation. Hyphal growth was evident in infected tissue of susceptible plants, but little or no growth was observed in resistant material. Results indicate that leaf disks can be used to evaluate downy mildew resistance. Further investigations will determine efficacy of this technique for screening seedling populations for field resistance.
A study was conducted to determine how resistance to downy mildew [Plasmopara viticola (Bert. & Curt.) Berl. & de Toni] is inherited in germplasm (Vitis vinifera L., V. labrusca L., V. rupestris Scheele, and V. riparia Michx.) used for breeding table grapes. Crosses, including reciprocals, among parents possessing different levels of downy mildew resistance were evaluated in 1994 and 1995. The proportion of foliar tissue with sporulation, chlorosis, or necrosis was used to measure resistance. All genotypes were rated for these characters on two separate dates in 1994 and 1995. Hypersensitive flecking was also evaluated in the 1995 seedlings to determine its relationship with downy mildew resistance. Crosses with at least one resistant parent had a larger number of resistant offspring than crosses between two susceptible parents. General combining ability (GCA) effects were highly significant for 1994 and 1995. Specific combining ability effects were significant, but were relatively small compared to GCA, suggesting additive gene action was a primary influence on downy mildew resistance. Heritability estimates for sporulation, chlorosis, and necrosis were the highest at the second rating in 1994 (0.88, 0.74, and 0.57, respectively) and 1995 (0.50, 0.60, and 0.60, respectively). Reciprocal crosses indicated that maternal inheritance did not influence downy mildew resistance. A small percentage of progeny with hypersensitive flecking were identified from the germplasm. Seedlings with the flecking characteristic tended to have lower sporulation, chlorosis, and necrosis ratings earlier in the growing season.
This research was conducted to compare an in vitro leaf disk technique with greenhouse and field evaluations for screening large populations of grape (Vitis sp.) seedlings for downy mildew (Plasmopara viticola Berk. & Curt. Berl. & de Toni) resistance. Seedlings produced by crossing resistant × resistant, resistant × susceptible, and susceptible × susceptible parents were rated for sporulation, chlorosis, and necrosis. Leaf disk sporulation ratings at the first and second rating were highly correlated with the second sporulation rating in the field. Necrosis ratings from the leaf disk evaluations were significantly correlated with field necrosis ratings, but leaf disk chlorosis ratings were not correlated with field ratings. Some correlations, including evaluations of chlorosis, between the greenhouse and field ratings were highly significant. Seedling ratings of 0 or 1 for sporulation, chlorosis, and necrosis in the leaf disk assay agreed with field evaluations 85.6% of the time vs. 80.3% agreement between greenhouse and field ratings. Sporulation was the parameter most highly correlated between leaf disk or greenhouse and field evaluation of resistance. The leaf disk procedure appeared to be a good predictor of field resistance, and is more practical than the greenhouse method for screening large populations.