‘Summerred’ apples (Malus domestica Borkh.) are highly susceptible to biennial bearing if not properly thinned. This results in erratic yields and also affects fruit quality adversely. Between 2003 and 2005, ‘Summered’/‘M9’ trees were treated with ethephon at concentrations of 250, 375, and 500 mg·L−1 when most king flowers opened (≈20% bloom) or at concentrations of 500, 625, and 750 mg·L−1 when the average fruitlet size was 10 mm in diameter. The experiment was conducted with 2.5-m height slender spindle trees sprayed to the point of runoff with a hand applicator only when temperatures exceeded 15 °C. Within 2 weeks after the second application, fruit set was reduced linearly with increasing concentrations of ethephon to less than one fruitlet per cluster at the highest concentrations used. Most thinning treatments reduced fruit set significantly compared with unthinned trees. Fruit numbers per tree decreased significantly with increasing ethephon concentrations, and the highest concentrations of ethephon applied during bloom or when the average fruitlet size was 10 mm in diameter resulted in overthinning. Yield results confirmed the fruit set response in which yield reductions were significant at the highest concentrations of ethephon (2.1 kg/tree) compared with hand-thinned trees (7.3 kg/tree) in 2005. All thinning treatments resulted in higher percentage of fruits larger than 60 mm diameter compared with unthinned control fruit. Thinning resulted in significantly higher soluble solid contents, and this was especially so for hand-thinned trees. Other fruit quality parameters like yellow–green background color did not show a clear response to thinning. Return bloom was, however, improved on all thinned trees. It is recommended that ethephon be applied at a rate of 375 mg·L−1 when king flowers open or at a rate of 625 mg·L−1 when the average fruitlet size is 10 mm in diameter. This thins ‘Summerred’ apples to a target of approximately five fruits/cm2 per trunk cross-sectional area or 50 to 70 fruits per 100 flower clusters without impacting on fruit quality, yield, or return bloom the next year.
Mekjell Meland, Oddmund Frøynes, and Clive Kaiser
In a cool, maritime Nordic climate, where fruit have been grown for many centuries, 13 early maturing peach (Prunus persica) cultivars from northern European climates were evaluated over a 6-year period. Peach leaf curl (Taphrina deformans) was the main disease affecting production. The most consistent yielding cultivar was Riga with 5.74 kg/tree in 2009, 2.06 kg/tree in 2010, and 4.71 kg/tree in 2011. Harnas yielded 6.93 kg/tree in 2010, but had very low yields in 2009 and no yield at all in 2011. ‘Harnas’ fruit quality was excellent when compared with all other cultivars, but trees were extremely susceptible to peach leaf curl. ‘Riga’ was somewhat tolerant of peach leaf curl but flowered early. We conclude that none of the cultivars tested are suitable for commercial production. However, looking forward, both ‘Riga’ and ‘Harnas’ may be suitable for commercial tunnel production and home gardens. Furthermore, ‘Harnas’ and ‘Riga’ could be considered for use in future breeding programs for this cool, mesic Nordic climate.
Clive Kaiser, Philip B. Hamm, Stacy Gieck, Nicholas David, Lynn Long, Mekjell Meland, and J. Mark Christensen
In vitro dose responses of several calcium and potassium salts were determined on some commercially significant plant pathogens, including: Helminthosporium solani, Fusarium oxysporum f. sp. pisi race 2, Colletotricum coccodes, Phytophthora cactorum, Phytophthora cinnamomi, Phytophthora erythroseptica, Phytophthora infestans, Phytophthora megasperma, Pythium ultimum, and Venturia inaequalis. Mycelial growth inhibition was both salt-specific and dose-related. Pythium ultimum was completely inhibited by 75 mg·L−1 or greater calcium propionate, but needed 300 mg·L−1 or greater of calcium acetate and 40 mL·L−1 or greater of potassium silicate for complete inhibition. Phytophthora infestans was completely inhibited by 150 mg·L−1 or greater calcium acetate, 150 mg·L−1 or greater calcium propionate, or 5 mL·L−1 or greater potassium silicate. Phytophthora cactorum was completely inhibited by 300 mg·L−1 or greater calcium propionate, but required 600 mg·L−1 or greater calcium acetate and 10 mL·L−1 or greater potassium silicate for complete inhibition. Phytophthora cinnamomi was completely inhibited by calcium propionate at 600 mg·L−1 or greater or by 10 mL·L−1 or greater potassium silicate. Only potassium silicate inhibited Phytophthora megasperma, Phytophthora erthroseptica, V. inequalis, and H. solani at concentrations of 5 mL·L−1 or greater, 20 mL·L−1 or greater, 40 mL·L−1 or greater, or 80 mL·L−1 or greater, respectively. Potassium acetate did not completely inhibit any of the pathogens in this study when tested at concentrations 1200 mg·L−1 or less.
Fuad Gasi, Silvio Simon, Naris Pojskic, Mirsad Kurtovic, Ivan Pejic, Mekjell Meland, and Clive Kaiser
There is a need for classifying and conserving local apple cultivars from two main regions in Bosnia and Herzegovina (B&H). Consequently, 71 local apple accessions (31 from Sarajevo and 40 from eastern Bosnia) were evaluated with a set of 10 simple sequence repeats (SSRs). These accessions were compared with 37 reference cultivars (24 traditional B&H and 13 international cultivars maintained at the ex situ collection Srebrenik) to determine synonyms, homonyms, and possible introgression of foreign genotypes into the local apple germplasm. Using 10 primer pairs of microsatellites, we were able to amplify 135 alleles for the 71 local apple accessions. Detection of more than two different alleles per locus was observed for 34 accessions. Fourteen different homonyms and 12 synonyms were identified among all the apple cultivars analyzed. Analysis of molecular variance (AMOVA) revealed a significant genetic differentiation between most of the groups analyzed but not between accessions from Sarajevo and eastern Bosnia. Bayesian method and admixture analysis of the allele frequency allowed classification of all accessions analyzed and found that they fell into two main groups [reconstructed panmictic populations (RPPs)]. Strong genetic differentiation between these two groups was detected using AMOVA (fCT = 0.130; P < 0.001). Analysis of the genetic structure indicates that overall, approximately half of the local apple cultivars from Sarajevo and eastern Bosnia (52% and 45%, respectively) grouped in the RPP1 consisting mainly out of international reference cultivars, whereas the other half grouped in the RPP2 with traditional B&H reference cultivars. Both neighbor joining (NJ) cluster analysis based on Bruvo genetic distance and factorial correspondence analysis (FCA) confirmed the results of the genetic structure analysis. The molecular data show that both apple accessions from Sarajevo and from eastern Bosnia represent an interesting source of diversity, which needs to be conserved.
Fuad Gasi, Naris Pojskić, Mirsad Kurtovic, Clive Kaiser, Stein Harald Hjeltnes, Milica Fotiric-Aksic, and Mekjell Meland
‘Ingeborg’ is currently the main commercial pear cultivar grown in Norway. However, fruit set and subsequent yields of this cultivar have proven to be variable and overall low averaging 10–20 t·ha−1. Pear seeds found in ‘Ingeborg’ fruits are often underdeveloped, suggesting that incomplete fertilization might be a major cause of poor fruit set. In some years, sporadically unfavorable environmental conditions during and immediately after pollination in Hardanger district, western Norway, have resulted in poor fruit set of ‘Ingeborg’. In this study, the pollinizer efficacy of several pollinizers, namely ‘Clara Frijs’, ‘Herzogin Elsa’, ‘Anna’, ‘Colorée de Juillet’, and ‘Belle lucrative’, from several orchards located in the Hardanger district was investigated using 12 microsatellite markers for two growing seasons (2014 and 2016). Pollinizer efficacy was estimated by genotyping ‘Ingeborg’, each individual pollinizer, as well as normally developed seeds from ‘Ingeborg’ fruit, and conducting gene assignment analyses to identify the pollen contribution from each of the pollinizer cultivars. In addition, S-allele genotyping was conducted, and only one pollinizer, ‘Anna’, was identified as being semicompatible with ‘Ingeborg’, whereas all other pollinizers were fully compatible. ‘Clara Frijs’ and ‘Belle lucrative’ were identified as the most efficient pollinizers probably because these cultivars were abundant compared with all other pollinizers within all, but one of the examined orchards. Higher yields could not be attributed to a particular pollinizer, and genetic effects associated with the triploid nature of ‘Ingeborg’ are most likely implicated as a cause behind the low and variable yield of this cultivar.
Fuad Gasi, Kenan Kanlić, Belma Kalamujić Stroil, Naris Pojskić, Åsmund Asdal, Morten Rasmussen, Clive Kaiser, and Mekjell Meland
Apple genetic resources in Norway are currently conserved within a number of local clonal archives. However, during establishment of these ex situ collections, primary focus was not on capturing as much of the diversity as possible, but instead on preserving cultivars of particular importance to specific fruit-growing areas. To identify redundancies within the collection as well as to assess the genetic diversity and structure of apple germplasm currently being conserved in Norway, eight microsatellites were used in genetic characterization of 181 apple accessions. Overall, 14 cases of synonym or possibly mislabeled accessions were identified, as well as several homonyms and duplicates within and among the analyzed collections. The information obtained should contribute to overall better management of the preserved germplasm. Bayesian analysis of genetic structure revealed two major clusters, one containing most of the foreign cultivars, while the other consisted mainly of traditional Scandinavian cultivars, but also some very winter-hardy genotypes such as ‘Charlamovsky’, ‘Gravenstein’, ‘Transparente Blanche’, and ‘Wealthy’. Analyses of molecular variance (AMOVA) detected a significant genetic differentiation among the clusters (f CT = 0.077; P < 0.01). The results of the Bayesian analyses do not indicate a strong differentiation between the foreign and the Norwegian apple accessions, however, they do suggest that climate adaptation has had a significant influence on the genetic structure of the preserved germplasm. Overall, apple accessions currently maintained ex situ in Norway represent a diverse germplasm which could be very valuable in future breeding programs, especially for the Scandinavian climate.