QTL mapping for brown rot (Monilinia fructigena) resistance in an intraspecific peach (Prunus persica L. Batsch) F1 progeny

Brown rot (BR) caused by Monilinia spp. leads to significant post-harvest losses in stone fruit production, especially peach. Previous genetic analyses in peach progenies suggested that BR resistance segregates as a quantitative trait. In order to uncover genomic regions associated with this trait and identify molecular markers for assisted selection (MAS) in peach, an F1 progeny from the cross "Contender" (C, resistant) 7 "Elegant Lady" (EL, susceptible) was chosen for quantitative trait loci (QTL) analysis. Over two phenotyping seasons, skin (SK) and flesh (FL) artificial infections were performed on fruits using a Monilinia fructigena isolate. For each treatment, infection frequency (if) and average rot diameter (rd) were scored. Significant seasonal and intertrait correlations were found. Maturity date (MD) was significantly correlated with disease impact. Sixty-three simple sequence repeats (SSRs) plus 26 single-nucleotide polymorphism (SNP) markers were used to genotype the C 7 EL population and to construct a linkage map. C 7 EL map included the eight Prunus linkage groups (LG), spanning 572.92 cM, with an average interval distance of 6.9 cM, covering 78.73 % of the peach genome (V1.0). Multiple QTL mapping analysis including MD trait as covariate uncovered three genomic regions associated with BR resistance in the two phenotyping seasons: one containing QTLs for SK resistance traits near M1a (LG C 7 EL-2, R2 = 13.1-31.5 %) and EPPISF032 (LG C 7 EL-4, R2 = 11-14 %) and the others containing QTLs for FL resistance, near markers SNP_IGA_320761 and SNP_IGA_321601 (LG3, R2 = 3.0-11.0 %). These results suggest that in the C 7 EL F1 progeny, skin resistance to fungal penetration and flesh resistance to rot spread are distinguishable mechanisms constituting BR resistance trait, associated with different genomic regions. Discovered QTLs and their associated markers could assist selection of new cultivars with enhanced resistance to Monilinia spp. in fruit

The permeability concept: a useful tool in analyzing water transport through the sweet cherry fruit surface

Rain cracking of sweet cherry fruit is thought to be related to a net transport of water into the fruit. This net transport may occur through the fruit surface and along the pedicel/fruit juncture as uptake into the fruit or transpiration from the fruit surface. In addition, vascular transport through the pedicel may contribute to rain cracking. Water transport through the fruit surface may be described quantitatively using Fick’s law of diffusion, where the amount of water taken up into or transpired from the fruit surface is expressed as the product of the fruit surface area, the driving force for water transport, and the permeability of the fruit surface. Analyzing water transport on this basis allows prediction of the effect of selected fruit factors on net water transport; the leakiness of the pedicel/fruit juncture, the effect of fruit size, the magnitude of the driving force, the skin permeability or the effect of environmental variables such as relative humidity or the percentage of the fruit surface area wet. This modeling approach may be extended and a complete fruit water balance established by including estimates for vascular transport through the fruit pedicel. Examples using literature sources are provided of how these calculations may be used to identify important determinants in the fruit’s water balance

The effects of N fertiliser application rates on red drupelet disorder (reversion) in ‘Ouachita’ thornless blackberries grown under tunnels

Red drupelet disorder (RDD), sometimes called reversion, red cell, or reddening, is a physiological occurrence in blackberries (Rubus fruticosus) which causes individual drupelets to revert to a red colour from black, reducing marketable yield. The disorder usually develops during postharvest cool storage. The objective of this trial was to assess the impact of nitrogen (N) application on fruit and foliar N concentration, with the aim to identify any link between N availability and RDD. Three levels of N (52.85, 105.7, 211.4 kg ha-1 total) were applied via weekly fertigation during the harvest period to ‘Ouachita’ blackberries grown under 150 µm high-UV transmittance polythene tunnels in northern Tasmania, Australia. The design consisted of three blocks of three 105.7 m long rows arranged in a complete randomised block design, with each row receiving a N application treatment. Fruit was sampled at five dates every 15 days between January 11 and March 11, 2016 by harvesting every fruit from four randomly selected 3 m sections of cane, from which subsamples of 20 fruit per row were taken for further analysis. Fruit were assessed for drupelet disorder by counting individual drupelets expressing RDD then weighed and homogenised to measure pH, titratable acidity, and total soluble solids (TSS) (°Brix). ‘High’ N treatment fruit had significantly increased average number of drupelets expressing RDD relative to the fruit of ‘low’ and ‘medium’ N treatments in the first four harvest dates. Over the season the average number of drupelets per fruit showing any level of RDD was 2.2, 1.63, and 1.41 for high, medium, and low N treatments, respectively. Early season fruit across all treatments had the highest levels of reversion averaging 3.19 drupelets per fruit in pick 1 with levels declining significantly at each subsequent harvest date (2.61, 1.41, 1.17, and 0.69 drupelets per fruit in picks 2-4 respectively). This study has shown that there is an association between high N fertigation during harvest and RDD expression, and time of the season and RDD expression

Red drupelet reversion in blackberries: A complex of genetic and environmental factors

Red drupelet reversion (RDR; also referred to as red drupelet disorder) in blackberries is a physiological disorder that causes the postharvest reddening of individual or groups of drupelets, resulting in economic loss due to a reduction in marketability. This paper reviews recent advances in the understanding of RDR including the physiochemistry, causes of expression and genotypic variation in its incidence. RDR is associated with a significant reduction in anthocyanin pigment concentration, which can vary in severity causing degrees of partial or full colour change. This is associated with observations of disrupted cellular structural integrity and loss of membrane integrity. Drupelets affected by RDR are characterised by anthocyanin species containing disaccharides or acylated sugar moieties that are not degraded as readily as those containing monosaccharides and non-acylated sugars. Susceptibility to RDR is genotypically influenced, with identified links between cultivar texture, postharvest weight loss and incidence of RDR. Current findings indicate that RDR is primarily caused by mechanical injury to the fruit that has induced cell decompartmentalization, for example in one study 85 % of handled fruit developed RDR relative to only 6% of non-handled fruit. We found that various methods have been employed for assessment of RDR and we propose a red drupelet index and/or image analysis approach that accurately reflects visual appearance and will enable comparison between studies. Gaps in knowledge are highlighted in relation to the mechanism for pigment degradation, and to investigate confounding genotypic and environmental effects (pre- and post-harvest) on incidence of RDR

Effects of climatic conditions during harvest and handling on the postharvest expression of red drupelet reversion in blackberries

Red drupelet reversion (RDR) causes individual drupelets on blackberries to revert from black at harvest to a red colour postharvest, reducing the quality and marketability of fruit. The objective of this trial was to assess the effects of time of harvest and associated climatic variables, as well as handling of fruit during harvest, on postharvest RDR expression and fruit quality. Fruit were harvested on ten occasions over two days by one of two methods: either hand-harvested into shallow buckets and transferred to industry standard 125 g clamshell punnets (standard practice) or harvested carefully without handling by cutting the pedicel and placing each fruit into individual cotton wool-lined trays. The number of partially red (PR) and fully red (FR) drupelets per fruit was counted, firmness was measured by compression, and skin firmness measured by a penetrometer. Air and fruit skin temperature, relative humidity, vapour pressure deficit, and soil water tension were all influenced by the time of day. 85% of fruit that was handled during harvest had at least one drupelet develop RDR, whilst only 6% of fruit not handled during harvest had any RDR. In handled fruit, warmer skin temperature at harvest was associated with increased RDR incidence and severity (P = 0.001). The skin firmness of fully black (FB) drupelets, measured by a penetrometer, also decreased significantly by an average of 0.56 N when harvested during warmer temperatures compared to fruit that was not handled. The data indicate that mechanical injury incurred during harvest is a major cause of RDR in fresh blackberries, and that harvest times associated with warmer temperatures result in significantly higher rates of RDR and reduced postharvest quality

Nitrogen application rate and harvest date affect red drupelet reversion and postharvest quality in ‘Ouachita’ blackberries

Red drupelet reversion (RDR) is a postharvest physiological disorder in blackberries that causes fruit that is black at harvest to subsequently turn red. This trial aimed to investigate the effects of nitrogen (N) fertiliser application rate on the expression of RDR and postharvest fruit quality.Nitrogen was applied weekly during the growing period via fertigation at a low, medium, and high rates (53, 106, and 212 kg N ha−1 respectively) to ‘Ouachita’ blackberries in 2016 and 2017. Yield, RDR, and postharvest quality were assessed. Harvest date, N application rate, and fruit mass were significant factors in the postharvest expression of RDR. In both years, fruit from the high N treatment exhibited significantly increased incidence and severity of RDR relative to the other two N application rates. Fruit temperatures during harvest of more than 23 °C were associated with higher incidence and severity of RDR in 2017, and smaller fruit were more likely to have no RDR in both years. The high N treatment produced more fruit than the low N treatment in 2016, and more and heavier fruit than both other treatments in 2017

Calcium application and impacts on cherry fruit quality

The most common mineral employed in the management of fruit cracking is calcium (Ca). Ca is a xylem mobile mineral, and as fruit xylem connections and pathways are thought to be reduced during maturation, so early accumulation is vital. Ca has been implicated in building resilience to cracking into fruit, with many studies exploring the impact of late season calcium chloride (CaCl2) spray applications on cracking. These trials have produced inconsistent results but early and repeated spray patterns for calcium uptake are supported by studies in apples. Enhanced Ca uptake rates have been recorded in the sweet cherry cultivar 'Van' with the use of thickeners and surfactants. A major limitation of using direct Ca application to prevent cracking is the unsightly residue left on the fruit, therefore the benefits must outweigh this disadvantage. To assess from where Ca was incorporated into fruit (vascular supply, or directly across the fruit skin) trials were undertaken in southern Tasmania, Australia which included Ca applications supplied via fertigation and/or foliar spray applications. Foliar sprays commenced either before or after Stage II (pit-hardening) of fruit development to assess when any uptake occurred. Ca levels were assessed using ICP-MS at both Stage II and at harvest maturity. It was expected that increased calcium would strengthen fruit tissue and reduce cracking resulting from rain. Relationships between fruit Ca and fruit quality (firmness) were explored, and the potential for continued use of Ca in mediating fruit cracking after rainfall discussed

Physiochemistry of blackberries (Rubus L. subgenus Rubus Watson) affected by red drupelet reversion

Red drupelet reversion (RDR) is a physiological disorder causing individual or groups of drupelets on blackberries that are black at harvest to turn red during postharvest cool storage. The objectives of this study were to examine and quantify the physiochemical changes occurring in flesh affected by RDR. Drupelets were classified as ‘fully black’, ‘partially red’, or ‘fully red’. The total anthocyanin concentration in black, partially, and fully red drupelets was 1841 mg kg−1, 1064 mg kg−1 and 769 mg kg-1 fresh weight respectively. Anthocyanins containing acylated or disaccharide sugar moieties were more stable than anthocyanins with non-acylated monosaccharide sugar moieties. The pH of partially red (3.05) and fully red drupelets (3.01) was lower than black drupelets (3.32). Firmness of partially red (1.90 N) and fully red drupelets (1.77 N) was lower than that of fully black drupelets (2.39 N). Examination by light and electron microscopy showed cell disruption, separation, and loss of integrity in the upper mesocarp of affected drupelets. Electrolyte leakage over 24 h was significantly higher from partially red (84.8%) and fully red (90.0%) than fully black drupelets (64.9%). The data are consistent with RDR in blackberries arising from mechanical damage that causes cell decompartmentalisation and subsequent anthocyanin degradation

To cut or not to cut: the role of extension growth in fruit quality

Cherries are a sweet fruit, with total soluble solids being a key determinant of quality in most fruit quality studies. It is also well known that leaves play a role in delivering assimilates to fruit, and that fruits are strong sinks. Less well known is the role of extension growth (current season shoot growth) on fruit quality.    Extension growth occurs throughout the fruit growing season and has the potential to be a strong source of assimilates. However, while growing they can also be a strong sink; hence the question; to cut or not to cut? Summer pruning can be useful to reduce vigour, or promote light interception but can it also be used to reduce competition for assimilates between growing fruit and extension growth?    Trials assessing fruit quality from trees (cvs. ‘Kordia’, ‘Sweetheart’ and ‘Satonishiki’) which had been pruned at various times throughout the growing season were undertaken. Timing had a significant impact on fruit quality with pruning after, but not at, Stage II increasing fruit sugars (TSS). Pruning at most stages had no impact on sugars in newly developing buds. Pruning during Stage III increased current season fruit sugars. Pruning did not affect the uniformity of bud burst or yield in the following season. Therefore, used appropriately, summer pruning of extension growth may be a useful tool in promoting sugar assimilation.    Furthermore, pruning during rainfall in the three weeks prior to harvest maturity reduced fruit cracking, while pruning prior to rainfall in the same period did not impact on cracking. This implicates a role of extension growth leaves in the supply of excess water to the fruit reducing both yield and quality such that pruning could increase both yield and quality