Terence L. Robinson

A comparison of ‘Empire’ apple fruit size and anatomy in unthinned and hand-thinned trees

SummaryA stereological method was developed to analyze the anatomical features of fresh ‘Empire’ apple fruit sectors cut in the transverse equatorial plane. Fruits were from unthinned trees or trees hand-thinned to one fruit per cluster at –7, 0, 10, 20 or 40 d after full bloom. At final harvest (140 DAFB), fruits representing the size range within each treatment were analyzed for the effects of thinning on fruit size, weight and cortex anatomy, namely, parenchyma cell size, cell number and the proportion of cortex volume occupied by intercellular space (IS). A dissecting stereobinocular microscope fitted with a ten-by-ten reticule was used to count cells and proportion of IS in three fields in each of two cortex sectors per fruit. Cell volume in each field was derived by knowing only the grid area, a point-count for proportion of IS and a count of cell numbers within the grid. Fruit size and weight decreased as thinning was prolonged and unthinned trees had the smallest fruit. Within a thinning treatment...

Summer pruning reduces whole-canopy carbon fixation and transpiration in apple trees

Summary Canopy size control is one of the major purposes of summer pruning. However, reducing canopy size might also result in less light interception, consequently decreasing canopy photosynthetic efficiency and carbohydrate production, which might lead to the imbalance of carbohydrate supply and fruit demand. To document the effectiveness of summer pruning on canopy control and the impact on canopy gas exchange, pruning treatments at four levels of intensity (unpruned, light, moderate, and severe) were carried out on mature ‘Empire’/M.9 slender spindle apple trees (Malus domestica Borkh.) on 30 July 1998 and 4 August 1999. Changes in canopy leaf area after summer pruning were estimated. Canopy net carbon exchange rate (NCER) and canopy transpiration before and after summer pruning were monitored. Canopy growth was suppressed by summer pruning and the post-pruning regrowth was insignificant. Canopy NCER was reduced in proportion to the amount of leaf area removed by summer pruning. The result suggests that commercial pruning intensity similar to the moderate to severe treatments in this study could cause a significant reduction in canopy NCER and carbohydrate production. In addition, canopy transpiration was reduced in proportion to pruning intensity. Lower water consumption and improved water status during the growing season after summer pruning might benefit fruit growth and relieve the potential detriment due to carbohydrate shortage.

Genetic Diversity of Prunus necrotic ringspot virus Isolates Within a Cherry Orchard in New York

A survey for Prunus necrotic ringspot virus (PNRSV) in an orchard of Prunus cerasus cv. Montmorency and Prunus avium cv. Hedelfingen in New York by enzyme-linked immunosorbent assay indicated an eightfold higher infection rate in sour cherry (33%, 32 of 96) than in sweet cherry (4%, 6 of 136) trees. The presence of PNRSV was confirmed by reverse transcription-polymerase chain reaction and amplification of the coat protein (CP) gene in total RNA from infected leaf tissue. Latent infection was prevalent in the majority of trees infected (87%, 33 of 38), while a few of them exhibited shock symptoms or had severely reduced growth (13%, 5 of 38). Asymptomatic PNRSV-infected trees clustered in spatial proximity to symptomatic trees. Sequence analysis of the CP gene (675 bp) indicated a population structure consisting of one predominant molecular variant for 10 isolates and six minor molecular variants for seven isolates. A high sequence identity was found between the CP gene of PNRSV isolates from cherry trees and other isolates from diverse hosts and various geographic origins at the nucleotide and amino acid levels (88 to 100%). Phylogenetic analyses showed a clustering of PNRSV isolates from cherry trees in New York in the predominant group PV-96.

Summer pruning effects on fruit size, fruit quality, return bloom and fine root survival in apple trees

Summary While many undesirable effects of summer pruning on apple (Malus domestica Borkh.) growth and development have been reported, the results are inconsistent and difficult to interpret. This study resolves the inconsistency by supplying a model that integrates pruning effects with tree physiological crop load, i.e. canopy net carbohydrate exchange rate per fruit. Our previous study suggests that the potential impact of reducing canopy photosynthesis after summer pruning depends on the balance of carbohydrate supply and demand. To test the hypothesis that summer pruning affects carbohydrate balance, we measured fruit growth, fruit quality, return bloom, and root growth in 20 year old slender spindle ‘Empire’/M.9 apple trees in response to different severities of summer pruning. Results were interpreted in relation to pruning severity, fruit number per tree, and the physiological crop load. Within commercial cropping ranges, light and moderate summer pruning had slight influences on fruit size and fresh weight. Summer pruning did not affect fruit colour, soluble solids content, starch, firmness, and internal breakdown after storage. Summer pruning alone did not affect return bloom or root growth. However, the potential negative effects of summer pruning on fruit growth, return bloom, and fine root survival can be predicted through their relationships with physiological crop load. This study suggests that the carbohydrate supply and demand balance model feasibly explains summer pruning influences. In addition, the impact of carbohydrate shortage after summer pruning is likely to be mediated by the reduction in canopy transpiration. However, the interaction between canopy carbon balance and water status after summer pruning is also likely to be manipulated by annual weather pattern.

Fire Blight Resistance of Budagovsky 9 Apple Rootstock

Erwinia amylovora, the causal agent of fire blight, can cause a fatal infection of apple rootstocks known as rootstock blight. Budagovsky 9 (B.9) apple rootstock is reported to be highly susceptible when inoculated with E. amylovora, although results from multiple trials showed that B.9 is resistant to rootstock blight infection in field plantings. Conflicting results could stem from genetic variation in the B.9 population, appearing as phenotypic differences in rootstock material. However, genetic testing, using 23 microsatellite loci, confirmed the clonal uniformity of B.9 in commerce. Variation in growth habit between B.9 rootstocks originating from two nurseries also has been discounted as a source of disease resistance. Instead, results indicate a possible novel resistance phenotype in B.9 rootstock. B.9 rootstock was susceptible to leaf inoculation by E. amylovora, statistically similar to the susceptible rootstock Malling 9 (M.9). Conversely, inoculation assays targeting woody 4- to 5-year-old tissue revealed a high level of resistance in B.9, whereas M.9 remained susceptible. Although the mechanism by which B.9 gains resistance to E. amylovora is unknown, it is reminiscent of age-related resistance, due to an observed gain of resistance in woody rootstock tissue over succulent shoot tissue. Durable fire blight resistance correlated with tissue development could be a valuable tool for rootstock breeders.

Crop load expressed in terms of intercepted photosynthetically-active radiation can be used as a covariate to compare peach tree performance

Summary When fruit tree performance is evaluated in rootstock, thinning or other management trials, tree size and fruit number (or crop load) are the most important factors that confound the real effects of the treatments that are being evaluated. An experiment to assess the effects of tree size and crop load on yield efficiency and fruit size was conducted in a ‘Ross’ peach orchard on ‘Nemaguard’ rootstock in Malloa, Chile. Two groups of 15 trees each were selected according to tree size and were hand-thinned at the beginning of pit hardening to a wide range of crop loads within each tree size group, but keeping the average crop load similar between the two groups of trees. Fruits per tree (crop load) was either normalised or not normalised for tree size, assessed either as cm–2 of trunk cross-sectional area (TCSA) or the fraction of above-canopy photosynthetically-active radiation (PARf) intercepted by the canopy at harvest. At harvest, all the fruits were counted and weighed, and the average fruit weight calculated. Analysis of variance and covariance (with and without using crop load as a covariate), and regression analysis were performed and the results were compared. Differences in fruit size, yield efficiency (yield normalised by tree size) and total production between the two tree size groups were detected by ANOVA; but, by selecting the appropriate covariate, no differences between groups were detected. Thus, for a correct interpretation of the effect of treatment, in this case tree size, on tree performance measured as fruit size, yield, or yield efficiency, the differences in crop load (number of fruits per tree) must be removed by performing covariance analysis. Normalising crop load (fruits per tree) to account for tree size was essential for proper interpretation of the data. Calculating crop load using the fraction of light intercepted at harvest as the normalising factor was better than using TCSA as the normalising factor.

Comparative Programs for Arthropod, Disease and Weed Management in New York Organic Apples

Organic apple production in the eastern US is small and is mostly based on existing varieties, which are susceptible to scab, and rootstocks, which are susceptible to fire blight. This requires numerous sprays per year of various pesticides to produce acceptable fruit. From 2014 to 2016, we tested different arthropod, disease and weed management programs in an advanced tall spindle high-density production system that included disease-resistant cultivars and rootstocks, in an organic research planting of apples in Geneva, New York. Arthropod and disease management regimens were characterized as Advanced Organic, Minimal Organic, or Untreated Control. Results varied by year and variety, but, in general, the Advanced program was more effective than the Minimal program in preventing damage from internal-feeding Lepidoptera, plum curculio, and obliquebanded leafroller, and less effective than the Minimal program against damage by foliar insects. Both organic programs provided comparable control of sooty blotch, cedar apple rust, and fire blight, with some variability across cultivars and years. The advanced selection CC1009 and Modi seemed to possess complete resistance to cedar apple rust, while Pristine had partial resistance. For weed control, bark chip mulch, organic soap sprays, and limonene sprays tended to be most effective, while mechanical tillage and flame weeding had lower success.