Dr. Sherif Sherif is an Assistant Professor of Horticulture at the School of Plant and Environmental Sciences at Virginia Tech, USA. He received both his bachelor's and master's degrees in horticulture from Alexandria University, Egypt, and his doctorate in plant agriculture from the University of Guelph, Canada. Dr. Sherif's broad research experience in molecular biology, plant development, and tree physiology has led him to lead several research projects focusing on the biotic and abiotic factors affecting fruit trees' production, sustainability, and competitiveness. Sherif's current research program at Virginia Tech focuses on frost mitigation strategies, precision crop load management, high-density production systems, molecular regulation of critical horticultural traits, and germplasm development.
The following is a link for a very brief survey that should take less than 2 min to complete. The results of this survey will help us assess our tree-fruit research and extension program at the AHS Jr. AREC. This survey will also help us identify priority areas and extension activities most preferred by Virginia tree fruit growers and stakeholders.
Below is the apple maturity report for the Winchester/Fredrick county area (Table 1). We measured fruit firmness (lb), fruit color, total soluble solids (Brix), and starch conversion rate (1-8 index) for 12 apple cultivars grown in the AHS Jr. AREC’s and we will keep sending similar reports throughout the harvest season to help you determine the optimal harvest time for your apple varieties. More varieties will be added to the table below as we move through the harvest season. We have assessed fruit firmness, Brix, and starch index according to the methods we described previously in another blog post (https://blogs.ext.vt.edu/tree-fruit-horticulture/2012/08/23/harvest-maturity-resources/). We used a DA meter to measure fruit coloration. The DA device measures the chlorophyll content in the fruit, just below the skin. As the fruit ripens, chlorophyll degrades, and DA readings decline accordingly. Starch index, fruit firmness and, ‘to some degree’, Brix values, are used either alone or in combination to determine the harvest date for most commercial apple varieties. In Table 2, you will find the estimated firmness and starch index values at which apples should be harvested for controlled atmosphere (CA) storage and fresh consumption. If you don’t see your variety in the list, you can generally harvest your crop at starch index (3-5), firmness (> 16 lb), and DA reading (0.60) for CA storage. But if the crop is intended for fresh market, then wait until starch index of (5.5-7), firmness (13-16 lb), DA reading (0.35) and Brix (12-14%). Again, there are varietal differences, so it’s always recommended to use more than one maturity index to determine the optimal harvest time.
The pre-harvest drop refers to the abscission of fruits from the tree before horticultural maturity. Depending on the cultivar and growing season, yield losses due to pre-harvest drop can reach 30%. Factors such as heat and drought stress, heavy insect infestation, and late summer pruning can increase the severity of fruit drop. Early-maturing cultivars (e.g., Gala and Honeycrisp) are usually more prone to fruit drop than late-maturing cultivars (e.g., Fuji and Pink Lady). Ethylene, the ripening hormone, is considered the primary driver of pre-harvest drop, and therefore ethylene inhibitors are used to control the pre-harvest drop in apple orchards. ReTain (from Valent Bioscience) and Harvista (from AgroFresh) are the two ethylene inhibitors labeled for pre-harvest drop control in Virginia. ReTain inhibits the biosynthesis/production of ethylene, whereas Harvista prevents ethylene reception and action. The purpose of this blog post is to share with you the results of two experimental trials we conducted in the 2018 and 2019 seasons to explore the effects of different rates and application timings on the efficacy of these materials.
In 2018: we examined two rates of ReTain (166g/acre and 333g/acre) and three application timings (1 week, 3 weeks and (1 & 3) weeks before the anticipated harvest date) on the percentage of fruit drop (%) of Gala apples. We used 6 trees per treatment. We flagged and counted 100 fruit/tree at 4 weeks before harvest (WBH), and we counted the fruits on these trees every week starting at 1 WBH through 4 WAH. The percentage of fruit drop (%) at each stage was calculated relative to the initial fruit count. In addition to fruit drop, we also investigated the effects of different treatments on fruit quality parameters (fruit weight, size, firmness, color, sugar content, starch index, and acidity). The findings of this trial can be concluded in four points: a) The effect of ReTain treatments on fruit drop was more evident at 1 and 2 weeks after harvest. b) Applying ReTain at the full rate (333g/acre) 3 weeks before the anticipated harvest date reduced fruit drop by 40-50% and extended the harvest season by 1-2 weeks. The same results were obtained when ReTain was used at half rate (166g/acre) at 1 and 3 WBH. C) fruits treated with ReTain (full rate) were firmer, had a lower starch index and lower brix than the untreated checks. However, ReTain-treated fruits were generally less colored than untreated checks. D) no differences in fruit size were found between ReTain- treated and untreated fruits. (Please refer to Tables 1 & 2).
Table 1: The effects of ReTain rate and application timing on the pre-harvest fruit drop (%) of Gala apples-Season 2018.
Table 2: Gala fruit quality as affected by ReTain applications- 2018.
In 2019: we examined 2 rates of ReTain (333 g/acre and 666 g/acre), 1 rate of Harvista (121 fl oz/acre), 3 application timings for ReTain (at 1, 3 and (1&3) weeks before harvest) and 2 application timings for Harvista (at starch index 2 and at starch index (1.5 and 3). Again, six trees were used per treatment. The total number of fruits on each tree was counted four weeks before harvest. The percentage of fruit drop was calculated starting at 1 WBH and through 3 WAH. We also examined the effects of different treatments on the fruit quality of Gala apples at harvest. Our results indicated that: a) a full rate (333 g/acre) of ReTain applied at 3 WBH was not statistically different than a double rate (666 g/acre); b) Harvista applied twice (at starch index 1.5 and 3) showed better results on fruit drop than the single application (at starch index 2); c) There were no significant effects on fruit size and weight when either ReTain or Harvista were used; d) Fruits treated with ReTain were generally firmer than control, but had poor coloration; e) Harvista did not improve fruit coloration, but also did not reduce it compared to control; f) ReTain applications at a full rate significantly reduced the fruit’s ethylene content, but it required two applications of Harvista to acquire the same effect. (Please refer to Tables 3 & 4)
Table 3: ReTain and Harvista effects on the pre-harvest fruit drop (%) of Gala apples-2019
Table 4: ReTain and Harvista effects on fruit quality of ‘Gala’ apples-2019
Other general notes:
6h drying time after ReTain and Harvista
applications is required for achieving better results.
For optimal results, ReTain and Harvista should
be applied with 100 gal/acre as a complete spray.
ReTain label allows the use of up to two pouches
per acre either as a single spray or as a split application for apple.
Harvista can be applied close to anticipated
harvest date (within 3 days prior to harvest).
Our fruit size averages for ‘Gala’, ‘Red Delicious’ and ‘Honeycrisp apples in Winchester are 15.66, 14.9 and 13.23 mm, respectively. If you applied thinning treatments on May 2 or 4, you should be able to see thinning responses by now. Fruitlets to be abscised will be loose and easily pulled off. In order to assess the response of thinning applications applied on May 4, we marked the fruitlets of 10 clusters of Gala, Honeycrisp and Reds and tracked their sizes in the last three days using a digital caliper. The idea behind this was to monitor the fruit growth rate and determine how many fruitlets will eventually abscise. Fruitlets to be abscised are believed to have a growth rate that is less than 50% of the fastest growing fruit on the tree. For instance, the fastest growth rate for Gala in the past three days was 0.6 mm per day. A fruit in the cluster that shows less than 0.3 mm/day will likely abscise soon. Based on this, we have found that our thinning applications on May 4, will result on 50%, 67%, 82% abscission rate for Reds, Gala and Honeycrisp, respectively. On other words, we should expect more triples, doubles and singles/cluster for these three cultivars, respectively. Our per acre thinning rates for Gala and Reds were (64-96 fl oz of Exilis + 1 qt of Sevin + 1 pt of Regulaid). For Honeycrisp, our rate was (4.5 oz of Refine + 1 qt of Sevin + 1 pt of Regulaid). If you applied similar rates on the same day for the same cultivars, your responses will be likely the same.
If, after assessing thinning responses in your orchard, you
still feel that another thinning application is required, it’s still possible
to apply 6-BA and NAA applications until your average fruit sizes are ~ 18 mm. NAA
would likely result in better thinning responses at this stage. As shown in the
figure below, the carbohydrate model predicts carbohydrate surplus in the next
three days and therefore the recommendations are to increase the rate of
thinning materials by 30%.