Juan Carlos Melgar

Clemson University | CU · Department of Plant & Environmental Sciences

Hyperspectral imaging (HSI) is an emerging technology being utilized in agriculture. This system could be used to monitor the overall health of plants or in pest/disease detection. As sensing technology advancement expands, measuring nutrient levels and disease detection also progresses. This study aimed to predict three different levels of potassi...

The purpose of this research is to examine and monitor factors affecting peach cultivation as a part of an overall research program for developing Integrated Pest Management strategies. Potassium fertilization and the practice of root-collar excavation have proven effective in increasing yield and fruit quality and in extending orchard longevity on...

The photosynthetic light response of commercial blackberry cultivars ( Rubus L. subgenus Rubus Watson) is largely unexplored, although they are frequently grown in full sun. In this experiment, light response curves of floricane leaves from the cultivars Natchez, Apache, Navaho, and Von were examined throughout the following production stages: befo...

Fruit bagging is an acceptable cultural practice for organic production that provides a physical barrier to protect fruit. It can reduce pest and pathogen injury for a variety of fruit crops, but quality attributes have been inconsistent for peach [Prunus persica (L.) Batsch] and other bagged fruit. A 2-year experiment on a U.S. Department of Agric...

Variable fall temperature and moisture conditions may alter leaf senescence of deciduous fruit trees, influencing carbon assimilation before dormancy and phenology the following spring. This study explored gas exchange of young peach trees (Prunus persica (L.) Batsch) when senescence proceeded normally or was delayed during the fall under two soil...

Horticultural performance of eleven blackberry cultivars, including two primocane-fruiting cultivars, was evaluated over three years after fruiting began at the Musser Fruit Research Center in South Carolina, United States. Results showed differences in yield between cultivars for each year of the study. ‘Natchez’ had the highest cumulative yield a...

The aim of this research was to assess how fruit tree age influences nutrient partitioning patterns in aboveground organs. We selected 6-year-old (mature) and 20-year-old (old) ‘Cresthaven’ peach trees and measured the macronutrient concentrations in organs removed during pruning, thinning, harvesting, and leaf fall for 3 years. Then, we calculated...

In this study, one-year old grapefruit trees grafted onto sour Orange (SO) and C22 rootstocks were exposed to NaCl-induced salinity (approx. 6 dS m-1) in pot culture for two months under greenhouse conditions. The experiment was laid out in a randomized block design with eight replicates. The trees were irrigated with saline solution containing 0.1...

Nutritional imbalances in plants influence their responses and defense mechanisms against abiotic stress, pests, and diseases, ultimately impacting crop production. Normal functioning and growth of the plant are affected due to insufficient availability of an essential nutrient(s). Plants have developed highly complex and specialized nutrient sensi...

Transgenic 'Hamlin' and 'Valencia' sweet orange and 'Ruby Red' grapefruit plants overexpressing a cyclic nucleotide gated ion channel (CNGC) gene were previously produced via Agrobacterium tumefaciens-mediated transformation. Artificial challenge inoculations on these transgenic plants under laboratory conditions revealed an enhanced tolerance to s...

A delay of leaf senescence resulting from variable fall climate may allow for additional nutrient resorption, and storage within reserve organs. Autumn leaves and reserve organs (<1 year shoots, >1 year shoots, stem above and below the graft union, the tap root, and fine roots) during dormancy of young peach trees were evaluated following warmer fa...

The objective of this research was to determine the influence of ripening season on nutrient concentration and nutrient partitioning of peach trees (Prunus persica L. Batsch). We selected peach trees from three different ripening seasons and measured: (1) the concentration of macronutrients in pruned wood, thinned fruitlets, harvested fruit, and le...

Maintaining shelf life and postharvest quality of blackberries (Rubus subgenus Rubus) from harvest to consumer is challenging for growers and packers due to several postharvest issues including fresh weight (FW) loss, red drupelet reversion, and fruit leakiness. The time of day blackberries are harvested, the time from harvest to cold storage, and...

Exogenous application of biochemicals has been found to improve water stress tolerance in herbaceous crops but there are limited studies on deciduous fruit trees. The goal of this research was to study if ascorbic acid applications could improve physiological mechanisms associated with water stress tolerance in young fruit trees. Ascorbic acid was...

Peach skin streaking is a previously undescribed skin discoloration affecting red-blush peach cultivars in Georgia and South Carolina. Streaked peaches have been observed in the field close to harvest. The cause of streaking is still unknown but one hypothesis is that atmospheric pollutants may be involved. The goal of this study was to establish p...

Partial rootzone drying (PRD) has been reported to potentially improve crop water use efficiency (WUEcrop) compared to full irrigation in different fruit trees; however, field studies on the effect of PRD in citrus are scarce. In this field study, three irrigation strategies were tested in an orchard of mature grapefruit trees during two consecutiv...

Subtropical and tropical climates are becoming more vulnerable to drought and salinity problems. Of the crops produced in these climates, citrus is especially sensitive to increasing salinity. In times of water scarcity and periodic drought alternative water sources that may be lower in quality may be required for irrigation, leading to reduced yie...

This experiment evaluates the potential of using parameters based on tree trunk fluctuations for detecting water deficit in citrus trees under two different water saving-irrigation strategies: sustained deficit irrigation and partial root-zone drying. Three irrigation treatments were applied: 1) Control: trees were irrigated with 100% of their evap...

The exogenous application of the plant hormone abscisic acid (ABA) may provide a new approach for preventing freeze damage to fruit tree crops. Exogenous ABA (1 mM) was foliarly applied to citrus trees under different irrigation regimes: well-irrigated trees or drought-stressed trees. Foliar applications of ABA increased the endogenous leaf ABA con...

Purpose of the review: This review describes the importance of considering biotic and abiotic factors in citrus production and highlights the relevance of understanding the effects of two of these factors (citrus greening and drought), as well as the effect of horticultural management practices on yield and fruit quality, so that different strategi...

Partial rootzone drying (PRD) is an irrigation strategy that consists in keeping half of the rootzone well-irrigated while the other half is allowed to dry in order to trigger root-to-shoot signaling to increase water use efficiency. A field experiment was conducted to evaluate the effect of PRD on growth and fruit production in mature grapefruit (...

The effect of partial rootzone drying (PRD) on whole plant water use efficiency (WUEwp = tree growth per water applied) and physiological responses of two-year-old split-root potted grapefruit trees was studied in a greenhouse. Four treatments were applied through above and below tree evapotranspiration (ETc) requirements for 12 weeks: optimum PRD...

As climate change impacts rainfall distribution and frequency many agricultural producers are turning to alternative water sources as supplemental irrigation. Often these sources are of low quality and high in salinity. Glycophytic crops, which include most agricultural crop species, are negatively impacted by salt stress. This results in a dramati...

The effectiveness of electrolyte leakage assays was tested in order to develop simple and sound procedures and to evaluate cold tolerance in fruit tree research studies. Divergences in specific methodological details that may impact the accuracy of these types of assays have been found in the literature. In this methodological study, leaves from gr...

The objectives of this study were: 1) to study the response of grapefruit trees to regulated deficit irrigation (RDI) and partial rootzone drying (PRD); and 2) to evaluate the use of different methods, including stem water potential, sap flow, and tree trunk micro-variations, to estimate tree water use. A greenhouse experiment was conducted at the...

The effects of alternate and fixed partial rootzone drying (PRD) on leaf gas exchange and water use efficiency (WUE) of citrus were evaluated. Three-year-old split-root potted Mexican Lime trees were grown in a greenhouse. Two irrigation trials were developed, and three treatments were applied in each trial. In the first trial, the treatments were:...

Water movement through cell membranes is facilitated by water channels called aquaporins (AQPs). These membrane proteins belong to the major intrinsic protein (MIP) family and play a central role in plant water relations. There are increasing evidences that AQPs are involved in the regulation of water transport in many physiological processes such...

Nitrogen plays an important role in plant nutrition and its positive effect on plant response under flooding conditions has been reported in several crops. One-month-old Cyphomandra betaceum seedlings were transplanted into 1-L plastic pots containing peat as substrate and fertigated with a complete nutrient solution containing two N concentrations...

Citrus production in Texas, currently valued at approximately $75 million, is the third largest citrus industry in the U.S., with Hidalgo and Cameron Counties in the Lower Rio Grande Valley (LRGV) accounting for more than 98% of bearing acres. In recent years, the citrus industry in the U.S. and the LRGV in particular, has been facing numerous chal...

Freezes have led to tremendous economic losses to the fruit production industry in the United States either by direct death of trees and rotting of fruits or indirect losses caused by decreased fruit quality. In leaves, freezes form ice crystals that draw out the water inside the cells and make water unavailable to plant tissues as well as cause da...

Cape gooseberry (Physalis peruviana L.) is a native plant of tropical America, principally of Colombia, Peru and Ecuador. It is common to find episodes of water stress in cape gooseberry, since it is mainly grown under rain-fed conditions in Colombia. Kaolin particle film applications has been used to improve water use efficiency (WUE) under water...

Water demand for irrigation is increasing in olive orchards due to enhanced yields and profits. Because olive trees are considered moderately tolerant to salinity, irrigation water with high salt concentration is often used without considering the negative effects on olive tree growth and yield. We studied salt effects in mature olive trees in a lo...

The loss of leaves, branches and flowers along with bark scuffing and root exposure that can occur during mechanical harvesting (MH) has not reduced long term yields of healthy trees. Nonetheless, such visible injuries have limited the widespread adoption of mechanical harvesting in Florida's citrus industry. There was little measurable physiologic...

Trees with different nitrogen status: low (1.1-1.3%), adequate (1.4-1.6%) or high (1.7-1.8%) leaf N concentration of July samples, were selected from a long-term experiment in which mature 'Picual' olive trees were subjected for 13 consecutive years to different nitrogen fertilization regimes. Leaves were sampled from each group of trees from Octob...

Leafy inflorescences in citrus have been reported to favor higher fruit set and fruit persistence than leafless inflorescences. In subtropical humid climates like Florida, drought stress during winter may be used as a management strategy to delay flowering in late-season sweet orange cultivars destined for late-season mechanical harvesting to avoid...

The effects of different levels of Fe-deficiency chlorosis on the fruit yield, appearance and composition of pear and peach trees grown in field orchards have been studied. The major effect of Fe deficiency in both species was a large yield reduction, even when chlorosis was moderate, associated to decreases in fruit tree load. Fruit size increased...

Oleocellosis or oil spotting on the peel of citrus fruit is a common post-harvest injury caused by improper handling. Mechanical injury allows phytotoxic oil to leak out of oil glands and cause injury to surrounding flavedo cells, resulting in oleocellosis. Mechanical harvesting (MH) of ‘Valencia’ sweet orange is conducted in late spring, when the...

The most prevalent nutritional disorder in fruit tree crops growing in calcareous soils is Fe deficiency chlorosis. Iron-deficient, chlorotic tree orchards require Fe-fertilization, since chlorosis causes decreases in tree vegetative growth as well as fruit yield and quality losses. When assessing the effectiveness of Fe-fertilizers, it is necessar...

The effects of deficit irrigation (DI) and partial rootzone drying (PRD) on the growth and mineral nutrition of citrus rootstock seedlings in the glasshouse were determined, as well as the potential of DI and PRD to trigger root-to-shoot signalling of abscisic acid (ABA) to increase the growth per amount of water used (water use efficiency (WUE))....

We determined if frequency of application of irrigation water plus fertilizer in solution (fertigation) could modify root and shoot growth along with growth per unit nitrogen (N) and water uptake of seedlings of the citrus rootstock Swingle citrumelo growing in a greenhouse. In the first experiment, all plants received the same amount of water with...

We determined if winter drought stress could delay flowering and fruit development of immature ‘Valencia’ sweet oranges to avoid young fruit loss during late-season mechanical harvesting. Beginning in December over three consecutive seasons (2007–2009), Tyvek� water-resistive barrier material was used as a rain shield groundcover under 13- to 15-ye...

Horticultural crops include a wide range of commodities, such as fruits and vegetables, that are highly valuable for humanity. They are extensively grown worldwide, and their production can be described as an open and highly complex system affected by many factors, among which we can count weather, soil and cropping system, as well as the interacti...

We determined if winter drought stress could delay flowering and fruit development of immature ‘Valencia’ sweet oranges to avoid young fruit loss during late-season mechanical harvesting. Beginning in December over three consecutive seasons (2007–2009), Tyvek® water-resistive barrier material was used as a rain shield groundcover under 13- to 15-ye...

In three separate experiments, the growth and water use of salinized citrus rootstock seedlings and grafted trees were modified using different growth substrates, elevated CO2, or 50% shade screen under field conditions. By reanalyzing previously published data, we tested the hypothesis that salinity tolerance in citrus can be characterized as the...

The interactive effects of root-zone salinity and sunlight on leaf biochemistry, with special emphasis on antioxidant defences, were analysed in Olea europaea L. cv. Allora, during the summer period. Plants were grown outside under 15% (shade plants) or 100% sunlight (sun plants) and supplied with 0 or 125 mM NaCl. The following measurements were p...

We evaluated effects of alternate and fixed partial root zone drying (PRD) on leaf photosynthesis and water use efficiency (WUE = photosynthesis per transpiration) of split-root ‘Carrizo’ citrange seedlings growing in autoclaved fine sandy soil in a green house. There were three treatments: 1) a well watered control where both root zone halves were...

Oleocellosis or oil spotting of citrus fruit, is a common injury on the flavedo caused by mechanical damage during harvesting and handling. Physical injury of oil glands of the peel allows the phytotoxic oil to injure the surrounding cells. Using ‘ Valencia’ orange trees, we evaluated the effects of winter time drought stress and late season mecha...

Water demand for irrigation is increasing in olive orchards due to enhanced yields and profits. Because olive trees are considered moderately tolerant to salinity, irrigation water with salt concentrations that can be harmful for many of fruit tree crops is often used without considering the possible negative effects on olive tree growth and yield....

Root-zone salinity stress and high solar irradiance concomitantly occurs in the Mediterranean basin, where Olea europaea is the dominating fruit-tree crop-species. Although the effect of each individual stressor on plant performance has been widely investigated, much less is known on the interaction effects of salinity stress and solar irradiance o...

The aim of this work was to evaluate long-term effects of different irrigation regimes on mature olive trees growing under field conditions. A 9-year experiment was carried out. Three irrigation treatments were applied: no irrigation, water application considering soil water content (short irrigation), or irrigation without considering soil water r...

The effects of salinity on growth, leaf nutrient content, water relations, gas exchange parameters and chlorophyll fluorescence were studied in six-month-old seedlings of citrus (Citrus limonia Osbeck) and rooted cuttings of olive (Olea europaea L. cv. Arbequina). Citrus and olive were grown in a greenhouse and watered with half strength Hoagland’s...

We compared growth, leaf gas exchange characteristics, water relations, chlorophyll fluorescence, and Na(+) and Cl(-) concentration of two cultivars ('Koroneiki' and 'Picual') of olive (Olea europaea L.) trees in response to high salinity (NaCl 100mM) and elevated CO(2) (eCO(2)) concentration (700microLL(-1)). The cultivar 'Koroneiki' is considered...

This review aims to offer the reader a comprehensive picture of how various mechanisms of response to excess soil salinity highly integrate in Olea europaea plants. We focus our attention on the relationships between (the effectiveness of) salt-exclusion mechanism and both carbon acquisition and whole-plant growth, reconsidering traits to be used t...

The responses of leaf water status, growth, and ion concentrations to water or to saline stresses were compared in olive cuttings of different Ca 2+ status. Mist-rooted 'Picual' olive cuttings were grown in a greenhouse in 2 l plastic pots containing perlite. A nutrient solution with or without 2.5 mM CaCl 2 was initially used to irrigate the plant...

Picual' olive cuttings were grown in a greenhouse under saline conditions in 2 L plastic pots containing perlite. Plants were irrigated with a nutrient solution plus 75 mM NaCl and 0, 2.5, 10 or 40 mM CaCl 2 . Vegetative growth, leaf and root Na + and Ca 2+ concentrations were measured. Na + toxicity symptoms were observed in plants non-treated wit...

The effects of shading in combination with salinity treatments were studied in citrus trees on two rootstocks with contrasting salt tolerance to determine if shading could reduce the negative effects of salinity stress. Well-nourished 2-year-old ‘Valencia’ orange trees grafted on Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citran...

The salinity tolerance of citrus rootstocks varies but grafted citrus trees are generally considered to be more sensitive to salinity stress than olive trees that are usually grown from cuttings. We compared the salt tolerance of 6-month-old seedlings of the relatively salt tolerant citrus rootstock Rangpur (Citrus limonia Osbeck) with similar size...