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E.W. Stover

The relationship between intensity of flowering and various aspects of cropping will be reviewed for fruit and nuts. Clearly, relatively light flowering can limit yield in most fruit and nut species. This commonly occurs before mature bearing commences and in “off” years for varieties that display alternate bearing. During mature bearing, many species will carry fruit numbers that exceed commercially desired levels, resulting in excessively small fruit and accentuating alternate bearing. The economic disadvantages of excess cropload have resulted in considerable research on fruit thinning and widespread commercial application of this practice. Heavy flowering intensity in some crop species results in economic disadvantages beyond the problems of excessive cropload and resultant small fruit size. Many species flower profusely and have initial fruit set far in excess of final tree capacity, resulting in abscission of numerous flowers and fruitlets. Abscised organs can represent a substantial investment in carbohydrates and nutrients, compromising availability at critical periods in flower and fruit development. The potential implications of this process are best exemplified in navel orange, where an increase in flowering beyond intermediate intensity results in a reduction in both initial fruit set and final fruit yield at harvest. In several crops, there is evidence that fruit size may be reduced by excessive flowering, even when cropload is quickly adjusted to an acceptable level. These data suggest that further research on the advantages of controlling flowering intensity is warranted.

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Kim E. Hummer and Ed Stover

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E.W. Stover and D.W. Greene

Plant response to foliar application of plant growth regulators (PGRs) is often variable, in part due to environmental factors. Weather prior to application is thought to influence cuticle development and thus PGR uptake. For example, in growth chamber studies foliar uptake of 1-naphthaleneacetic acid (NAA) is sometimes increased when fruit trees are placed in low temperature and high humidity several weeks prior to application. Environmental conditions over an extended period of time after application may influence PGR conversion to active form (e.g., ethephon), PGR metabolism, or metabolic factors that affect PGR activity in the plant. The effects of environmental conditions on PGR uptake have been investigated extensively in laboratory studies. In many cases, uptake is clearly increased by high temperatures immediately after application. Laboratory studies report a linear positive correlation between temperature and uptake and greater temperature response above 25 °C (77.0 °F). High humidity and longer drying time often are also reported to increase PGR uptake in laboratory studies. These results are consistent with many grower observations on effects of weather on chemical thinning and have been incorporated into many product labels and extension recommendations. However, relatively few field experiments have been reported in which the relationship between PGR response and environmental conditions were assessed. Wash-off studies have demonstrated that rain shortly after application may reduce efficacy of NAA. Several studies demonstrate environmental interaction with metabolic activity involved in PGR action. For example, shading after thinner application is reported to increase fruitlet abscission and enhance effectiveness of some thinning agents. Chemical thinning of apples (Malus ×domestica) with ethephon is reported to correlate strongly with temperature in the days after application, while studies suggest that higher temperatures after aminoethoxyvinylglycine (AVG) application may reduce control of preharvest drop. However, the stage of fruitlet development at apple thinning often appears to be more important than environmental conditions at the time of PGR application. In addition, field experiments indicate that longer drying times at lower temperatures seem to largely compensate for greater uptake rates at higher temperatures. This paper discusses data from published and previously unpublished experiments in order to understand the effects of environment on PGR response variability.

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E.W. Stover and T.G. McCollum

The diseases huanglongbing [HLB, associated with Candidatus Liberibacter asiaticus (CLas)] and Asian citrus canker [ACC, caused by Xanthomonas citri (Xcc)] are widespread in Florida and many other citrus-growing areas, presenting unprecedented challenges for citrus breeding. Because HLB and ACC weaken trees and compromise cropping, breeding is much less efficient using seed parents that have been exposed to these diseases. Therefore, it would be highly desirable to use unique disease-exposed selections only as pollen parents with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this potential, abundance of the pathogens associated with these diseases was assessed in anthers and flowers using quantitative polymerase chain reaction. Because CLas is systemic, levels on mature leaves from the flower source trees were assessed to see if the presence of CLas in flowers was associated with leaf levels. Disease-exposed trees were tested in 10 genotypes from each of three broad genotypic categories, which reflect different levels of susceptibility to the diseases associated with the pathogens studied: Poncirus trifoliata hybrids (most resistant to HLB), Citrus maxima and hybrids (susceptible to both diseases), and C. reticulata and hybrids (considerable resistance to ACC). Of the 30 samples of each tissue type analyzed for CLas, 88% of mature leaves, 69% of flowers, and 88% of anthers had one or more CLas bacterium per sample. The trifoliate genotypic group had significantly lower levels of CLas than the pummelo and mandarin groups in mature leaf samples, but CLas levels were more similar between groups in anther and flower samples, and the pathogen was present in most of the trifoliate hybrids tested. Mean numbers of CLas detected per nanogram nucleic acid were 100 to 800 times higher in mature leaf samples, most characteristic of HLB symptoms, compared with anther samples. Xcc DNA was detected in 30% of flower samples and 23% of anther samples. No significant differences in Xcc levels were found between tissue type or genotypic group. However, regressions between Xcc levels in flowers and percent of plant pedigree derived from mandarin had a negative correlation and an r 2 of 0.159 (P = 0.029). The biology of CLas is consistent with the pathogen being present in anthers from unopened flowers, whereas the ACC pathogen detected inside flowers was likely the result of contamination despite great care in sample collection and handling. Where exceptional diligence to exclude HLB and ACC is appropriate, results suggest that there may be a risk of spreading these pathogens through use of pollen from trees on infected farms.

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E.W. Stover and C.S. Walsh

Trees of each of five rootstock genotypes (M.7a, M.9, M.26, MM.111, Mark,) were inoculated above and below ground with three strains of Agrobacterium tumefaciens. These were compared to controls that were uninoculated or inoculated with sterile deionized water. All rootstocks tested were quite susceptible to crown gall, but M.9 and Mark were consistantly among the most susceptible genotypes. Percent of inoculated sites forming galls above ground ranged from 43% in M.7a to 77% in M.9 and the mean size of galls that formed ranged from 3.7 mm in M.26 to 7.7 mm in M.9. All rootstocks except M.9 formed galls at a higher percentage of inoculated sites that were below ground. Percent of below ground inoculations forming galls ranged from 67% in MM.111 to 100% in Mark while mean size of galls underground ranged from 4.2 mm in MM.111 to 15.3 mm in M.9. The proportion of inoculated sites forming galls below ground in M.7a was twice as high as in above ground sites For rootstock × strain means, each measure of crown gall susceptibility above ground was significantly correlated with corresponding below ground data at the 0.01 level. to three rootstocks, some trees inoculated with sterile deionized water also produced apparent galls at sites below the soil line (100% in Mark, 60% in M.7a, 22% in M.26) although none of the above ground control inoculations produced galls. Uninoculated controls showed no gall formation.

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E.W. Stover and C.S. Walsh

A factorial planting of three scion varieties (`Gala', `Fuji', and `Braeburn') on each of four rootstocks (Mark, M.26, M.7a, and MM.1 11) was established at two locations in MD in 1990. In 1993, trees were scored for trunk circumference (TC), extension growth, leaf color, burrknots (BK) and gall-like swellings (GLS) both above and below ground, and presence of pests on the shanks. Mark was the only rootstock with GLS; 75% of the trees examined had these tissue proliferations, Mark grafted with `Fuji' had 2-3X higher %TC composed of GLS below ground than was observed with other scion varieties. Across rootstocks, %TC composed of BK below ground was significantly affected by location. ln each rootstock, regressions between growth parameters and % of TC with BK or GLS were almost all negative and many bad significant correlations. ln Mark, % of TC with GLS below ground (but not above) was significantly correlated with reduced TC at 25 cm. The occurrence of GLS was much greater below ground than above. Orchard surveys of trees on Mark were also conducted at several commercial sites in MD and NY. The percentage of trees showing GLS ranged from 0 to 90%. There was a significant reduction in TC at 25 cm correlated with increasing levels of GLS. The percentage of trees with GLS and mean % of TC with GLS significantly correlated with percentage of trees having visible woolly apple aphids or their distinctive galls.

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E.W. Stover, T.E. Paine and W.C. Stiles

Damage to xylem subtending apple buds is often observed following very low winter temperatures. Reports suggest that prebloom application of boron, zinc, and urea facilitate recovery. Prebloom nutrient treatments were applied to `McIntosh' and `Empire' at three sites in Spring 1994. The following treatments were applied to drip at half-inch green: boron (22.8 mM, solubor); Zn-EDTA (0.75 mM); boron and Zn-EDTA; boron, Zn-EDTA, and urea (59.4 mM). Another treatment used boron and Zn-EDTA at half-inch green, followed by boron, Zn-EDTA, and urea at pink. Spur leaf area, fruit set, fruit size, and seed number were determined. There were no clear treatment effects at the warmest site (mid-winter low –32C); however, this orchard was more variable than other treatment sites. The intermediate site (mid-winter low –37C) had a strong trend of increasing fruit set in `Empire' and `McIntosh' as more nutrients were applied. The combined half-inch green and pink treatment significantly increased fruit set by 23.8% compared to the untreated control. At the coldest site (mid-winter low –42C), `Empire' again displayed a strong trend of increasing fruit set with additional nutrients. All treatments combining boron and zinc significantly increased fruit set. The combined half-inch green and pink treatment increased fruit set by 43%. At this site `McIntosh' did not respond to treatment. However, `McIntosh' trees had continued active growth into late Fall 1993 and sustained severe cold injury in November. Data suggest that, when they were effective, nutrient treatments resulted in increased retention of flower buds on damaged spurs.

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E.W. Stover, M. Myers, R.M. Sonoda and Z. Guo

Stylar-end russetting (SER) is a cosmetic defect of Florida citrus fruit most frequently associated with navel orange. SER is evident as spots or streaks of corky tissue that often form a network of intersecting lines. Occurrence of SER is reported to vary widely from year to year, but some orchards have a history of severe SER, with fruit culled annually for this defect. Growers report that SER is typically first evident in August. The cause of SER has not been determined. Reports of yeast-like fungi inducing russet in pome fruit suggest that similar organisms may be implicated in SER. Yeast-like fungi were isolated on acid PDA from navel oranges in an orchard with frequent severe SER. Strains were selected with a wide range of colony morphology, but were not identified taxonomically. These strains, and strains of Aureobasidium pullulans and Rhodotorula glutinis that caused russetting in pome fruit, were grown in liquid suspension and sprayed on navel orange trees with three repeated applications during July and Aug. 1998. No increase in SER was observed on strain-inoculated trees compared to controls. Two broad-spectrum fungicides were sprayed on other navel orange trees to further explore the possibility that fungi may be involved in SER. GA (gibberellic acid) was also applied in this experiment because it can reduce russetting in apples. All applications were made five times at 3-week intervals in June through Sept 1998. SER was assessed in fruit harvested late Sept. 1998. The proportion of fruit with less than 10% of the surface exhibiting SER was 51% for controls, increased to 69% where myclobutanil was applied at 74 mg a.i./L and increased further to 93% where manganese ethylenebisdithiocarbamate was applied at 1775 mg a.i./L. GA did not significantly influence SER.

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E.W. Stover, S.M. Ciliento and X. Yang

In Florida's subtropical climate, citrus floral induction is primarily stimulated by cool weather in the fall and winter. Frequently, inductive periods are separated by warm conditions conducive to bud development, resulting in prolonged and multiple bloom. Large variability in date of fruit-set creates an array of problems, and prolonged bloom contributes to severity of postbloom fruit drop. GA applied during the inductive period inhibits bloom in citrus. This study was conducted to determine whether different GA application timings could shift bloom intensity and duration to reduce problems associated with prolonged bloom. GA was applied via airblast sprayer to mature `Navel' on sour orange rootstock near Ft. Pierce, Fla., at 49.4 g GA/ha, 0.05% Silwet L-77, and 2340 L/HA spray volume. Six single-tree experimental units per treatment were blocked by size and vigor in a randomized complete block design. Treatments were time of GA application: 23 Nov. 1998; 17 Dec. 1998; 6 Jan. 1999; 25 Jan. 1999; 23 Nov. and 17 Dec.; 23 Nov. and 25 Jan.; 6 Jan. and 25 Jan.; and a control. Bloom began one month earlier on trees that received the two January applications compared to trees that received GA on 23 Nov. and 17 Dec.. Trees sprayed 25 Jan. (or 6 Jan. and 25 Jan.) had 2 weeks shorter bloom duration compared to controls. Total flowering on GA-treated trees ranged from 13% to 55% of the controls. Single GA applications on 17 Dec. and 6 Jan. increased fruit yield at harvest by 42% and 25%, respectively, while GA on 6 and 25 Jan. yielded 27% less than controls. All other timings had no effect on yield.

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X.E. Yang, X.X. Long, W.Z. Ni and E.W. Stover

Vegetables play an important role in the human diet, and production in suburban areas has increased as populations have become more urbanized. However, heavy metal pollution of soils has enhanced in such areas, and metal accumulation in vegetables may pose a human health risk when consumed. Zinc is an essential micronutrient for plants and humans, but it is toxic to plants and humans at high levels. Although a maximum Zn tolerance for human health has been established for edible parts of vegetables (20 mg/kg DW), little information is available for predicting vegetable Zn concentration based on soil and water Zn levels. The objectives of this study were to determine the critical Zn concentrations in nutrition solution and soil to reach maximum Zn tolerance concentrations in Chinese cabbage, bok choy, and celery. Five Zn levels were used for both solution and soil culture experiments, with three replicates of each. Shoot growth was significantly inhibited at Zn concentrations above 50 mg/L in nutrition solution and above 180 mg/kg in soil. The sensitivity of crops to zinc toxicity, in term of shoot and root growth, decreased in the order: celery > Chinese cabbage > bok choy. Zinc accumulation in shoots and edible parts varied with Zn supply levels and type of vegetables. A negative correlation was noted between Zn accumulation and dry matter yields, with r-squared values of 0.980** for nutrient solution and 0.960* for soil culture. Zinc concentrations in shoots or edible vegetable parts were below 20 mg/kg (human health threshold) when they were grown at DTPA extractable Zn in the soil less than 75, 100, and 175 mg/kg for bok choy, celery, and Chinese cabbage, respectively.