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Pamela Korczynski, James E. Faust and Robert Klein

Paclobutrazol drenches (1 ppm, 118.4 mL per pot) were applied to Poinsettia `Freedom' Red' on 1, 11, 21, and 31 Oct. in 1997 and 1998. Plant heights were recorded twice weekly throughout the experiment, and internode length and bract area were measured at harvest. The total bract area of the three true bracts and the top three transitional bracts was reduced by 5.8%, 13.6%, 4.2%, and 2.3% for the 1, 11, 21, and 31 Oct. application dates, respectively; however, all plants were highly marketable. At the time of each drench application, the most newly unfolded leaf was marked. The internode lengths for the three internodes below this leaf and the internodes that developed after the drench application were typically between 5 and 10 mm in length, while the internode lengths of the control plants were typically 10 to 25 mm, depending on node number. Plant height increased 62, 51, 47 and 19 mm following application on the 1, 11, 21, and 31 Oct. application dates, respectively. The 1, 11, 21, and 31 Oct. drench applications reduced total stem elongation from 1 Oct. to anthesis by 64%, 49%, 28%, and 15%, respectively. Paclobutrazol drenches did not affect time to anthesis.

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James E. Faust, Pamela C. Korczynski and Robert Klein

Experiments were conducted to evaluate the effects of paclobutrazol drenches on poinsettia (Euphorbia pulcherrima) `Freedom Red' height and flowering. In 1997 and 1998, paclobutrazol drenches [(a.i.) 0.118 mg/container; (28,350 mg = 1.0 oz)] were applied to poinsettias grown under natural photoperiods on four dates from 1 Oct. to 2 Nov. On plants receiving the paclobutrazol drench application during the second week in October, bract area was reduced by 15% and 12% compared with that of the control in 1997 and 1998, respectively; however, the bract area reduction was commercially acceptable. Anthesis date was not significantly affected during either year. Plant height and internode length measurements indicate that paclobutrazol drench applications had both a rapid and a long-term impact on poinsettia stem elongation. Paclobutrazol drenches applied in late October or early November are an effective tool for controlling late-season stem elongation of `Freedom Red' poinsettias grown under natural photoperiods. These late-season applications have the least risk for negatively affecting bract size while still reducing stem elongation in the last few weeks of the crop. Chemical names used: (±)-(R*,R*)-b-[(chlorophenyl)methyl]-a-(1,1-dimethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

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Alireza Pourreza, Won Suk Lee, Mark A. Ritenour and Pamela Roberts

Citrus black spot (CBS) is a fungal disease caused by Phyllosticta citricarpa (synonym Guignardia citricarpa). CBS causes fruit lesions and significant yield loss in all citrus (Citrus) species. The most distinguishing CBS symptom is called hard spot, which is a circular lesion with gray tissue at the center surrounded by a black margin. The spectral characteristic of CBS lesions was investigated and compared with the spectral signature of healthy fruit tissue to determine the best distinguishing wave band. Healthy and CBS-affected samples presented similar reflectance below 500 nm and above 900 nm. However, healthy samples reflected more light between 500 and 900 nm, especially within the visible band. Also, spectral reflectance of the same symptomatic lesion was acquired six times over a 2-month period to determine the variation of symptom’s spectral signatures over time after being harvested. A two-sample t test was employed to compare each pair of consecutive repetitions. The results showed that the spectral signature of the CBS lesion did not change significantly over 2 months. The wavelengths between 587 and 589 nm were identified as the distinguishing band to develop a monochrome vision–based sensor for CBS diagnosis. A support vector machine (SVM) classifier was trained using the spectral reflectance data at the selected bands to identify CBS-affected samples in each repetition. The overall CBS detection accuracies varied between 93.3% and 94.6%.

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Les D. Padley Jr, Eileen A. Kabelka and Pamela D. Roberts

The various disease syndromes caused by Phytopthora capsici Leonian can be devastating to squash (Cucurbita spp.) production areas of the United States. In some growing seasons, yield loss has been reported up to 100%. A recently developed University of Florida Cucurbita breeding line, #394-1-27-12, resistant to the crown rot syndrome of P. capsici, was used to determine the inheritance of resistance to this disease. Data from F1, F2, and backcross progeny from crosses of a P. capsici-susceptible butternut-type winter squash (C. moschata) with #394-1-27-12 indicated that resistance is conferred by three dominant genes. The introgression of P. capsici crown rot resistance from #394-1-27-12 into morphologically diverse domesticates within Cucurbita will aid in the management of this economically important pathogen.

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Robert E. Rouse, Monica Ozores-Hampton, Fritz M. Roka and Pamela Roberts

Citrus trees affected by huanglongbing (HLB) become diminished, weak, and develop dieback resulting in reduced production. Decline in fruit yield ultimately prevents economically acceptable commercial citrus production. The objectives of this study were to evaluate the effects of severe pruning in combination with an enhanced foliar nutritional treatment on growth, yield, and juice quality of HLB-affected orange trees. The bacterial titer within the trees was monitored before and after treatments, and a cost–benefit analysis provided an economic evaluation of the treatments. Fifteen-year-old ‘Valencia’ orange (Citrus sinensis Macf.) trees on Swingle citrumelo rootstocks [C. paradisi × Poncirus trifoliata (L.) Raf.] with 100% incidence of HLB, confirmed by real-time polymerase chain reaction (PCR), were severely pruned back to the main scaffold branches. Between 2010 and 2015, foliar nutrients were sprayed on both pruned and nonpruned trees to target new flush growth. Three enhanced nutritional foliar treatments were evaluated and compared with a control foliar nutritional treatment that was considered to be a standard practice before endemic HLB. The enhanced nutritional treatments included a mixture of micro- and macronutrients commonly known as the “Boyd cocktail,” a micronutrient package labeled Fortress © (Florida Phosphorus LLC, Key Largo, FL) sprayed with potassium nitrate (KNO3), and the Fortress © micronutrient package sprayed with urea. The experiment was a split-plot with seven replications, with pruning as the main plots, and a foliar nutritional treatment as subplots. Tree pruning was performed in Feb. 2010 before the spring flush. Pruned trees grew longer shoots than the controls the year after pruning. Canopy volume and leaf area were greater with nonpruned trees, but the chlorophyll content per cm2 leaf area was higher in the pruned trees compared with nonpruned trees in 3 years of the 5-year experiment. Pruned and nonpruned trees bloomed and set fruit the first year of the experiment in the spring of 2010–11. The fruit crop for the 2010–11 and 2014–15 seasons, and the overall total fruit crop for the 2010–15 season on pruned trees were significantly lower than those on nonpruned trees. However, no significant yield differences were found between pruned and nonpruned trees in the 2011–12, 2012–13, and 2013–14 growing seasons. Fruit yields from pruned trees never surpassed the yields from nonpruned trees, and this was possibly due to the severe-pruning treatment. Thus, severe pruning, as used in this trial, was not cost effective through the first 5 years after pruning. The rapid regrowth response of the pruned trees, however, may indicate that a reduced pruning approach could be effective at rejuvenating the HLB-affected trees, and an alternative to tree removal and replanting. Enhanced foliar nutrition treatments provided some yield benefits, especially in the early years of the trial. However, the enhanced foliar nutrition treatments did not prove to be cost effective.

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Jiaqi Yan, Megan M. Dewdney, Pamela D. Roberts and Mark A. Ritenour

Citrus black spot (CBS), caused by Guignardia citricarpa, is a fungal disease that was first described in Australia in the 1890s and has since been discovered in Southwest Florida in 2010. The current study evaluated the effects of hot water treatments on mycelial growth of G. citricarpa in vitro and also evaluated postharvest hot-water dips and fungicide treatments on CBS development on ‘Valencia’ oranges. In vitro exposure to 56 °C for 120 seconds, 59 °C for 60 seconds, or 62 °C for 30 seconds suppressed mycelial growth of all three G. citricarpa isolates by >30%. These treatments did not significantly reduce disease incidence or severity of CBS lesion development on whole ‘Valencia’ oranges from CBS-infected trees when the fruit already had visible CBS symptoms before treatment. On asymptomatic fruit, while the treatments did not significantly reduce the incidence of CBS lesion development, fruit dipped in 56 °C water for 120 seconds significantly reduced disease severity after 2 weeks of storage compared with the control. None of the treatments caused peel scalding or fruit quality deterioration. Postharvest application of azoxystrobin, imazalil, or thiabendazole significantly reduced CBS disease severity on fruit that were asymptomatic at harvest, but did not affect disease incidence. These fungicides were not effective on fruit harvested later in the season (April), possibly because most lesion expression had already occurred before harvest, with little left to develop after harvest. On fruit showing CBS symptoms at harvest, postharvest fungicide treatments did not significantly affect disease incidence or severity after storage. Heating the fungicide solutions did not significantly improve fungicide effectiveness. These results demonstrated that fungicide azoxystrobin, imazalil, or thiabendazole could reduce CBS severity, but not incidence, on orange fruit that are still asymptomatic at harvest.

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Les D. Padley Jr, Eileen A. Kabelka, Pamela D. Roberts and Ronald French

Phytophthora capsici causes several disease syndromes on Cucurbita pepo L. (squash, pumpkin, and gourd), including crown rot, foliar blight, and fruit rot, which can lead up to 100% crop loss. Currently, there are no C. pepo cultivars resistant or tolerant to this pathogen, which can aid in disease management strategies. The objective of this study was to evaluate a select group of C. pepo accessions for resistance to the crown rot syndrome of P. capsici. One hundred fifteen C. pepo accessions, from 24 countries, were evaluated for their disease response to inoculation with a suspension of three highly virulent P. capsici isolates from Florida. Replications of each accession, including susceptible controls, were planted in the greenhouse using a randomized complete block design. At the second to third true leaf stage, each seedling was inoculated at their crown with a 5-mL P. capsici suspension of 2 × 104 zoospores/mL. Fourteen days after inoculation, the plants were visually rated on a scale ranging from 0 (no symptoms) to 5 (plant death). Mean disease rating scores (DRS) and sds were calculated for each accession and ranged from 1.3 to 5.0 and 0 to 2.0, respectively. Eight accessions with the lowest mean DRS were rescreened. Results paralleled those of the initial study with one accession, PI 181761, exhibiting the lowest mean DRS at 0.5. Further screening and selection of accessions from the C. pepo germplasm collection should aid in the development of breeding lines and cultivars with resistance to crown rot caused by P. capsici.

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Chandrasekar S. Kousik, Scott Adkins, William W. Turechek, Craig G. Webster and Pamela D. Roberts

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Charles S. Vavrina, Pamela D. Roberts, Nancy Kokalis-Burelle and Esa O. Ontermaa

Six greenhouse trials of five commercial products marketed as systemic resistance (SR) and plant growth promotion (PGP) inducers were evaluated on tomato (Lycopersicon esculentum Mill.) over a 21-month period. The effect of the inducers on treated plants was measured by monitoring plant growth and disease suppression after inoculation with either plant pathogenic bacteria or nematodes. The commercially available SR/PGP inducers included a bacterial suspension [Companion (Bacillus subtilis GB03)], two plant defense elicitors with nutrients (Keyplex 350DP plus Nutri-Phite, and Rezist with Cab'y), natural plant extracts (Liquid Seaweed Concentrate and Stimplex), and a synthetic growth regulator (Actigard 50W). Growth enhancement was noted in some trials, but the parameter of growth affected often varied with trial. Response to Actigard treatment included significant suppression of bacterial spot [Xanthomonas campestris pv. vesicatoria (Xcv)] in three of the six trials. Companion, Keyplex 350DP plus Nutri-Phite, Rezist and Cab'y, and seaweed products induced only partial disease suppression of bacterial spot in inoculated tomato plants. The alpha-keto acids plus nutrients (Keyplex 350DP plus Nutri-Phite) increased plant growth by 14.3% and improved root condition compared to the untreated control following exposure to nematodes. Results are encouraging, if not consistent, and with a greater understanding of the SR system and the conditions related to product efficacy, such materials may become effective tools for production agriculture.

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Chandrasekar S. Kousik, Scott Adkins, William W. Turechek and Pamela D. Roberts

Watermelon vine decline (WVD) is a new and emerging disease caused by the whitefly-transmitted squash vein yellowing virus (SqVYV). The disease has become a major limiting factor in watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] production in southwest and west–central Florida and is estimated to have caused more than $60 million in losses. Symptoms of WVD typically occur at or just before harvest and are manifested as sudden decline of the vines, often with a reduction in fruit quality. In this study, we present results of greenhouse and field evaluations of U.S. plant introductions (PIs) for resistance to SqVYV. Of the 218 PIs we evaluated for resistance to SqVYV, none were completely immune, but several showed varying levels of resistance and these were further evaluated in two greenhouse and field trials. Disease progress was significantly slower on the selected PIs compared with disease progress on susceptible watermelon cultivars Mickey Lee and Crimson Sweet. Moderate resistance was observed in two C. colocynthis (PI 386015 and PI 386024), a Praecitrullus fistulosus (PI 381749), and two C. lanatus var. lanatus PIs (PI 482266 and PI 392291). Variability in the resistant reaction to SqVYV within most PIs was observed. The identification of potential sources of partial resistance to SqVYV suggests that watermelon germplasm with moderate resistance can be developed by careful screening and selection of individual resistant plants within these PIs for use in breeding programs.