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  • Author or Editor: Peter C. Andersen x
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The influence of bilateral cordon (BC) and cane training systems and level of pruning severity on vegetative and reproductive characteristics of Vitis hybrid `Suwannee' were determined from 1987 to 1989. In 1987, yield and quality were similar on BC- and cane-trained vines. In 1988, shoot count, yield, and quality were similar regardless of training system and pruning severity (50, 70, or 90 nodes per vine). When data from both training systems were combined, yield was related to the number of shoots. Vines pruned more severely compensated by producing more shoots from non-count (non-node) positions on the canes, cordon, or spurs. Similarly, in 1989 yield and berry quality were not affected by training system or levels of pruning severity (50, 70, 90, or 110 nodes), although berry weight was affected by training system, and shoot count and shoot length were affected by level of pruning severity. Interactive effects of training system and pruning level were not significant in either year. An analysis of combined data for 1989 indicated that yield was affected by the number of nodes and shoots. Thus, `Suwannee' may be trained to the BC system, which is more readily adapted to mechanization. Pruning to a specific number of nodes per vine was not critical.

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Abstract

Diurnal characteristics of the grape bleeding phenomenon were investigated for four muscadine (Vitis rotundifolia Mich.) grape cultivars. Shoots 6 to 12 mm in diameter of 8-year-old ‘Fry’, ‘Noble’, ‘Cowart’, and ‘Jumbo’ were cut to 60- to 140-mm-long spurs on 3 Apr., and the naturally occurring exudate was collected in foil-covered Erlenmeyer flasks. Water flux (Jv), osmotic potential (Ψs), and total solute flux (Js) were monitored diurnally from bleeding spurs. A distinct diurnal trend in Jv occurred with a minimum during early evening and a maximum during late morning. The Ψs of the exudate varied only slightly diurnally, thus the total quantity of solute moved from the root to the shoot was several times greater during the day than during the night. Water flux and Ψs were cultivar- and time-dependent. The Ψs of the exudate varied from −53.4 to −27.7 kPa. Maximum Js occurred at 1000 hr and was ≈ 0.250 mmol·s−1·m−2 for all cultivars except ‘Cowart’.

Open Access

There is increasing interest in the culture of satsuma citrus in the states bordering the northern Gulf of Mexico. Yield, tree size, and fruit quality of mature ‘Owari’ and ‘Brown Select’ satsuma (Citrus unshiu Marcovitch) on Poncirus trifoliata (L.) Raf. (‘Rubidoux’ and ‘Flying Dragon’) were evaluated in north Florida. Canopy area and volume, yield, and fruit quality data were analyzed as a 2 × 2 factorial design with scion and rootstock as the main effects. There were no scion × rootstock interactions. Overall average yield per tree was 16, 88, 91, 143, and 101 kg in 2010, 2011, 2012, 2013, and 2014, respectively. Yield was influenced by scion (higher for ‘Brown Select’) in three of five years, and by rootstock every year (higher for ‘Rubidoux’). Yield per tree was ≈2-fold greater for trees on ‘Rubidoux’ compared with ‘Flying Dragon’; the highest yield was recorded for ‘Brown Select’ on ‘Rubidoux’. Yield per m2 canopy area was often similar since canopy area was often ≈2-fold greater for trees on ‘Rubidoux’. For three of the five years, fruit weight was greater for ‘Brown Select’ (average = 157 g) than ‘Owari’ (average = 146 g), with no rootstock effect. Soluble solids of juice averaged 10.0 °Brix and were higher for trees on ‘Flying Dragon’ than on ‘Rubidoux’ in three of five years. Juice pH averaged 3.67 and was unaffected by scion or rootstock. Trees were not subjected to freeze protection and were not damaged by minimum temperatures as low as −9.4 °C, except for 2014/15. A rating of defoliation after a freeze on 19 Nov. 2014 (−5.6 °C) indicated that cold hardiness varied by scion (greater for ‘Brown Select’) and rootstock (greater for ‘Rubidoux’). Differences in cold hardiness did not persist when assessed later in the winter. Successful satsumas production can be achieved in north Florida in the absence of a severe freeze event.

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Leaf physiology and growth parameters of Aucuba japonica (Thunb.) cv. Variegata were assessed under conditions of full sun (photosynthetic photon flux = 1531 to 2073 μmol·m-2·s-1) and under shadecloth (light transmittance of 69%, 47%, and 29% full sun) over 2 years. Two days after treatment initiation, net CO2 assimilation (A) was proportional to light level, although stomata1 conductance to water vapor (gs) was not influenced by shading. Subsequently, A, gs, transpiration rate, and water use efficiency of 100% sun-grown plants were often <50% that of shade-grown plants. After 1 month of exposure to 100% sun, leaves were chlorotic and necrotic; plant appearance was normal for plants grown under shadecloth. The growth index and total, leaf, stem, and root dry weights were inversely related to light level. Relative water content and chlorophyll concentration in leaves that had expanded before treatment initiation were reduced with increasing levels of irradiance, but these variables were not altered in leaves produced after treatment initiation. Plants from all treatments experienced dieback when transplanted to the field under conditions of full sun. We conclude that A. japonica is shade obligate, performing best with exposure to 47% of full sunlight.

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Homalodisca coagulata (Say) is a xylem-feeding leafhopper that is the principal vector of many economically important diseases resulting from infection by Xylella fastidiosa (i.e., plum leaf scald, phony peach disease). We have previously established that high abundances and high consumption rates of H. coagulata occur on host species with high amide concentrations in the xylem fluid. Several lines of research suggest that selection of “marginal hosts” (those that typically have low abundances of leafhoppers) may be influenced by ovipositional, as well as feeding, preferences. In northern Florida, Euonymus japonica consistently has the highest densities of eggs and young nymphs, but is only a marginal host for adults. Adults caged on this host feed little and have a short longevity. In contrast, young insects (second instar) caged on the host have high survivorship rates and assimilate dietary nutrients with high efficiency. H. coagulata are abundant on Prunus germplasm in northern Florida during the month of June, but only occasionally visit Prunus after this period. In a study of 10 Prunus scion/rootstock combinations, we established that abundances of H. coagulata on Prunus during the peak period were correlated to leafhopper consumption rates. During summer, when Prunus serves as a marginal host, leafhopper abundances are tightly coupled to fecundity rates. Understanding of ovipositional preference may be central to our understanding of Xylella acquisition. These preliminary experiments suggest that leafhoppers may sample xylem fluid during ovipositional selection, as they preferentially select ovipositional sites that have proper nutrient profiles for development of young nymphs (“mother knows best”). Although consumption rates are low for marginal hosts, repeated probing for ovipositional preference may contribute to the spread of diseases caused by X. fastidiosa.

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Homalodisca coagulata (Say), a xylem-fluid feeding leafhopper, vectors diseases induced by the bacterium Xylella fastidiosa such as phony peach disease and Pierce's disease. The purpose of this study was to investigate plant factors that influence feeding. H. coagulata were confined to stems of peach [Prunus persica (L.) Batsch] and crape myrtle (Lagerstroemia indica L.). Osmolarity, amino acid and organic acid concentrations of xylem fluid were maximum during the morning for peach and declined thereafter; xylem fluid chemistry of crape myrtle followed a less distinct trend. Irrigated plants had higher concentrations of organic constituents and feeding rates were higher on these plants. Feeding rates and xylem fluid tensions, were maximum during midday; feeding did not occur at night. In separate experiments feeding rates were greatly reduced at xylem tensions >1.5 MPa.

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The xylophagous leafhopper Homalodisca coagulata Say is an important vector of diseases caused by the bacterium, Xylella fastidiosa (e.g., Pierce's disease, phony peach disease, plum leaf scald, etc.). Neither leafhoppers or X. fastidiosa can be controlled by chemical sprays. For many plant species there is no resistant germplasm. H. coagulata is highly polyphagous, and within Prunus spp. host preference ranges from moderate (plum) to low (peach). The abundance, feeding and performance of H. coagulata on many unrelated plant species have been previously correlated with the amino acid profile, and particularly the amides in xylem fluid. We tested Prunus scion/rootstock combinations, which provided for a range of xylem fluid chemistry, on the behavior (abundance, feeding) and performance (survivorship, fecundity, body weight and body composition) of H. coagulata. Leafhopper abundance on various rootstock/scion combinations was determined seasonally. During the period of peak abundance (June 14 to 30) feeding rates and performance indices were determined. Leafhopper abundance and feeding increased with an increased concentration of amino acids. Abundance and feeding rate were most highly correlated with the amides in xylem fluid; performance indices were influenced by the amides and certain essential amino acids. Lower consumption rates decreased survivorship, reduced body dry weight and the carbon concentration of surviving insects.

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