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C.A. Powell, A. Hadidi, and J.M. Halbrendt

The ability of 32P-labeled transcribed cRNA probes to detect tomato ringspot virus (TmRSV) RNA in nucleic acid extracts from roots, bark, and leaves of nectarine (Prunus persica [L.] Batsch) trees with the Prunus stem-pitting disease was assessed and compared with detection of TmRSV antigen by enzyme-linked immunosorbent assay (ELISA) in the same tissues. Neither TmRSV-specific nucleic acid nor antigen was detected in nectarine leaf tissue. ELISA detected TmRSV antigen in root extracts from 71% of the diseased trees, while dot hybridization detected virus-specific nucleic acid in 18% of the same samples. However, ELISA detected TmRSV antigen in only 47% of bark extracts; whereas TmRSV-specific nucleic acid was detected in 100% of the bark extracts from samples collected at or near the soil line. When nucleic acid extracts from bark were prepared from various locations on diseased trees and tested for TmRSV-specific nucleic acid by dot hybridization, there was an almost perfect correlation between the presence of stem-pitting symptoms and the detection of TmRSV nucleic acid. Detection of TmRSV RNA from the bark tissue of rootstock suckers from TmRSV-infected `Delicious'/MM.lO6 apple (Malus × domestica Borkh.) trees was unsuccessful using dot hybridization. The viral RNA, however, was usually detected in either leaf or root tissue of these same trees.

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C. Gupton, J. Clark, D. Creech, A. Powell, and S. Rooks

Rabbiteye (Vaccinium ashei Reade) and southern highbush (mostly V. corymbosum L.) type blueberry selections were evaluated in regional trials at five locations. Entry × location interactions (E × L) were significant for all traits in the rabbiteye type and all except plant productivity, plant volume, Julian date of 50% ripe fruit, and berry weight at harvest 3 in the southern highbush type. Despite the significant interactions, selection FL80-11 and `Gulfcoast' were the earliest flowering rabbiteye and southern highbush entry, respectively, at each location. Significant E × L for plant volume and yield suggests that adaptation to the local environment is important in the selection of potential cultivars. Fruit quality traits appear less affected by environment than fruit production traits for the entries tested.

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Arlie A. Powell, Scott Goodrick, Ed Tunnell, and Richard Murphy

Inadequate winter chilling periodically becomes a serious problem for the commercial peach industry in the Southeast, especially along the Gulf Coast. A number of countries around the world are using hydrogen cyanamide (Dormex-SKW) to replace lack of chilling in peaches and other fruit plants. Studies were conducted over 3 years (1990-1992) to evaluate the effects of hydrogen cyanamide on replacing lack of winter chilling in 'Ruston Red' peach, (850 hour chill requirement). Findings indicated full tree sprays in early fall and late winter (after buds had become active) caused excessive bud thinning and crop reduction. Applications made when 65 to 85% of chilling requirement was satisfied (no visible bud activity) were very effective at concentrations of 0.5 to 1.0% V/V of 49% Dormex. Rates above 2% were very toxic causing crop loss. Dormex effectively replaced a shortage of 265 chilling hours of 'Ruston Red' during one season resulting in full cropping while controls failed to crop.

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Youjian Lin, R.H. Brlansky, and Charles A. Powell

Six severe and six mild Florida isolates of citrus tristeza virus (CTV) were used to evaluate the transmission efficiency of the virus from grapefruit seedlings by single brown citrus aphids (Toxoptera citricida Kirkaldy) (BrCA) from colonies initiated by aphids obtained from citrus groves in Fort Pierce, Fla. The transmission rate to 2120 receptor plants [`Mexican' lime (Citrus aurantifolia)] from grapefruit by single BrCA was 1.5%. Single BrCA transmitted four of the six severe isolates and three of the six mild isolates of CTV. The average transmission rate of severe isolates was 1.8%, higher than that (0.9%) of mild isolates. Severe isolate Y-7 had the highest transmission rate among six severe isolates, 3.6%. Mild isolate Y-23 had the highest transmission rate among the mild isolates, 3.0%. The transmission rates of CTV by alatae, apterae, or nymphs of BrCA were 1.5%, 1.5%, and 1.0%, respectively. The results suggested that BrCA is an inefficient vector of CTV when the source plant is grapefruit.

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Harry S. Paris, Peter J. Stoffella, and Charles A. Powell

`Striato d'Italia' (cocozelle group) and `Clarita' (vegetable marrow group) summer squash were grown in the greenhouse and field in the presence of sweetpotato whiteflies (Bemisia tabaci Germ.) and their susceptibility to leaf silvering was compared. Silvering was less severe in `Striato d'Italia' in the greenhouse and field.

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Arlie A. Powell, Karl Harker, Roger Getz, and Eugene H. Simpson

In order to provide timely weather information to county agents (CEA) and growers, a sophisticated user friendly weather information program was developed that provides over 900 weather files daily to users. This program uses a 420 Sun Server that automatically downloads files from the NWS office on the AU campus and makes them instantly available to CEA offices via the Extension Network. Growers may obtain information from CEAS or use their personal computers to access a “Weather Board”. A chilling/growing degree hour (GDH) model (mod. 45) has been developed for peaches that provides a good estimate of when rest is completed and allows prediction of phenological stages through flowering. This information assists growers with orchard management decisions. Studies with peaches were conducted using the chilling/GDH model to properly apply hydrogen cyanamide (Dormex) to replace lack of chilling. This work resulted in an effective application timing based on chilling accumulation and allowed development of a forecast model for grower use.

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Harry S. Paris, Peter J. Stoffella, and Charles A. Powell

Summer squash (Cucurbita pepo L.) plants were grown in pots with high (290% capacity) or low (45% to 70% of capacity) soil moisture. The plants were exposed or not exposed to sweetpotato whiteflies (Bemisia tabaci Genn.). Only the plants exposed to whiteflies developed leaf silvering. Silvering was more severe in plants subjected to low soil moisture.

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Arlie A. Powell, Roger Getz, and Eugene H. Simpson III

An agricultural weather program has been developed in Alabama and is available on the ACENET computer network of the Alabama Cooperative Extension Service (ACES). This program involves the coordinated efforts of the National Weather Service (NWS), ACES and grower organizations. The program began in March 1987 and has been upgraded several times. Hardware now being used includes a Sun Microsystem SPARC station by NWS and a Sun Microsystems Server Model 4/280 by ACES. Existing and experimental NWS forecast products are disseminated to each of Alabama's 67 county agents offices (CEAs) and to local producers using ACES' computer network. A comprehensive selection of climate and weather related information is available to ACES staff including a widely used freeze alert program. Very detailed freeze forecasts and related information is available to users hourly, 7 days a week. A specialist prepared commentary further enhances use of information during each freeze event. Considerable cost savings have been realized by producers. A pilot program is being initiated in 1991 to incorporate data from several real time weather stations into the system.

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Charles A. Powell, Peter J. Stoffella, and Harry S. Paris

Zucchini squash (Cucurbita pepo L.) fruit yield and the incidence of sweetpotato whitefly (SPWF) [Bemisia tabaci (Gennadius)], squash silver leaf (SSL) disorder, and zucchini yellow mosaic virus (ZYMV) were measured during Spring and Fall 1991 in experiments containing various plant populations. In both experiments, as the within-row spacing increased from 30.5 to 76.2 cm or the number of plants per hill decreased from three to one, the number of marketable fruit per hectare decreased, and the marketable fruit per plant increased. Adult SPWF populations increased with decreased within-row spacing in the spring but not the fall experiment. The incidence of SSL or ZYMV infection was not affected by plant population in either experiment. The results indicate that increasing zucchini squash plant population can increase yield without affecting the incidence of SSL or ZYMV.

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W.L. Mountain, C.A. Powell, and L.B. Forer

A trough system was developed to study rates of plant virus transmission by plant parasitic nematodes. Perforated plumber's polyvinyl chloride pipe, 5 cm in diameter, was cut into 48-cm lengths, split longitudinally, and fashioned into troughs to hold soil and common dandelion (Taraxacum officinale Webber) transplants. The first plant in each trough was infected with tomato ringspot virus (TmRSV), followed by 10 uninfected plants spaced at 4-cm intervals. The soil contained a high concentration of Xiphinema rivesi (199 per 100 cm3), a low concentration (16 per 100 cm3), or none. Plants were assayed biweekly for TmRSV. After 42 weeks, transmission rates between the low and high concentrations of nematodes were not significantly different. The subirrigated trough system provided excellent soil conditions for plant growth and sufficient nematode survival to detect virus transmission through 36 weeks.