Showing papers in "Hortscience in 1991"

Light-emitting Diodes as a Radiation Source for Plants

Abstract: Development of a more effective radiation source for use in plant-growing facilities would be of significant benefit for both research and commercial crop production applications. An array of light-emitting diodes (LEDs) that produce red radiation, supplemented with a photosynthetic photon flux (PPF) of 30 micromoles s-1 m-2 in the 400- to 500-nm spectral range from blue fluorescent lamps, was used effectively as a radiation source for growing plants. Growth of lettuce (Lactuca sativa L. Grand Rapids') plants maintained under the LED irradiation system at a total PPF of 325 micromoles s-1 m-2 for 21 days was equivalent to that reported in the literature for plants grown for the same time under cool-white fluorescent and incandescent radiation sources. Characteristics of the plants, such as leaf shape, color, and texture, were not different from those found with plants grown under cool-white fluorescent lamps. Estimations of the electrical energy conversion efficiency of a LED system for plant irradiation suggest that it may be as much as twice that published for fluorescent systems.

Fruit Quality Characteristics of Transgenic Tomato Fruit with Altered Polygalacturonase Activity

Abstract: We have analyzed the quality and composition of transgenic tomato fruit (Lycopersicon esculentum Mill.) cultivar Ailsa Craig modified by the expression of antisense RNA to polygalacturo nase (PG). Tomato plants with <1% of normal PG activity in the fruit were grown under commercial glasshouse conditions and shown to have reduced levels of PG activity throughout ripening compared with untransformed lines. The firmness of fruit, as measured by vertical compression throughout ripening, was not altered in the transgenic samples when compared with controls. However, storage life and ability to withstand transport without damage were improved in the transgenic tomatoes. Tomato juice made from the PG antisense fruit by a cold-break method had significantly higher consistency (distance flowed per unit time) than juice from control fruit.

Bound versus Free Water in Dormant Apple Buds—A Theory for Endodormancy

Abstract: thidiazuron Abstract. Intact apple (Malus domestica Borkh.) buds were examined by magnetic resonance imaging (MRI). MRI did not excite water in unchilled apple buds and could not image it. When chilling was satisfied, images were produced. We interpret this difference to mean that water is in bound and/or structured form in dormant apple leaf buds before the chilling requirement is satisfied. Conversion of bound to free water occurred equally in the low-chilling-requirement cultivar Anna and the high-chilling- requirement cultivar Northern Spy only after 600 and 4000 hours of chilling, respec- tively. It appears that processes involved in satisfying chilling requirement are also converting water in buds from bound to free form. Absence of free water in dormant buds during the winter signifies endodormancy, whereas when the water is in free form, buds are ecodormant. Thidiazuron, a dormancy-break ing agent, applied to par- tially chilled buds is instrumental in converting water to the free form within 24 hours. Summer-dormant buds contain free water, and they could be classified only as para- dormant. Based on proton profiles, ecodormant and paradormant buds cannot be distinguished but endodormant buds can be readily identified.

Evaluation of Strawberry Cultivars for Ellagic Acid Content

Abstract: Ellagic acid tissue extracts of green and red-ripe strawberries (Fragaria × ananassa Duch.) was detected and quantified by HPLC. Ellagic acid content of green fruit pulp ranges from 1.32 to 8.43 mg.g −1 of tissue dry weight (mean 3.36 mg.g −1 ) and in achenes of green fruit from 1.32 to 20.73 mg.g −1 (mean 7.24). Ellagic acid content of red fruit pulp at one location for 35 cultivars and selections ranged from 0.43 to 4.64 mg.g −1 of dry weight (mean 1.55) and from 0.43 to 3.47 mg.g −1 (mean 1.45) for 15 clones at another location (...)

Electrolyte Leakage, Respiration, and Ethylene Production as Indices of Chilling Injury in Grapefruit

Abstract: Storage of «Marsh» white seedless grapefruit (Citrus paradisi Macf.) for 2 weeks at 5C resulted in the development of chilling injury (CI). Electrolyte leakage from chilled fruit did not increase significantly until CI had become severe, and was therefore considered to be of limited value as an early indicator of CI. In contrast to electrolyte leakage, respiration and ethylene evolution were significantly higher in chilled than in nonchilled fruit, even before the onset of visual symptoms of CI

Germination and seedling emergence of primed tomato and asparagus seeds under adverse conditions

Abstract: Seeds of 'Ace 55' tomato (Lycopersicon esculentum Mill.) and 'Mary Wash- ington' asparagus (Asparagus officinalis L.) were primed in -0.8 MPa (20C, 1 week, dark) polyethylene glycol 8000 (PEG), synthetic seawater (INO), or NaNO 3. Primed seeds of both species had a higher percentage of germination than untreated seeds only at 10C in nonsaline ( - 0.05 MPa) medium, while in saline medium ( - 0.6 MPa) priming increased the percentage of germination of tomato seeds at 10 and 30C, and of primed asparagus seeds at 10 and 20C. Sodium nitrate was superior to PEG or IN0 for priming tomato seeds since it resulted in fewer days to 50% germination and higher final germination percentage in saline media at all temperatures. IN0 was a satisfactory alternative to PEG or NaNO 3 for priming asparagus seeds since priming agent had little or no effect on germination. Seedling emergence from NaNO 3-primed seeds of both species sown in a seedbed provided saline (- 0.39 MPa) irrigation was faster than from untreated dry-sown seeds. In the saline seedbed, priming increased final emer- gence percentage (FEP) from asparagus seeds, provided they were not subsequently dried, but had no effect on the percentage emergence of tomato seeds. Fluid-drilling primed or germinated seeds of either species enhanced seedling establishment in the saline seedbed by reducing time to 50% emergence and/or increasing FEP relative to primed, dried-b&k or untreated seeds. Priming involves exposing seeds to an ex- ternal water potential low enough to restrict germination and yet permit pregerminative physiological and biochemical activities (Bradford, 1986). Commonly used priming agents include inorganic salts such as CaCl 2,

Movement of Boron Out of Tree Fruit Leaves

Abstract: Foliar B sprays (500 mg·liter -1 ) had increased the B content of apple (Malus domestics Borkh), pear (Pyrus communis L.), plum (Prunus domestics L.), and cherry (Prunus cerasus L.) leaves 90%) to 185% 3 days after treatment. Boron levels in treated apple, pear, and plum leaves decreased to levels similar to nontreated leaves by 9 days after application, whereas cherry leaves required 33 days to approach levels in non- treated leaves. Movement of applied B was also studied by treating cherry leaves with B solutions enriched in the stable isotope, 10 B. Isotope analysis indicated that applied B moved out of leaves and into subtending tissues. The highest concentrations of applied B were found in buds, followed by bark and wood.

Nitrogen, Phosphorus, and Potassium Fertility Regimes Affect Tomato Transplant Growth

Abstract: Tomato (Lycopersicon esculentum Mill.) seedlings were nutritionally condidioned with solutions factorial combinations of N at 25, 75, and 225 mg.liter -1 , P at 5, 15, and 45 mg.liter -1 , and K at 25, 75, and 225 mg.liter -1 to determine the effect of nutritional regimes on tomato transplant growth and quality. As N increased from 25 to 225 mg.liter -1 , fresh shoot weight, plant height, stem diameter, leaf number, leaf area, shoot and root dry weights, and total chlorophyll increased. Nitrogen accounted for the major source of variation. Phosphorus effects were significant only in 1988; P at 45 mg.liter -1 increased fresh shoot weight, plant height, stem diameter, leaf number, and leaf area in comparison to 5 and 15 mg.liter -1 (...)

The Use of Zeatin to Initiate in Vitro Cultures of Vaccinium Species and Cultivars

Abstract: Explants of mature pot-grown Vaccinium corybosum L. cultivars were tested for initiation of new shoots using two growing conditions and four cytokinin treatments. Initiation tests with 12 genotypes showed significantly higher rates of new shoot growth on modified woody plant (MWPM) medium with 4 mg zeatin/liter at 25C under low light intensity than on any other treatment. Explants at 25C in light with 10 or 15 mg 2iP/liter initiated at a moderate rate, but significantly lower rates were found for all controls and at 4C in darkness. To determine the utility of zeatin for initiation of diverse genotypes, 96 Vaccinium accessions from the National Clonal Germplasm Repository, representing 22 species and 44 cultivars, were screened using 25C and low light intensity. Initiation rates higher than 60% were achieved for 89 of 96 accessions tested. Chemical name used: N6-(2-isopenteny1) adenine (2iP), 6-(4-hy- droxy-3-methylbut-2-eny1amino)purine (zeatin).

Calcium inhibition of polyacrylamide gel hydration is partially reversible by potassium

Abstract: Hydration of a commercial hydrophilic polyacrylamide gel in 20 meq Ca(NO 3 ) 2 /liter was reduced to <10% of the maximum hydration in deionized water. Repeated soaking with deionized water to remove soluble salts restored hydration to ≃30% of maximum. Incorporating KNO 3 at concentrations ranging from 5 to 40 meq.liter -1 with the Ca(NO 3 ) 2 in the hydration solution partially reversed the Ca 2+ inhibition of hydration following repeated soaking (...)

Zinc Deficiency Inhibits Chlorophyll Synthesis and Gas Exchange in `Stuart' Pecan

Abstract: Leaves of 'Stuart' pecan (Carya illinoensis (Wangenh.) C. Koch) with var- ious levels of Zn deficiency were analyzed for physiological indicators of leaf vigor. Leaf chlorophyll content, stomatal conductance, and net photosynthesis were adversely affected by Zn deficiency. In leaves with severe Zn deficiency, each of these indicators increased 3- to 5-fold with a doubling of leaf Zn concentration , but stabilized as leaf Zn approached the sufficiency range (14 µg·g -1 ). High intercellular CO2 associated with low net photosynthesis indicates that stomatal aperture was not the cause of the reduction of net photosynthesis under Zn deficiency.

Resistance to Races 0, 1, and 2 of Fusarium Wilt of Watermelon in Citrullus sp. PI-296341 -FR

Abstract: Fusarium wilt of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is caused by Fusarium oxysporum Schlecht. (emend. Snyd. & Hans.) f. sp. niveum (E.F. Sm.). The disease was first described by Smith (1894) from South Carolina and Georgia in the United States. It is now well-established throughout the watermelon-growing regions of the world. Once a field is infested, the Fusarium wilt pathogen may survive for many years and can seriously limit watermelon production in those fields. Long-term crop rotation (5 to 10 years) and leaving the soil fallow for several years helps reduce the pathogen population in the soil, but generally does not eradicate the pathogen. Even in cases where the pathogen is reduced to noneconomic levels, reintroduction via contaminated soil or seed is possible. Over the years, attempts to control Fusarium wilt have focused on several approaches, including soil polarization (Martyn and Hartz, 1986), induced resistance (Biles and Martyn, 1989; Davis, 1967; Shimotsuma et al., 1972), cultural practices (Hopkins and Elmstrom, 1976; Jones et al., 1975; Sun and Huang, 1985), fumigation (Hopkins and Elmstrom, 1979), and grafting (Kuniyasu, 1981). However, genetic resistance has

Sour Cherry Trees Respond to Foliar Boron Applications

Abstract: Seven trials were conducted over 3 years in several Michigan locations to study the response of sour cherry (Prunus cerasus L. cv. Montmorency) to foliar B sprays. Orchards ranged in age (6 to 12 years) and leaf B concentrations (19 to 32 μg B/g dry weight). Treatments consisted of a 500 mg B/liter spray applied to leaves in late September or early October, and an untreated control. Boron sprays increased B concentrations in dormant buds and flowers by 94% and 54%, respectively, but did not consistently change leaf levels

Thinning Pattern and Light Environment in Peach Tree Canopies Influence Fruit Quality

Abstract: The effects of shoot length, fruit density, thinning patterns, and light levels on crop quality of peach (Prunus persica Batsch.) were investigated. On shoots 50 and 75 cm long, fruits were thinned to produce several distribution patterns, with peaches concentrated in the bottom, middle, or distal portion of the shoot, or uniformly spaced along the shoot at two fruit densities (≃10 or 15 cm between fruit). The lower fruit density on the longer shoots resulted in larger fruits (...)

Response of Young Olive Trees to Sodium and Boron Excess in Irrigation Water

Abstract: The influence of sodium and boron excess in the irrigation water on shoot growth and on the distribution of these elements within various leaf types was studied on rooted olive cuttings (Olea europaea L.). 'Lechin de Granada' was more tolerant than 'Manzanillo' to sodium excess, as indicated by greater shoot growth and lower accumulation of sodium, especially in the young leaves. 'Picual' was more tolerant to boron than 'Manzanillo', with less accumulation in adult leaves. The results suggest the avoidance of toxicity by an ionic exclusion mechanism that is more effective in some cultivars than others. Also, the results reveal cultivar differences in the tolerance of olive to sodium and boron excess in the culture medium. The saline tolerance range of crops is very wide, extending from very sensitive species, such as beans, to highly tolerant ones, such as barley and cotton (Maas and Hoffman, 1977). Olive has been described as a tolerant species that may be cultivated in saline soils where other fruit trees cannot grow (El Gaz- zar et al., 1979). However, some symptoms of toxicity and cultivar differences in sus-

Modifying apple tree canopies for improved production efficiency

Abstract: Canopy development in perennial crops has a seasonal and a lifetime developmental pattern. The sum of development over individual seasons results in the final canopy dimensions and form. In apple, there is a natural progression from a single-shoot tree toward the tall (10 m), umbrella-shaped tree (Fig. 1). There are many disadvantages to this large umbrella-shaped tree: 1) it is difficult to spray, prune, and hand-harvest; 2) it has poor distribution of light throughout the canopy; and 3) it has a low early life light interception, leaf area index, and fraction of land covered by canopy, leading to delayed cropping. These disadvantages have resulted in widespread efforts to reduce tree size, increase tree density, accelerate canopy and yield development, and improve canopy form to overcome the limitations of the large tree. In apple, there are several practical approaches to tree size control and canopy modification, including the use of rootstocks, scions, pruning, and tree training. Rootstocks are available that will give a wide range of tree sizes ranging from the 10-m tree described above to fully dwarf trees <2 m high. There is also available a range in scion types, ranging from the nonbranching, columnar types to the spreading, nonspur types. In addition, apples respond well to canopy modification by pruning and training. These four variables make possible a myriad of tree forms, planting arrangements, and tree heights, widths, and geometric forms (spheric, rectangular, conic V, T, and A forms). The practical value of canopy modifications depends on their effect on orchard production efficiency.

A Rapid and Nondestructive Method for Estimating Leaf Area of Onions

Abstract: zLeaf circumference at 25%, 50%, and 75% of the leaf length, respectively. NS,** Not significant or significant at P ≥ 0.01.

Field Performance of Sweet Corn Seed Bio-primed and Coated with Pseudomonas fluorescens AB254

Abstract: In field experiments, bio-priming and coating with Pseudomonas fluorescens AB254 consistently protected sweet corn (Zea mays L.) seeds from preemergence damp- ing-off caused by Pythium ultimum Trow. The bio-priming seed treatment was evalu- ated under various disease pressures and with seeds of three sweet corn genotypes: shrunken-2 supersweet (sh-2), sugary enhancer (se), and sugary (su). While no damping- off occurred in the su sweet corn, bio-priming protected sh-2 and se sweet corn seeds at a level equivalent to that obtained by treatment with the fungicide metalaxyl. Bio- priming increased seedling height of all three sweet corn genotypes at 4 weeks post- planting. Coating of sweet corn seeds with P. fluorescens AB254 provided an equivalent degree of protection from damping-off under all but the most severe conditions.

The Longevity of Seeds

Abstract: INTRODUCTION Ewart (1908) and Harrington (1972) provided summary lists of species with seeds that were short-lived, survived for moderately long periods, or survived for very long periods. Time is only one of several variables of concern, however; seed longevity is influenced markedly by the storage environment. Ambient environments in tropical regions are generally poorer for seed storage than those of temperate regions, and so the maintenance of viability during storage is generally a greater problem in tropical regions. However, there is no need to distinguish among crop species with regard to region of production or origin when considering the physiology of seed survival. The major difference transcends these areas; it is a consequence of different responses to desiccation. Roberts (1973) classified species into two categories with regard to seed storage behavior: orthodox and recalcitrant. Orthodox seeds tolerate considerable desiccation, and storage longevity is greatly increased thereby (Roberts, 1973). Similarly, considerable increase in orthodox seed longevity results from reduction in storage temperature, particularly since the seeds can be stored at subzero temperatures once they have been dried to a sufficiently low moisture content. These aspects of the physiology of orthodox seeds provide seedsmen with considerable scope to enhance storage life by environmental modification. If these responses are quantified, then the benefits to seed longevity can be assessed against the costs of modifying seed storage environments. In contrast, recalcitrant seeds do not survive desiccation (Roberts, 1973; Chin, 1988). In addition, the storage temperature cannot be reduced greatly because very moist seeds are damaged by cooling to subzero (and in some cases higher) temperatures. Thus, the scope for modifying seed storage environment is limited, and so it is difficult to improve the storage life of recalcitrant seeds beyond comparatively short periods (Ring and Roberts, 1979; Roberts et al., 1984). Fortunately, most crops have orthodox seeds (Ring and Roberts, 1979; Roberts, 1973; Roberts et al., 1984), including most

Container Size and Shape Influence Growth and Landscape Performance of Marigold Seedlings

Abstract: Seeds of marigold (Tagetes erecta L. 'Janie') were sown in flats of three cell sizes (inverted pyramids, Todd 080A, 100A, or 175; volume 7, 24, or 44 cm 3 , respectively) or in flats of different root cell configurations [Todd 100A, Grow-Tech (GT) 200, or Growing Systems (GS) 135; shaped as inverted pyramid, cylinder, or cylinder with a bottom lip, respectively]. During 2 consecutive years, plants grown in Todd 080A trats had 60% less leaf area and shoot and root dry weights than plants grown in Todd 175 trays. Plants grown in Todd 100A trays had 30% less leaf area and shoot and root dry weights than plants grown in the larger volume tray (...)

Onion Plant Growth, Bulb Quality, and Water Uptake following Ammonium and Nitrate Nutrition

Abstract: -1 ) affected onion (AIlium cepa L.) plant growth when grown in solution culture. Nitrate alone or in combination with NH4-N increased leaf fresh and dry weight, leaf area, root fresh and dry weight, and bulb dry weight when compared to growth with NH4-N as the sole N source. Bulb fresh weight was highest with an NH4-N : NO3-N ratio between 1:3 and 3:1. Maximum leaf fresh weight was not necessary to produce maximum bulb fresh weight when onions were subjected to different N-form ratios. Precocious bulbing resulted when NH 4-N was the sole N source; however, high bulbing ratios early in plant development were not correlated with final bulb fresh weight. Nitrogen form also influenced water uptake and pungency, as measured by enzymatically developed pyruvate concentration, but did not affect bulb sugar concentration.

Accelerated germination of pepper seed by priming with salt solutions and water

Abstract: Sweet pepper (Capsicum annuum L. CV. Keystone Resistant Giant #3) seeds were imbibed (primed) in salt solutions to determine a) what concentrations would inhibit radicle emergence and b) the influence this delay in radicle emergence would have on subsequent germination. Seeds were primed for 17 days at 23C in petri dishes with KNO3, KCl, NaCl, K2SO4, Na 2SO4, 1 NaCl: 1 CaCl 2 (mol/mol), Ca(NO3)2, CaCl 2, Na2HPO4, and K2HPO4 in 10, 25, 50, 100, 200, or 300 mM of the salts. Germination was not inhibited in the 10- to 100-m M salt range, although most 200- and all 300-mM solutions reduced radicle emergence to <5.0%. The time to 50% germination (T 50) of these primed seeds in water significantly (P < 0.01) decreased, when compared to unprimed seeds, and a negative correlation (r = - 0.98) was observed between this reduction and the osmotic potential of the solutions. Solutions with the highest osmotic potentials most severely reduced T50 without reducing the final germination percentage. For seeds primed in K2SO4 or Na2SO4 (200 and 300 mM) through 18 days, the reduction in T 50 and duration of priming were negatively correlated (r = - 0.99). Seeds soaked in double distilled water and then dried germinated faster than controls, but not as fast as seeds primed in salt solutions. Priming of pepper seeds in this study was de- pendent on the osmotic potential of the solution, rather than a specific salt, and the duration of treatment.

Water Use and Growth of Honey Locust and Tree-of-Heaven at High Root-zone Temperature

Abstract: Honey locust (Gleditsia triacanthios var. inermis Willd.) and tree-of-heaven [Ailanthus altissima (Mill.) Swingle] sometimes are exposed to high root-zone temperatures in urban microclimates. The objective of this study was to test the hypothesis that seedlings of these species differ in how elevated root-zone temperature affects growth, leaf water relations, and root hydraulic properties. Shoot extension, leaf area, root: shoot ratio, and root and shoot dry weights were less for tree-of-heaven grown with the root zone at 34C than for those with root zones at 24C

Postharvest Response of Two Strawberry Cultivars to Foliar Application of CaCl2

Abstract: The objective of this study was to compare the effects of the foliar application of CaCl 2 on the shelf life and Ca content of the fruit of the strawberry (Fragaria×ananassa) cultivars Kent and Glooscap, which differ in fruit firmness. Calcium was applied repeatedly, 3 days, 3 and 6 days, or 3, 6, and 9 days before harvest at 0, 10, or 20 hg.ha -1 . Calcium treatment influenced amounts of free sugars and organic acids, color, texture, and disease development during storage in air at 4C

Mechanical conditioning for control of growth and quality of vegetable transplants

Abstract: Physical disturbance of plants or plant parts results in a mechanical stress response from the plant. Mechanical stress applied naturally or under controlled conditions inhibits growth in mass and dimensions of major plant parts. Wind and precipitation are important natural sources of mechanical stress. Under field conditions, cultivated plants experience these natural mechanical stresses plus those caused by irrigation and contact with farm machinery or workers during cultivation. Some horticultural practices, such as staking of trees or potted plants, pinching or pruning stems, deadheading flowers, bagging potted plants, or training branches, are mechanical stresses that affect plant growth. Handling practices in plant science research, such as cross-pollination or tagging of flowers (Frizzell et al., 1960) or repeated attachment of leaf chambers for measurement of water status or photosynthetic rates (Marler and Mickelbart, 1992), also can affect plant growth and physiology. Grace (1977) extensively studied effects of wind on the growth and physiology of grasses. Jaffe (1973) termed plant response to tactile (touching, rubbing, brushing) mechanical stress “thigmomorphogenesis,” whereas plant response to a nontactile (shaking, vibration, wind) treatment was termed “seismomorphogenesis” by Mitchell et al. (1975). However, only recently has plant response to mechanical stress been studied to control plant growth in controlled environments, such as greenhouses and growth chambers (Beyl and Mitchell, 1977b; Hammer et al., 1974).

Ethylene and Polyamine Metabolism in Climacteric and Nonclimacteric Carnation Flowers

Abstract: We have compared the ethylene and polyamine metabolism of senescing flowers from two cultivars of carnation (Dianthus caryophyllus L.), one showing climacteric (Arthur') and the other nonclimacteric behavior (Killer'). Arthur' carnations showed the first symptoms of senescence at day 7, coinciding with maximum ethylene and CO 2 production, a peak in the ethylene-forming enzyme (EFE) activity, and a 7-fold increase in free ACC content in respect to the initial value. In Killer' carnations, however, onset of senescence was 15 days after harvest, and no ethylene or CO 2 peak was detected (...)

Drought Avoidance Mechanisms of Selected Bermudagrass Genotypes

Abstract: This study was conducted to determine rooting characteristics, root car- bohydrate content, and performance of 10 bermudagrass (Cynodon dactylon (L.) Pers.) genotypes exposed to drought. A greenhouse study was conducted twice to determine root distribution and carbohydrate content throughout the soil profile during simulated drought stress. Root distribution among genotypes and accumulation of total nonstruc- tural carbohydrate within roots differed with depths. Root mass at 30, 60, 90, and 150 cm was significantly correlated with turf quality during drought stress (r = 0.72, 0.86, 0.80, and 0.81, respectively) only for one of the two tests. Root carbohydrate distri- bution was not significantly correlated with turf quality for the selected bermudagrass genotypes.

Association of the Sweetpotato Whitefly with a Silverleaf Disorder of Squash

Abstract: The sweetpotato whitefly, Bemisia tabaci (Gennadius), was associated with symptoms of a silverleaf disorder of acorn squash (Cucurbita pepo L. cvs. Table King Bush and Table Ace) in cage studies in the greenhouse. Symptoms appeared on un- infested leaves that developed after plants were infested with the whitefly. When the infested lower leaves were removed and the young leaves protected from infestation with insecticides, new growth was asymptomatic or nearly so and symptomatic leaves remained symptomatic. Symptom expression was related more to nymphal density than to adult density since the relationship between log nymph density and symptoms was linear when adult densities were equal.