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Chon C. Lim, Rajeev Arora, and Edwin C. Townsend

Seasonal patterns in freezing tolerance of five Rhododendron cultivars that vary in feezing tolerance were estimated. Electrolyte leakage was used, and raw leakage data were transformed to percent leakage, percent injury, and percent adjusted injury. These data were compared with visual estimates of injury. Percent adjusted injury was highly correlated (0.753) to visual estimates. Two asymmetric sigmoid functions—Richards and Gompertz—were fitted to the seasonal percent adjusted injury data for all cultivars. Two quantitative measures of leaf freezing tolerance—Lt50 and Tmax (temperature at maximum rate of injury)—were estimated from the fitted sigmoidal curves. When compared to the General Linear Model, the Gompertz function had a better fit (lower mean error sum of squares) than Richards function. Correlation analysis of all freezing tolerance estimates made by Gompertz and Richards functions with visual LT50 revealed similar closeness (0.77 to 0.79). However, the Gompertz function and Tmax were selected as the criteria for comparing relative freezing tolerance among cultivars due to the better data fitting of Gompertz function (than Richards) and more descriptive physiological representation of Tmax (than LT50). Based on the Tmax (°C) values at maximum cold acclimation of respective cultivars, we ranked `Autumn Gold' and `Grumpy Yellow' in the relatively tender group, `Vulcan's Flame' in intermediate group, and `Chionoides' and `Roseum Elegans' in the hardy group. These relative rankings are consistent with midwinter bud hardiness values reported by nurseries.

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Mercy A. Olmstead, Robert Wample, Stephanie Greene, and Julie Tarara

Traditionally, vegetative cover has been subjectively assessed by visual assessment. However, visual assessment is thought to overestimate percent vegetative cover. Thus, a repeatable method to objectively quantify percent cover is desirable. In two vineyards near Prosser, Wash., the percentage of ground surface covered by up to 15 different cover crops was assessed both visually and by computer-assisted digital image analysis. Quadrats in the cover crop were photographed digitally and the images analyzed with commercially available software. Areas of green vegetation in each image were identified and measured. Weeds in some images were differentiated from the cover crop by user-defined thresholds. Subjective visual estimates of percent vegetative cover were generally higher than those digitally estimated. Values for the visual estimates ranged from 5% to 70% in 1998 (mean = 52.4%) and 7.5% to 55% in 1999 (mean = 30.7%), compared to digital readings ranging from 0.5% to 24% (mean = 11.1%) and 10.3% to 36.6% cover (mean = 20.1%), respectively. The visual assessments had lower coefficients of variability in 1998 (cv 28.1) than the digital image analysis (cv 52.3), but in 1999, the values for the two techniques were similar (cv 41.2 vs. cv 45.7). Despite initial variations between the two methods, the accuracy of digital image analysis for measuring percentage vegetative cover is superior.

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R.L. Green, R.C. Hartwig, W.E. Richie, R.H. Loeppert, and J.B. Beard

Iron-deficiency (Fe-deficiency) stress, characterized by chlorosis of leaf tissue, is a major limiting factor in turfgrass production on calcareous soils. The objectives of this study were to: 1) evaluate ferrihydrite-amended growth media and the threshold amount of Fe initially added for use in a whole-plant screening procedure for selecting cultivars that are tolerant to Fe-deficiency stress conditions; 2) measure and evaluate whole-plant growth characteristics that could be an index of Fe deficiency stress; and 3) assess the potential of using a synthetically produced Fe oxide, ferrihydrite, as a slow-release Fe fertilizer source. Iron-stress sensitive `Raleigh' St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] and Fe-stress tolerant `Tifway' bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Davy] cultivars were grown under glasshouse conditions in a medium consisting of quartz sand, 5% (m/m) CaCO,, and a ferrihydrite amendment providing Fe in concentrations of 0, 15, 30, 46, or 120 mg·kg-1 media, (equivalent to 2, 3, 4, 5, or 10 mg DTPA-extractable Fe/kg media). There also was a nonlimiting iron control. St. Augustinegrass was first rated for iron chlorosis 83 days after planting (DAP) while bermudagrass was first rated at 294 DAP. Initial Fe levels equivalent to 5 mg DTPA-extractable Fe/kg media showed potential for screening genotypes. Visual estimates of iron chlorosis and chlorophyll contents of leaves were the best indicators of low soil Fe availability. A single ferrihydrite soil amendment at 10 mg DTPA-extractable Fe/kg media was adequate in preventing chlorosis for the duration of the study (174 and 509 days for St. Augustinegrass and bermudagrass, respectively). Chemical name used: Diethylenetriaminepentaacetic acid (DTPA).

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Susan C. Miyasaka, Charles E. McCulloch, and Scot C. Nelson

Taro leaf blight (TLB), caused by the oomycete pathogen Phytophthora colocasiae, is a worldwide disease that threatens the sustainable cultivation of the tropical root crop taro (Colocasia esculenta). To evaluate taro germplasm from Asia, Hawai‘i, and several South Pacific Islands for resistance to TLB, 119 cultivars were planted along the Hamakua Coast of Hawai‘i (mean annual rainfall of 130 inches) in plots containing five or 10 plants that were replicated over time from 1993 through 2005. Fresh and dry weights of corms were measured after about nine months, with rotten portions removed and weighed. When epidemics of TLB occurred (in nine out of 12 years), visual estimates of disease severity on leaves were assessed using a modified Horsfall–Barratt scale. The correlations between mean dry weight yields for each cultivar and mean severity of TLB, and, respectively, between mean yields and mean severity of corm rots were calculated. As severity of TLB or severity of corm rots increased (suggesting increased susceptibility of particular cultivars to TLB or corm rots), mean dry weight yields decreased significantly (r 2 = 0.37 and 0.22, respectively). “Multiple comparisons with the best” (MCB) were conducted on fresh and dry weight yields, severity of TLB, severity of corm rots, percentage dry matter of corm, and consumer acceptance. Five cultivars were found to be “among the best” with: 1) fresh or dry weight yields that did not differ from the highest level; 2) severity ratings for TLB that were significantly lower than the highest level, suggesting TLB resistance; and 3) percentage of corm rots that were lower than the highest level, suggesting disease resistance. These cultivars, four of which originated from Palau, were Dirratengadik, Merii, Ngesuas, Ochelochel, and Sawa Bastora. Two commercial cultivars from Hawai‘i, Bun Long and Maui Lehua, had fresh and dry weight yields that were significantly lower than the maximum and severity of TLB injury that did not differ from the highest level, indicating that conventional breeding of taro to improve TLB resistance could improve yields of commercial taro cultivars, particularly in areas where epidemics of TLB occur.

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Scott R. Kalberer, Rajeev Arora, Norma Leyva-Estrada, and Stephen L. Krebs

Dehardening resistance and rehardening capacity in late winter and spring are important factors contributing to the winter survival of woody perennials. Previously the authors determined the midwinter hardiness, dehardening resistance, and rehardening capacities in deciduous azalea (Rhododendron L.) floral buds in early winter. The purpose of this study was to investigate how these parameters changed as winter progressed and to compare rehardening response at three treatment temperatures. Experiments were also conducted to measure bud water content during dehardening and chilling accumulation of 10 azalea genotypes. Buds of R. arborescens (Pursh) Torr., R. canadense (L.) Torr., R. canescens (Michx.) Sweet, and R. viscosum (L.) Torr. var. montanum Rehd. were acclimated in the field and were dehardened in the laboratory at controlled warm temperatures for various durations. Dehardened buds were rehardened for 24 hours at 2 to 4 °C, 0 °C, or –2 °C. Bud hardiness (LT50) was determined from visual estimates of freeze injury during a controlled freeze–thaw regime. The midwinter bud hardiness in the current study was ≈4 to 8 °C greater than in early winter. R. canadense and R. viscosum var. montanum dehardened to a larger extent in late winter than in the early winter study whereas R. arborescens and R. canescens did not. The rehardening capacities were larger in early than in late winter. Even though rehardening occurred throughout the first 8 days of dehardening (DOD) in early winter in the previous study, in the current study it was only observed after 10 DOD (R. viscosum var. montanum) or 15 DOD (R. arborescens). There was no difference among the rehardening capacities at the three rehardening temperatures for any genotype. Water content decreased throughout dehardening in all four genotypes examined. R. canadense had the lowest chilling requirement (CR) [450 chilling units (CU)], followed by R. atlanticum (Ashe) Rehd., R. austrinum (Small) Rehd., R. canescens, and R. calendulaceum (Michx.) Torr. with intermediate CR [820, 830, 830, and 1000 CU respectively). The CR of R. arborescens, R. prinophyllum (Small) Millais, R. prunifolium (Small) Millais, R. viscosum var. montanum, and R. viscosum var. serrulatum (Small) Millais exceeded 1180 CU. Results of this study indicate that the dehardening kinetics (magnitude and rate) and the rehardening capacity of azalea buds are influenced by the progression of winter and the depth of endodormancy.

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Matthew D. Jeffries, Travis W. Gannon, W. Casey Reynolds, Fred H. Yelverton, and Charles A. Silcox

’s protected least significant difference test at P < 0.05 level and Pearson correlation coefficients ( P = 0.05) were determined to quantify the relationship between bermudagrass cover determined by visual estimate and line-intersect analysis. Results and

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M. Lenny Wells, Eric P. Prostko, and O. Wendell Carter

pecan tree canopy in June 2013. Visual estimates of herbicide injury severity were made in July 2013, Oct. 2013, and July 2014. Oct. 2017 severity estimates were made to assess for further injury symptoms before natural defoliation. Visual estimates of

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A. Douglas Brede

area was fertilized with 15 g·m −2 /year nitrogen. Bentgrass surface area estimates were taken by visual estimates. All plots started the trial at 0% bentgrass. Note that a proportion of bentgrass observed in the control and probably the ‘Southshore

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Peter C. Andersen and Brent V. Brodbeck

experiment, except after an extremely early 19 Nov. 2014 freeze (−5.6 °C) and subsequent freeze events during 2015. On 4 Dec. 2014, cold injury was assessed for each tree as a visual estimate of defoliation from 1 to 10, with 1 corresponding to severe damage

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W. Carroll Johnson III, David B. Langston Jr., Daniel D. MacLean, F. Hunt Sanders Jr., Reid L. Torrance, and Jerry W. Davis

during these trials. Other than the weed control treatments being evaluated, there was no supplemental weed control, including handweeding, at any point during these trials. Visual estimates of weed control and weed counts were determined mid-March of