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models built from on-site duplicate sampling or ground-truthing ( Catchpole and Wheeler, 1992 ; Hatfield et al., 2008 ; Im and Jensen, 2008 ). Digital images are gathered with various types of equipment. Inexpensive, consumer-based digital cameras

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. Digital image analysis (DIA) is another method used to quantify plant area. The process of DIA requires periodic photographing of plants and then digital analysis of the images to quantify plant cover. Additionally, DIA has been used to quantify plant

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have been obtained with measurements using digital photographic images to determine crop cover and radiation interception in soybean ( Purcell, 2000 ) and lettuce ( Klassen et al., 2003 ), crop cover in turfgrass ( Richardson et al., 2001 ), and canopy

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of the latest developments in digital technology, it is now possible to measure the evolution of vegetation cover through digital photography and to determine the PGC using image interpretation techniques ( Campillo et al., 2008 ; Rodríguez et al

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Digital images are becoming an essential part of computer “slide” presentations, identification of plants and problems from a distant location, and adding visual elements to Web pages. The use of digital video images allows capturing of single frames for individual or sequence photographs as well as “mass” storage of digital images. There are also some uses of short “video clips” to be included in slide or Web presentations. A discussion of digital image quality and demonstration of equipment used will be included in the presentation.

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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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Digital image analysis (DIA) was evaluated for use in assessing size and shape attributes of sweetpotato [Ipomoea batatas (L.) Lam.] storage roots in herbicide studies. Digital image files of U.S. no. 1 storage roots were taken using a digital camera. Image analysis was performed using a publicly available software package. Eight size and shape attributes were measured and subjected to univariate and multivariate procedures. DIA revealed differences for storage root width and roundness attributes. Principal component analysis suggested that storage root length, width, and roundness best described the variability of the storage root sample. The results demonstrate the potential use of DIA in augmenting data from sweetpotato herbicide trials as well as other investigations that require information about storage root size and shape responses.

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For multi-stemmed shrubs, especially those with fine foliage, obtaining measures of leaf area or density of foliage and twigs within the crown may be both difficult and time-consuming. However, this measure may be an indication of the ornamental quality of a species. A method of photographic analysis was developed to perform repeated measures within the crown of woody shrubs. Slides of 5 species of arid land woody shrubs were analyzed by use of a Visual Image Processor system. This digital imaging technique may be applied where comparative measures over time for individual plants is useful. Comparisons were made of slides taken in the fall of 1989 and the spring and fall of 1990. The use Of slides limited handling or removal of any portion of the plants. Initial care in slide production and continuity of photographic techniques permits consistent results between measurement dates. This computerized method al lows comparative analysis of the growth and “fullness” of plant crowns.

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A personal computer-based method was compared with standard visual assessment for quantifying colonization of sweet cherry (Prunus avium L.) leaves by powdery mildew (PM) caused by Podosphaera clandestina (Wallr.:Fr.) Lev. Leaf disks from 14 cultivars were rated for PM severity (percentage of leaf area colonized) by three methods: 1) visual assessment; 2) digital image analysis; and 3) digital image analysis after painting PM colonies on the leaf disk. The third technique, in which PM colonies on each leaf disk were observed using a dissecting microscope and subsequently covered with white enamel paint, provided a standard for comparison of the first two methods. A digital image file for each leaf disk was created using a digital flatbed scanner. Image analysis was performed with a commercially available software package, which did not adequately detect slight differences in color between PM and sweet cherry leaf tissue. Consequently, two replicated experiments revealed a low correlation between PM image analysis and painted PM image analysis (r2 = 0.66 and 0.46, P ≤ 0.0001), whereas visual assessment was highly correlated with painted PM image analysis (r2 = 0.88 and 0.95, P ≤ 0.0001). Rank orders of the 14 cultivars differed significantly (P ≤ 0.05) when PM image analysis and painted PM image analysis were compared; however, rankings by visual assessment were not significantly different (P > 0.05) from those by painted PM image analysis. Thus, standard visual assessment is an accurate method for estimating disease severity in a leaf disk resistance assay for sweet cherry PM.

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Options for acquiring digital images are explored. Photo CDs, scanned images, and video capture are the most common sources of images. Photo CDs produce the highest-quality images, but require more time to get the digitized images due to commercial processing. For Photo CDs, the images are photographed with a 35-mm camera and sent for processing and digitizing. Slide and flat bed scanning is time consuming when working with bulk quantities of images. With live video capture, a video camera is directly connected to a computer and images are digitized in real time. Tape-recorded images can be also be used, but the image quality is less than live video. VWeb server allows rapid dissemination of the materials. This procedure greatly reduces the production time to a finished product, gives flexibility in revising publications and allows a greater variety of materials to be produced.

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