Whole-shoot water potential, osmotic potential of the xylem fluid, and bark water potential were examined from late winter through early spring for six peach [Prunus persica (L.) Batsch.] cultivars varying in relative susceptibility to Leucostoma canker. There were significant differences among cultivars for whole-shoot water potential on all 11 dates tested in 1986, but not in 1985. The date effect was not consistent among cultivars, although when averaged across dates, the whole-shoot water potential of `Loring' was significantly more negative than that of `Candor' or `Garnet Beauty'. There were significant differences among cultivars for xylem fluid osmotic potential on one of five dates tested in 1985 and three of 11 dates tested in 1986, although cultivar differences were not consistent between years. Cultivars exhibited differences in bark water potential on three of five dates tested in 1985, with `Loring' exhibiting the least negative values when averaged across dates. There were only occasional significant correlations of the water status characteristics with relative susceptibility to Leucostoma canker or suberin accumulation. Measurements of plant water status among cultivars or genotypes in peach do not appear to be reliable indicators of susceptibility to Leucostoma spp. or wound response.
Three apple (Malus domestics Borkh.) cultivars varying in susceptibility to Erwinia amylovora (Burr.) Winslow et al., the causal agent of tire blight, were inoculated at biweekly intervals during the growing season. Data were collected on percent infection, canker length, and canker margin quality (a reflection of the overwintering status of the infection). There was a significant cultivar × noculation date interaction, indicating that cultivars that are more susceptible to the pathogen are more likely to develop cankers with indeterminate margins. For `Cortland' and `Jonathan', but not `Delicious', there was a significant linear trend toward forming indeterminate cankers as inoculations were made later in the season. Cankers initiated earlier in the season were more likely to be determinate, which suggests that later-season infections on susceptible cultivars carry over inoculum to the following season. No specific switch-over period from determinate- to indeterminate-type cankers could be identified, and canker margin qualities changed gradually during the growing season. As expected, `Delicious' appeared resistant to tire blight in this study, based on percent infection, canker length, and canker margin type, whereas `Cortland' and `Jonathan' appeared moderately and highly susceptible, respectively. `Delicious' was more likely to form cankers with determinate margins, which suggests that cankers formed on this cultivar are less likely to produce inoculum in the spring than the other two cultivars.
The phellogen of cultivated apple, sweet cherry, and peach trees was wounded at regular intervals beginning in early May and ending in late Aug. 1983. Bark tissue supporting the wounds was excised, sectioned, treated with phloroglucinol + HCl, and examined under a bright field to determine the extent of lignification. The same sections were examined under ultraviolet epi-illumination to determine the extent of suberin deposition in the boundary zone tissue formed from cells present at the time of wounding. Mean daily temperature, time post-wounding, and accumulated degree days (base = −5°, 0°, 5°, and 10°C) were used to predict the percentage of wounds lignified and suberized. A segmented quadratic equation incorporating accumulated degree days (base = 0°) was the best model for predicting lignification for the 3 species and for suberin deposition in peach and sweet cherry.
A.R. Biggs and R. Scorza
Suberin accumulation in mechanically wounded bark tissue was determined fluorimetrically in greenhouse-grown peach [Prunus persica (L.) Batsch] and F2 progeny from peach × almond [P. amygdalus (Mill.) DA. Webb] hybrids. In general, suberin accumulation following wounding was significantly greater for progeny from almond-type than for peach-type hybrids. Hybrids from parents with almond tree type combined with peach fruit type accumulated the highest suberin levels. These data may partially explain the differences observed among peach and peach × almond hybrids in relative susceptibility to Leucostoma canker [Leucostoma persoonii Hohn. and L. cincta (Fr.) Hohn.] and injury caused by lesser peachtree borer. The association of higher suberin accumulation with specific phenotypic characteristics could simplify the selection of desirable seedlings in a breeding program that includes canker resistance as an objective.
A. R. Biggs and D. D. Davis
Seedlings of 3 birch species were exposed to either 0.3, 0.6, 0.9, or 1.2 ppm SO2 for 1, 2, 3, or 4 hours. Stomatal conductance rate measurements of 10 plants were taken prior to and immediately following each exposure. The percentage of leaf tissue injured by SO2 was estimated 72 hours after exposure. Stomatal conductance rates of European white birch (Betula pendula Roth.) and yellow birch (B. lutea Michx. f.) increased after exposure to 0.3 ppm SO2 for 1 and 2 hours, and decreased in response to all other doses of SO2. Stomatal conductance rates of gray birch (B. populifolia Marsh.) increased only after exposure to 0.6 ppm SO2 for 1 and 3 hours and decreased in response to all other dosages. European white birch was slightly more susceptible to SO2than gray birch, whereas yellow birch was tolerant.
T. Auxt Baugher, H.W. Hogmire, A.R. Biggs, S.I. Walter, D.W. Leach, T. Winfield, and G.W. Lightner
Apple packout audits were conducted during 1991 to 1993 to assess effects of five orchard systems (three cultivars, two age groups) on fruit packout and determine if relationships exist between light quality and productivity. Cultivar/rootstock combinations on 1979 T-trellis and central-leader systems had the lowest light levels and relative yields. Trees on either 1979 3-wire trellis, 1986 MIA, or 1985 West Virginia spindle had the highest light transmission, and trees on 1979 or 1985 West Virginia spindle systems had the highest yields. Extra fancy/fancy packouts across systems ranged from 40% to 85%. `Empire', regardless of system, had the highest packouts, and `Golden Delicious' on 1979 or 1986 central leader had the lowest packouts. A regression analysis comparing percentage packout in grades below fancy to percentage full sun indicated that reduced packouts were related to low light conditions. Orchard system influenced the number of fruit downgraded due to color, russet, bruises, bitter pit, cork spot, apple scab, rots, sooty blotch/fly speck, and tufted apple budmoth. Regression analyses comparing defects to field data indicated that bitter pit decreased as yield efficiency increased, and rot and sooty blotch/fly speck incidence were related to low canopy light penetration. Revenue losses were disproportionate to percentage of downgraded fruit because some defects had a greater impact on grade than others. The greatest revenue losses were for russet in `Golden Delicious' on 1986 central leader ($1656.60/acre) and for bitter pit in `Golden Delicious' on 1979 T-trellis ($1067.30/acre). Total losses in returns for individual systems ranged from $453.71/acre for `Empire' on 3-wire trellis to $3145.49/acre for `Golden Delicious' on 1986 central leader. The comparisons of young versus mature system yields and packouts indicate that medium- to high-density vertical or inclined canopy systems are superior to horizontal or low-density vertical freestanding systems. The cost-benefit analyses prescribe areas where management can be changed in existing systems to increase profitability.