Thermotolerance of Hemlock (Tsuga spp.) and Investigation of a Foliar Disorder in Crapemyrtle (Lagerstroemia indica ×fauriei)
Type of Degreethesis
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Foliar thermotolerance of six Tsuga species to direct heat injury was evaluated using electrolyte leakage. Species evaluated included: T. canadensis, T. caroliniana, T. chinensis, T. diversifolia, T. heterophylla, and T. dumosa (formerly T. yunnanensis). Species were exposed to 12 temperatures (25-60°C) for 30 minutes and electrolyte leakage from needle tissue was measured. T. canadensis and T. dumosa had similarly, higher predicted temperature midpoints (Tm) and k-values than T. caroliniana suggesting a higher resistance to direct heat injury when exposed to brief, supra-optimal temperatures. While differences were found in Tm and k-values between T. canadensis and T. dumosa when compared to T. caroliniana, no differences were found in Tm and k-values for T. canadensis or T. dumosa when were compared to T. chinensis, T. diversifolia, or T. heterophylla. Similarly, no differences were found for Tm and k-values of T. caroliniana when compared to T. chinensis, T. diversifolia, or T. heterophylla. An online survey was conducted in cooperation with seven nursery and landscape associations in the southeastern U.S. to quantify the impact of a foliar disorder in Lagerstroemia spp. on the ornamental horticulture industry. Industry members were asked 16 questions to obtain demographic, disorder familiarity, and disorder significance information. No significant correlations between gross business income of participants and the ratings for the significance of “rabbit tracks” to the industry were found. Measures requiring a large amount of resources, expense, and costly solutions are not recommended for diagnosis work or treatment, as industry members did not indicate significant losses or customer concern for effects of “rabbit tracks” on plant material quality. A preliminary nutrition study was conducted on five Lagerstroemia indica ×fauriei cultivars using six modified nutrient solutions to evaluate foliar disorder occurrence in relation to treatment. Nutrient solutions were modified to exclude one macronutrient sulfur (S) or four micronutrients; copper (Cu), iron (Fe), manganese (Mn), or zinc (Zn), as prescribed by treatment. No occurrence of “rabbit tracks” for complete nutrient solution-treated plants was seen. There was no significant difference between any of the nutrient solutions for the occurrence of “rabbit tracks” foliar disorder on the five cultivars tested. Additionally, no significant relationship between modified nutrient solutions and percent chlorosis coverage, chlorosis stage, chlorosis progression, spot occurrence, spot pattern, or leaf distortion ratings were found. Chlorosis occurrence ratings due to treatment were found to be significant across all cultivars with iron-deficient nutrient solutions having higher incidence of chlorosis than zinc-deficient and complete nutrient solution treatments.