Masters Theses

Date of Award

12-1998

Degree Type

Thesis

Degree Name

Master of Science

Major

Landscape Architecture

Major Professor

Susan L. Hamilton

Committee Members

Mary Lewnes Albrecht, Mark Windham, Hugh Conlon

Abstract

Zinnia elegans Jacq. is prized for its large flowers, wide diversity in flower color, plant form and ease of culture. Many cultivars of Z elegans have been developed by plant breeders since the plant's introduction to culture in Europe around 1796. In the last twenty years, however, zinnias have become more widely known for their disease problems than for their garden attributes. The overall objective of this research was to determine whether cultivar selection and cultural practices during commercial production as well as in the landscape can affect the development and suppression of three pathogens: Altemahazinniae Rape, Erysiphecichoracearum DC ex Merat, and Xanthomonas campestris pv. zinniae (syn. X. nigromaculansl. sp. zinniae Hopkins and Dowson) on the popular bedding plant and cutflower. Zinnia elegans. This research consisted of three studies: 1) field cultivar trials, 2) pregermination seed treatment, and 3) field irrigation Practices.

The objective of the field cultivar trials study was to determine if any of the 57 cultivars of zinnia grown under uniform field conditions showed resistance to foliar pathogens. At week 4, all cultivars showed signs of being infected with Altemaria blight and bacterial leaf and flower spot, but disease severity was low enough to make them acceptable in the landscape. Powdery mildew was not a factor at week 4. At week 10, eleven cultivars were judged acceptable in the landscape with regard to all three pathogens. By week 17, none of the 57 cultivars tested was considered acceptable in the landscape due to disease severity from one or more pathogens.

In the pre-germination seed treatment study, seeds of seventeen cultivars of zinnia were treated with one of eight treatments and planted in 16.5 cm diameter (1180 cc) plastic pots containing soilless growing medium, placed on greenhouse benches, and allowed to grow until plants reached flowering stage. The objective of this study was to determine the effect of seven commercial greenhouse disinfectants, fungicides and biological control agents on seed germination and disease suppression during greenhouse production. It was found that pre-germination had no effect on plant height, number of buds and flowers, long-term survival of plants in the study, or disease incidence or severity for the three diseases studied. Pre-germination seed treatment did have a significant negative effect on germination percentage for some treatments.

In the irrigation field practices study, seventeen cultivars of zinnia were subjected to either sub-irrigation, overhead-sprinkler irrigation or natural rainfall to determine if irrigation method affects disease incidence and severity of foliar disease. It was found that regardless of irrigation method, all cultivars were rendered unacceptable in the landscape by either Alternaria blight and/or bacterial leaf and flower spot by week 6 of the study. While watering regime had little consequence in disease severity, overhead irrigation produced greater disease severity than drip irrigation or the control for Alternaria blight and bacterial leaf and flower spot. For powdery mildew, drip irrigation produced greater disease severity than overhead irrigation or the control.

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