2018 ASHS Annual Conference

Polyploidy – also known as whole genome duplication – has long been associated with changes to ornamental traits in plants. Increasing the number of chromosome sets in plant cells often leads to thicker stems and leaves, a deeper green foliage color, larger and more textured flowers, and increased resistance to diseases and environmental stress. Ploidy level has been shown to influence morphological traits in Hydrangea macrophylla, with triploids having larger inflorescences, thicker stems, and increased stomatal area. The objective of this study was to determine if ploidy has an influence on resistance to powdery mildew, a common foliar disease of Hydrangea. Five diploid and five triploid H. macrophylla genotypes from related full-sibling families were placed in the greenhouse in a randomized complete block design with six replications. Data collection began one week after the first symptoms of natural powdery mildew infection occurred and continued for nine weeks. Powdery mildew disease severity was evaluated using a scale of 0-100% foliage. Area Under the Disease Progress Curves (AUDPC) was calculated using the formula AUDPC = ∑[(Xi+1 + Xi)/2](ti+1–ti). Percent affected leaf tissue was measured using the Leaf Doctor plant image analysis application on the final week of data collection. Triploid plants had lower severity of powdery mildew infection in all weeks of observation. By week nine, AUDPC scores were significantly lower for triploids (7.7 ± 10.3) than diploids (19.5 ± 15.3). Triploids remained uninfected longer than diploids, with symptoms appearing an average of two and four weeks after the study onset, respectively. Triploids had significantly less leaf spot damage (2.5 ± 1.1%) than diploids (6.4 ± 1.2%). There were significant differences in powdery mildew resistance among genotypes within ploidy levels, with two genotypes remaining uninfected for the duration of the study. These data support the hypothesis that ploidy manipulation will be an effective breeding strategy to improve H. macrophylla for disease resistance traits.