Zinnia Variety Evaluation for Powdery Mildew and Bacterial Leaf Spot

A performance evaluation of Zinnia (Zinnia elegans) was conducted in the coastal plain (southern) and piedmont (northern) regions of New Jersey to determine disease incidence and severity, and the hypothesis that newer variety selections would display greater disease resistance than ‘Benary's Giant', an industry standard in the cut flower market. Five varieties, ‘Benary's Giant', ‘Oklahoma', ‘Peppermint Stick', ‘Whirligig', and ‘Zowie! Yellow Flame' were included in this study. This field experiment was conducted as a 5x5 Latin square with the five cultivars in a one-way treatment structure, repeated at the Rutgers NJAES Agricultural Research and Extension Center in Bridgeton, NJ (coastal plain) and the Rutgers NJAES Snyder Research and Extension Farm in Pittstown, NJ (piedmont). Zinnias were transplanted into the field on 17-May-2006 in Bridgeton and 26-May-2006 in Pittstown. No fungicides were applied during the growing season. Disease incidence of powdery mildew and bacterial leaf spot were measured using the Horsfall-Barrat rating scale (0-12), where higher numbers indicate higher disease incidence. The MIXED procedure of the SAS system was used to perform analysis of variance for disease incidence data from each evaluation date separately. Pairwise comparison of means was performed using Fisher's LSD (α=0.05). At both locations ‘Whirligig' exhibited high variability including both the highest and lowest bacterial leaf spot ratings with the following means: 3.7 (RAREC 19-Jun), 4.4 (RAREC 27-Jun), 5.3 (RAREC 6-Jul), 5.9 (Snyder 19-Jul) and 7.2 (RAREC 25-Jul) . ‘Zowie! Yellow Flame' consistently exhibited the most severe incidence of bacterial leaf spot with its season low (3.8) on 27-Jun at Snyder and its highest (7.3) at RAREC on 6-Jul. At the Snyder farm location on 27-June and 1-Aug no ratings were significantly different for bacterial leaf spot. Bacterial leaf spot was more prevalent at the start of the season and powdery mildew severity increased dramatically later in the season. ‘Zowie! Yellow Flame' consistently exhibited the lowest severity of powdery mildew throughout the season, at both locations: 1.9 (RAREC 1-Aug), 1.4 (Snyder 1-Aug) and 7.9 (Snyder 1-Sep). Excepting RAREC on 31-Aug when no ratings were significantly different for powdery mildew. ‘Oklahoma' and ‘Peppermint Stick' exhibited significantly high scores throughout the season at the two locations.