No abstract

Annually, about 130 ha of field-grown bulbous iris (Iris x hollandica) are produced in western Washington. The production of iris bulbs and cut flowers has an estimated value of $3.4 to 5.6 million. Leaf spot or "fire", caused by Mycosphaerella macrospora, is the most important foliar disease on this crop. Two trials were conducted to compare the efficacy of several standard and newer "reduced risk" fungicides for their ability to control leaf spot on two-year-old plantings of 'Saturnus' and 'Apollo' iris. Applications of Compass 50W (trifloxystrobin), Daconil Ultrex 82.5WP (chlorothalonil), Decree 50WDG (fenhexamid), Heritage 50WG (azoxystrobin), Insignia 20WP (pyraclostrobin), Medallion 50WP (fludioxonil), Phyton-27 (copper sulfate), Switch 62.5WG (cyprodinil + fludioxonil), and 26GT 23.3 (iprodione) were applied to plants at approximately 14 day intervals from 12 February through 21 May, 2003. The plot design was a randomized complete block with three blocks per cultivar. Treatments were applied to 1.5 m of row in each block. Disease pressure was high during late May and early June. On 'Apollo' iris, applications of Insignia, Heritage and 26GT provided excellent disease control and reduced the percentage of foliage that was killed. Insignia and Heritage also provided excellent disease control on 'Saturnus' iris. Applications of Medallion, Daconil Ultrex, and 26GT also reduced the level of disease compared to the unsprayed check. The results of these trials indicate that Insignia and Heritage have the potential to provide control of iris leaf spot that is equal to or better than that achieved by the standard materials used by growers.