Heat-Induced Flower Abnormalities in Vero and the Marble Cultivars of Dendranthema grandiflora. R. H. Lawson, Florist and Nursery Crops Laboratory, Beltsville Agricultural Research Center, United States Department of Agriculture, Beltsville, MD 20705-2350. M. M. Dienelt, Florist and Nursery Crops Laboratory, Beltsville Agricultural Research Center, United States Department of Agriculture, Beltsville, MD 20705-2350. Plant Dis. 76:728-734. Accepted for publication 10 December 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992. DOI: 10.1094/PD-76-0728.

The Marble cultivars of the florists’ chrysanthemum, Dendranthema grandiflora, often develop green bracts and displaced ligulate florets in the center of the inflorescence where normally only tubular florets are produced. These abnormal inflorescences were induced in plants of cultivars Pink Marble, Florida Marble, and the control Vero by exposing them to high (32 C) day temperatures, 6 and 12 days after they had been induced to flower by exposure to a 10-hr photoperiod at a cool (21 C) temperature. Plants remaining at 21 C, or exposed to 27 C day temperatures for 6 or 12 days after the cool-temperature induction, produced normal inflorescences. The normal floral receptacle contains outer ray (ligulate), pistillate florets and central disk (tubular), perfect florets. Abnormal structures produced on the floral receptacle of heat-treated plants included green bracts; displaced ligulate florets, and florets with characteristics of both tubular and ligulate florets. These included tubular florets with elongating corolla lobes and ligulate florets with sterile anthers. Scanning electron microscopy, light microscopy, and transmission electron microscopy revealed that green bracts in disks of abnormal inflorescences were indistinguishable from green bracts (phyllaries) composing inflorescence involucres. Pathogenic agents were not observed in the phloem or other cell types. Marble cultivars appear susceptible to “heat delay,” a condition in which inflorescences revert to, or maintain, a more juvenile state when high temperatures occur during inflorescence development.