Occurrence of Stem Rot of Chrysanthemum Caused by Sclerotinia sclerotiorum in Argentina

Chrysanthemum (Dendranthema × grandiflorum (Ramat.) Kitam.) is one of the most popular flowering plants in Argentina. A previously undescribed stem rot disease was observed in cvs. Alba and Palisade in greenhouses near Buenos Aires and La Plata, an area of intensive floriculture production. The stem was killed within 10 to 15 days causing the plant to wilt and die. Necrotic tissues were covered with whitish mycelium that produced black, irregular shaped (3 to 7 mm diameter) sclerotia. The pathogen was isolated from symptomatic stem sections, surface disinfested for 1 min in 2% NaOCl, and plated on potato dextrose agar (PDA) (1, slightly modified). The organism isolated produced white aerial mycelia and large number of sclerotia characteristic of Sclerotinia sclerotiorum (Lib.) de Bary. Symptoms were reproduced in the greenhouse by inoculating stems of 10 3-month-old plants with five mycelial plugs per plant from 7-day-old PDA cultures. Inoculated plants were enclosed in transparent plastic bags for 6 days with near saturation humidity and incubated in a growth chamber at 22 to 24°C with a 12-h photoperiod. Control plants were treated similarly except agar disks did not contain the fungus. After 6 to 9 days, symptoms were similar to those previously observed, and infected plants died 3 weeks after inoculation. No disease symptoms were observed on uninoculated plants. Koch's postulates were satisfied after reisolating the fungus. To our knowledge, this is the first report of the occurrence of white mold caused by S. sclerotiorum on chrysanthemum in Argentina. The disease has been previously observed in Argentina on lisianthus (Eustoma grandiflora (Raf.) Shinn.) in 1988 (2) and on carnation (Dianthus caryophyllus L.) in 1991, among other floriculture crops of economic importance. References: (1) A. Garibaldi et al. Plant Dis. 85:446, 2001. (2) S. Wolcan et al. Plant Dis. 80:223, 1996.