Cercospora leaf spot on Impatiens spp. in Argentina

Impatiens balsamina (balsam) and Impatiens x hawkeri (New Guinea impatiens), are cultivated worldwide because of their ornamental value. They are used in bedding, borders and as pot plants. Since 2001, plants growing in gardens and in commercial nurseries of La Plata (Buenos Aires Province) were found heavily affected by amphigenous leaf spots. As the disease progressed, the lesion's appearance varied among hosts. Lesions on balsam plants were brown, with circular (up 10 mm in diameter) to irregular in shape (Fig. 1). In most cases, they coalesced and necrotic tissues dropped out of the leaves. On this cultivar disease occurred during late summer and autumn. New Guinea red leaf cultivars had circular (up to 15 mm) and zonate spots of alternating light brown and dark brown areas with light ochre centers, distributed mainly near the leaf margins (Fig. 2). New Guinea green leaf cultivars had circular (up to 7mm) or irregular grayish spots with defined dark reddish-brown borders, randomly distributed on the leaf (Fig. 2). Autumn to late spring were favorable seasons for disease development in these two cultivars. Heavily infected leaves wilted and died. Leaf spots contained scattered black, undistinguishable stromata with conidiophores that were similar on all hosts. Conidiophores were mostly straight, geniculate and unbranched. Conidia were hyaline, straight to slightly curved, with apices rounded and visible scars on the basal cells. Conidia, indistinctly multiseptate measured 33-115 mm x 3-3.75 mm. The pathogen was determined to belong to the genus Cercospora. The fungus was easily recovered on PDA either by transferring of mature conidia or by routine isolation technique. The pathogen was identified as C. apii sensu lato (incl. C. fuckushana) by Dr U. Braun, Martin-Luther University Institute of Geobotany and Botanical Garden Herbarium, Halle (Saale), Germany. The pathogen was previously reported on I. balsamina and I. noli-tangere (Chupp, 1954; Guo & Jiang, 2000)

Pathogenicity tests were conducted in the greenhouse by spraying 3-months-old plants of the three different hosts with an aqueous suspension of conidia (1 x 10⁴ per ml). Control plants were treated with distilled water. They were incubated in a moist chamber for 48 h and then maintained at 17 to 22°C. After 40 days, leaf symptoms developed into lesions similar to those produced on naturally infected leaves and the pathogen was again isolated. No symptoms appeared on control plants.

To our knowledge, this is the first report of C. apii s. lat. (=C. fukushiana) on I. balsamina in Argentina and the first record of this fungus on Impatiens x hawkeri in the world.