Blackleg of geranium

Geranium plants with black stems and leaf petioles, wilt, and overall plant collapse were observed. This Alert describes blackleg of geranium caused by Pythium sp. To diagnose blackleg, submit plant samples to your preferred diagnostic lab.

An eight-week-old zonal geranium (Pelargonium × hortorum) crop was inspected because greenhouse-grown plants had started to wilt during the day, recover at night, and wilt again the following day. Most plants exhibited lower leaf chlorosis (yellowing) and root tip browning. The outer root tissue (cortex) easily pulled off exposing vascular tissue, typical of Pythium root rot disease. Upon closer inspection, a few plants exhibited blackening of the crown, lower stem, upper stem, and leaf petioles. Few plants exhibited stem rot at the substrate surface.

Overall plant collapse was also observed. These stem and crown symptoms are characteristic of the blackleg disease of geranium, which is caused by several species of the Pythium pathogen. According to the grower, initial wilting symptoms were observed during week six of the geranium crop cycle. By week eight, significant wilting was observed along with discoloration and plant collapse.

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