Occurrence of Stem Rot on Canola Caused by Sclerotinia sclerotiorum in Argentina

Abstract

Canola (Brassica napus) was introduced as an alternative crop for wheat in Argentina. During 2003, typical symptoms of stem rot disease were observed on canola plants in two commercial fields located at Bragado, in northern Buenos Aires Province in Argentina. Average disease incidence across four canola cultivars was 21% (range = 13 to 29%). Symptoms included chlorosis and wilting of foliage and necrosis of basal stems. The disease appeared singly or in patches consisting of 4- to 5-month-old plants. The first visible symptom noticed was chlorosis and wilting of the foliage beginning from the basal leaves. Infection of the main stem at ground level typically was followed by a grayish white discoloration that progressed above the soil line to the apex. In advanced stages of the disease, stems and branches became bleached and eventually died. Black and irregularly shaped sclerotia (average size 5.5 × 2.8 mm) inside necrotic stem tissue were the typical signs of the pathogen. From September to October 2003, four samples consisting of six affected plants per sample were arbitrarily collected from two commercial fields located at Bragado. Sclerotia were taken from diseased stems, dipped in 70% ethanol, surface sterilized with 1% sodium hypochlorite for 1 min, and rinsed in sterile water. Each sclerotium was blotted dry on sterile Whatman's filter paper and placed on potato dextrose agar. Plates were incubated in the dark at 25°C for 2 to 3 days, followed by incubation under 12-h NUV light/12-h dark for 6 to 8 days. Six resulting colonies were identified as Sclerotinia sclerotiorum (Lib.) de Bary on the basis of taxonomic characteristics of the plant pathogenic species of Sclerotinia (3). Koch's postulates for three fungal isolates from infected plants were carried out on 6-week-old canola plants (cvs. Eclipse, Impulse, Master, and Mistral) by placing a colonized agar disk into wounds made in the basal stem region with a sterile scalpel. Pathogenicity tests, which included five inoculated and three control plants potted in a sterilized soil mix (soil/sand, 3:1), were conducted in a greenhouse at 23 to 26°C and 75% relative humidity with no supplemental light. Characteristic symptoms identical to the original observations developed within 12 days after inoculation on 100% of the inoculated plants for three isolates. Symptoms included wilted foliage, collapsed plants, and plant death. White mycelium and sclerotia developed on infected tissues, and the pathogen was successfully reisolated from symptomatic plants in all instances. Control plants, which were treated similarly except that the agar disk did not contain fungal growth, remained healthy. The experiment was repeated, and the results were identical to the first inoculations. Canola stem rot disease incited by S. sclerotiorum was first reported in Argentina during 1995 at experimental field plots in Buenos Aires. S. sclerotiorum, which has been reported to cause disease in canola in Canada (2) and the United States (1,4), currently represents a serious problem to the main canola cultivars grown in Argentina. To our knowledge, this is the first report of the occurrence of S. sclerotiorum causing a high incidence of stem rot in commercial crops of canola in Argentina.