First Report of Charcoal Rot on Canola Caused by Macrophomina phaseolina in Western Australia.

Abstract

In the spring of 2006, canola (Brassica napus L.) plants suffering from wilt were observed in an experimental plot at Merredin, Western Australia. Symptoms on the affected plants were tan-brown, longitudinal streaks along the main stem and on some lateral branches. Lesions on the stem were predominantly unilateral but sometimes covered the entire stem. Some of the lateral branches were completely wilted, and if present, pods were either shriveled or contained small seed. At the base of the stem, the lesions were grayish brown streaks that caused longitudinal splitting of the stem base. Small spherical (55 to 75 μm in diameter) and elongated (75 to 120 μm long) microsclerotia were seen in the pith and vascular region. Roots appeared to be symptomless, but upon removing the epidermis, grayish streaks were also seen on the roots and small sclerotia were observed in the pith and the vascular region of roots. One hundred and four small pieces (1 to 2 cm) of stem and root from 10 symptomatic plants were surface sterilized with 1.25% NaOCl, rinsed twice in sterile distilled water, and plated on potato dextrose agar (PDA) supplemented with 10 ppm of aureomycin. These were incubated under a blacklight at 22°C. Macrophomina phaseolina (Tassi) Goid. was isolated from 80% of the pieces as identified by colony morphology and the size of microsclerotia that ranged between 50 and 190 μm (3). Eight-three isolates were obtained. None of the isolates produced pycnidia on PDA. However, pycnidia (100 to 190 μm) with pycnidiospores (17.5 to 30 × 7.5 to 10 μm) were produced on the affected stems collected from the field. Pathogenicity tests with one of the isolates were conducted on seven 2-week-old canola plants (cv. Stubby). Three uninoculated plants served as the control. Roots of 2-week-old plants were dipped in an aqueous conidial suspension (1 × 104 conidia/ml) of M. phaseolina for an hour while roots of control plants were dipped in sterile water. Inoculated and control plants were repotted in separate pots and transferred to a glasshouse. A week after inoculation, M. phaseolina produced chlorosis of the leaves, and subsequently, complete wilting and death of the inoculated plants. M. phaseolina was successfully reisolated from roots and stems of symptomatic plants. No symptoms developed on the control plants. Pathogenicity was also tested by soaking seeds of cv. Stubby with an aqueous conidial suspension of M. phaseolina for one-half hour and incubating on agar media after drying. Germinating seeds were colonized by the growing mycelium and seedlings were completely killed within a week. Abundant microsclerotia were produced on the dead seedlings. M. phaseolina has been previously reported on canola in the United States (1) and Argentina (2) and more recently has been reported on canola in eastern Australia (4). To our knowledge, this is the first record of occurrence of M. phaseolina on canola in Western Australia and its impact on canola yield needs to be determined.