A bedding plant grower in southwest Florida reported severe losses in potted Catharanthus roseus (Madagascar periwinkle) transplanted on various dates in December 1997. Symptoms included yellowing of lower leaves, defoliation, black root rot, and plant death. Thielaviopsis basicola was consistently isolated from blackened roots on a selective medium (1). A disease survey was conducted during mid March 1998 of 7,500 periwinkle plants in the two cultivar series Pacific (cvs. Punch, Red, and White) and Cooler (cvs. Grape, Icy, Peppermint, Pink, and Raspberry). Black root rot symptoms were observed in 21 to 53% of periwinkles transplanted between 3 and 11 December, and in 0 to 19% of plants transplanted between 21 and 29 December. The two cultivar series did not appear to differ in their susceptibility to T. basicola. The fungus was consistently isolated from symptomatic periwinkle roots, from roots of chlorotic violas (Viola cornuta), and from a potting medium used for early December transplanting. Pathogenicity of isolates of T. basicola from periwinkle, viola, and the contaminated medium was investigated. Inocula were produced by growing the fungus on acidified potato dextrose agar for 18 days. Conidia, chlamydospores, and hyphae of T. basicola were suspended in deionized water (100 ml/95 mm culture plate). One milliliter of inoculum suspensions of each of the three isolates was used to inoculate 12 plants of pansy (Viola × Wittrockiana) cv. Majestic Giant Blue Shade. An equal number of plants served as noninoculated controls. In addition, 5 ml of each isolate suspension was used to inoculate three six-plant replications of periwinkle cv. Pink Cooler. Plants were incubated in a controlled environment chamber at day/night temperatures of 24/22°C with a 12-h photoperiod. After 22 days, moderate to severe root discoloration was produced in pansy by the viola isolate of the fungus, while periwinkle and potting medium isolates produced only minimal discoloration. Chlamydospores typical of T. basicola were observed in the roots of all inoculated pansies, and in the roots of one of 12 non-inoculated controls. After 38 days, periwinkle plants were rated for foliar yellowing (0 = no yellowing, to 3 = severe yellowing) and percent defoliation, and fresh weights of shoots and roots were obtained. T. basicola was recovered on the selective medium from roots of all inoculated plants and from 68% of the noninoculated controls, indicating that secondary spread had occurred. All three isolates of the fungus caused yellowing and defoliation, but only the periwinkle and potting medium isolates significantly reduced both shoot and root weights in periwinkle, compared with control plants. T. basicola has not been previously reported to cause black root rot in C. roseus, and a contaminated potting medium used during early December may have provided initial inocula for the outbreak. The unseasonably cool and rainy weather prevalent during winter 1997 and spring 1998 (“El Niño”) may have also promoted the unusual appearance of the disease.
Reference: (1) L. P. Specht and G. J. Griffin. Can. J. Plant Pathol. 7:438, 1985.
