Pythium irregulare can cause root rot of Platysace lanceolata

Abstract

Cultivated Platysace lanceolata cv. Valentine Lace plants in south-east Queensland are affected by root rot that reduces shoot growth and flower production, and leads to plant death. Pythium irregulare, Rhizoctonia solani and Cylindrocarpon lichenicola were isolated from diseased roots using selective media and their pathogenicities tested under glasshouse conditions. Parameters used to assess disease severity included the proportion of the root system visibly rotting, the weight and size of the root system, and the proportional increase in total shoot length. Glasshouse plants inoculated and colonised with R. solani and C. lichenicola did not develop the symptoms of the field disease. Inoculation and colonisation with P. irregulare caused a detectable increase in root rot only when roots were wounded at the time of inoculation. However, root size, root weight and shoot length increases of plants inoculated with P. irregulare were all lower than those of non-inoculated plants, either with or without wounding. The combination of wounding and inoculation with P. irregulare resulted in rotting of ∼11% of the root length.