Biological control of Pythium root rot in soilless potting mixes

The objective of this research was to gain an understanding of the ecological basis for the biological control of Pythium root rot in soilless potting mixes (SPM). A strain of Pythium ultimum var. ultimum isolated from a geranium (Pelargonium hortorum) plant with symptoms of blackleg disease was identified based on its morphological characteristics and the internal transcribed spacer (ITS) region of its ribosomal DNA gene sequence. Several microbial inoculants (MI), including Companion (Bacillus subtilis GB03), Actino-Iron (Streptomyces lydicus WYEC 108), RootShield/PlantShield (Trichoderma harzianum T-22) and SoilGard [Trichoderma (formerly Gliocladium) virens GL-21], persisted in SPM throughout the crop production cycle. In dual cultures in vitro, MI showed strong inhibition of P. ultimum var. ultimum . Some MI also strongly inhibited P. ultimum var. ultimum in SPM, but inhibition was not consistent. In greenhouse trials, MI were not effective in controlling blackleg disease of seed geranium. MI did not benefit geranium plants in the absence of P. ultimum var. ultimum. Actino-Iron (S. lydicus WYEC108) predisposed plants to disease in two trials. When infected with P. ultimum var. ultimum, higher plant mortality was observed in SPM with low air porosity than in other SPM, suggesting that aeration of the root environment is a significant factor in root disease severity in conducive SPM. SYBR® Green real-time PCR assays were developed for the detection and quantification of B. subtilis, S. lydicus, T. harzianum and P. ultimum var. ultimum in SPM in order to overcome the limitations of the quantification of these microbes using microbiological culture media. This study showed that physical properties of SPM might be a critical factor in the success of MI in SPM. Compaction of SPM increased the population densities of P. ultimum var. ultimum and resulted in significantly higher seedling mortality in compacted conducive SPM, while the population densities of MI were not affected by compaction. A better understanding of the ecology of MI and the pathogen as well as their interactions in SPM is an important key to the effective use of MI in plant disease control. ^