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Disease Progression by Active Mycelial Growth and Biocontrol of Pythium ultimum var. ultimum Studied Using a Rhizobox System

This study demonstrates that outward growth of mycelium from primary foci through bulk potting mix to roots of adjoining plants can be an important means of spread of damping-off and root rot caused by Pythium ultimum. The use of a rhizobox system, which confines plant roots, enabled us to study the spread of actively growing mycelium between root systems placed at precise distances from each other. In steamed potting mix, hyphae of P. ultimum on average grew 9.6 cm from diseased root tissue compared to 5.3 cm in raw potting mix. The density of mycelium was highest within the first 2 cm from the infected root tissue, decreasing with increasing distances from the roots. Accordingly, the disease on adjacent plants decreased as the distance from infected roots increased. The time required for damping-off of adjacent plants was 3 days slower in raw as compared to steamed potting mix and increased by 2 days for each additional centimeter between the rhizoboxes. The presence of Trichoderma harzianum diminished the production of secondary inoculum and reduced the ability of P. ultimum hyphae to extend through bulk potting mix. In conclusion, the concentration of the primary inoculum, the plant density, the distance separating diseased from healthy roots, the resident microflora, and the presence of an antagonist were shown to be important factors affecting disease spread by mycelial growth.