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 t hrough bulk potting mix to roots of adjoining plants can be an important me ans of spread of damping-off and root rot caused by Pythium ultimum. The us e 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 po tting mix. The density of mycelium was highest within the first 2 cm from t he infected root tissue, decreasing with increasing distances from the root s. Accordingly, the disease on adjacent plants decreased as the distance fr om 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 incr eased by 2 days for each additional centimeter between the rhizoboxes. The presence of Trichoderma harzianum diminished the production of secondary in oculum 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 res ident microflora, and the presence of an antagonist were shown to be import ant factors affecting disease spread by mycelial growth.