Pot experiments with pasture soils to detect soilborne pathogens of white clover and lucerne, and effects of field application of fungicides

Abstract

Seedlings grown under glasshouse conditions in pots of pasture soil from sites in the North Island of New Zealand were used to detect pathogenic soilborne fungi and nematodes. White clover (Trifolium repens L.) seedlings grown in soil sampled from pastures of low productivity on three North Auckland farms showed damping-off caused by Thanatephorus cucumeris (Frank) Donk, and root rot caused by Chalara elegans Nag Raj & Kendrick. These pathogens were also detected in soil samples from pasture trial plots sited in the Waikato, Taupo, and Bay of Plenty regions. Lucerne (Medicago sativa L.) seedlings grown in soil from these plots were affected by damping-off caused by Pythium irregulare Buis. Less damping-off occurred in soil from plots treated with the fungicides metalaxyl and triadimefon than in soil from untreated plots. Seed treatment with metalaxyl or pyroxyfur also reduced damping-off. Cultivars of lucerne and diploid red clover (T. pratense L.) were susceptible to more isolates of P. irregulare than were those of white clover, tetraploid red clover, ryegrass (Lolium perenne L.), or tall fescue (Festuca arundinacea Schreb.). Seedling roots of white clover in all soil samples became infested with clover cyst nematode (Heterodera trifolii Goffart) and often with root knot nematode (Meloidogyne spp.). The lengths of primary roots and the number of lateral roots and rhizobium nodules on seedlings decreased with increasing numbers of H. trifolii females infesting roots.

Keywords:

• white clover
• Trifolium repens
• lucerne
• Medicaga sativa
• root rot
• fungicides, nematodes
• Heterodera trifalii
• plant pathology
• seedling baiting