Greenhouse technique to evaluate onion resistance to pink root

An assay for resistance of onion to pink root Pyrenochaeta terrestris was developed, employing infested vermiculite medium with an adequate inoculum concentration of the fungus to minimize escapes of susceptible plants. Plant material other than seed was used. Either root-excised 6-8 week-old seedlings or onion sets were planted in infested vermiculite, and kept for two successive 2-week periods of growth differing in light and temperature regimes. During the first phase a short-day regime of 10 h and 17±1°C was kept, under which a vigorous root system was obtained. For an additional 12-14 days under 26±1°C and 12 h light the prolific roots were then exposed to the optimal temperature for disease development. This technique enables one to distinguish easily between susceptible Allium cepa material and a resistant line of A. fistulosum.