Effects of pre-inoculation with a vesicular-arbuscular mycorrhizal fungus on growth of onions transplanted to the field as multi-seeded peat modules

A 1984 field experiment tested the effect of inoculation with a vesicular-arbuscular mycorrhizal fungus on yield of onions (Allium cepa L. cv. Balstora) grown under commercial conditions from seedlings raised in peat modules. Roots in commercial blocking compost (M 64) could not be infected, so a modified peat, containing 50% of sterilized clay soil, was used to produce mycorrhizal seedlings. Treatments to seedlings were: uninoculated in M64 compost (K), uninoculated in modified medium (NM) and inoculated with Glomus mosseae in modified medium (M). There were two blocks of plots, one irrigated, one not. At harvest the yields of marketable (> 20 mm bulb diameter) onions from M seedlings were generally about twice those from NM seedlings. On non-irrigated plots M seedlings yielded 30.3 t ha⁻¹, slightly less than did K seedlings (36.6 t ha⁻¹). On irrigated plots M seedlings yielded 35.3 t ha⁻¹ and K seedlings 34.9t ha⁻¹, but this difference was not significant. Differences in size of bulbs at harvest were small even though rates of vegetative growth differed markedly between treatments during crop development. Variations in final yield arose largely from differences in numbers of onions that failed to bulb (thicknecks). Irrigation increased mean bulb weight in all treatments but also markedly increased the number of thicknecks. Unexpectedly, the increase in thicknecks was much less in inoculated plants. This effect of mycorrhizal infection did not seem to be related to improved phosphorus nutrition.