Effects of Pythium and Cold Storage on the Survival of Southern Pine Seedlings

Abstract

Cold storing bareroot loblolly pine (Pinus spp.) seedlings for > 1 week after lifting from October to mid-December has been associated with poor outplanting survival compared to when seedlings are lifted and stored in January. In contrast, container-grown seedling survival is not affected when stored for > 1 week during the same period. The practice of lifting bareroot seedlings can wound root systems, which pathogenic fungi, particularly Pythium spp., could use as infection sites. Once seedlings are placed in storage, the cool, moist environment may be conducive for fungal growth which could lead to seedling mortality after outplanting. The objective of this research was to evaluate bareroot and container-grown seedling survival after inoculations with Pythium dimorphum Hendrix and Campbell and Pythium irregulare Buisman and cold storage. Bareroot longleaf pine (Pinus palustris) survival decreased as a result of the Pythium inoculations but bareroot loblolly (Pinus taeda) and slash pine (Pinus elliottii) were not affected by Pythium. Container-grown seedling survival was similar to non-inoculated seedlings even after wounding root systems. When a peat-mix was packed around inoculated bareroot loblolly pine roots and used as a container media, seedling survival did not improve. This suggests that something other than antagonistic fungi in peat may improve container-grown seedling storability. As a fine feeder root pathogen, Pythium can kill the fine feeder roots that are critical for seedling establishment after outplanting, and therefore, the effects of Pythium on seedling root growth potential (RGP) were tested. Pythium reduced bareroot loblolly and slash pine RGP after storage, but only P. irregulare-inoculated slash pine experienced reductions in survival after outplanting. Pythium must be present in nursery soils if bareroot seedlings are infected with Pythium during fall lifting. Bareroot nursery soils were assayed to quantify Pythium populations during the fall and winter seasons. Pythium populations were variable between nurseries with more Pythium being recovered from samples taken in the fall of Year 1 and winter of Year 2. It was determined that both P. dimorphum and P. irregulare act as storage pathogens. If Pythium spp. are actively present in the soil at lifting, infect seedling roots through wounds, and grow on seedling roots in cold storage, reductions in seedling root growth potential and survival are possible.