Damping off is a potential problem and general nursery precautions to lessen this threat should be in place; I use appropriate contact anti-fungal sprays preventatively. Studies on other plants indicate that a low concentration of a smoke water spray has a lowering of the Pythium impact. There is a measure of protection afforded by the cotyledon while it is still supplying nutrition to the seedling, but this stops when exhausted. At this stage I feel the seedling goes into a general nutrient deficiency shock that markedly weakens it until an adequate root system has been developed.
Planting into 1 litre pots at radicle emergence has a greater chance of damping off while the 500 ml size less so, but tapered cells of about 70ml with a greater aeration potential to volume came out tops. (Fig 2) Possibly the much smaller surface area of the smallest pot lessens the area of penetration for air-borne pathogens. Perhaps a larger pot does not eliminate excess water effectively (or holds more water as the radical does not use much), resulting in over watering.
My observation is that over-watering is a significant cause of failure at the seedling stage, especially if the surface area is constantly wet after potting up into I litre pots. To avoid this requires setting up a separate environment and watering system. (Fig. 9) A clear plastic rain roof needs to be in place as the warmer temperatures during spring rain is the prime time for pathogens.
After potting up into 1 litre pots, the water should be delivered via a dripper system which delivers the water to below the surface of the potting medium. I use the Netafim system with emitters delivering 2 or 4 litres/hour (6 and 8 litres/hr emitters are also available). via an 8 or 4-way manifold to arrow drippers that are spiked in close to the roots. The timing of the pulse should be about 8 seconds during the night up to first light and about 55 minutes apart. This should deliver somewhere in the region of 10 to 15ml of water per arrow dripper in each pulse from an eight-way combination manifold and is an excellent way of reducing overall water consumption. A specialist timer is needed, available from good irrigation specialists.
Water quantity is then dependent on the frequency of the longer “dry” time, as well as the “water on” time. The idea is to deliver as little water as needed to the roots rather than the surface as it seems that a constantly damp surface provided by an overhead spray causes problems with pathogens. This should be monitored by having a drip tray under the pot, the idea is to have less water penetrating through the medium and then wasted by dripping out. One of the arrow drippers should be in a clear container to monitor the volume of water passing through. The amount required depends on ambient temperatures as well as the species. The different emitters can be utilised to service different sized plants on a long dripper feedline. Fertigation can be delivered with this system but adds complications. I prefer to use 8-1-5 (N-P-K) slow-release granular fertiliser in the medium when potting up the seedlings.