While searching I stumbled on some references to this relatively "new" fungal disease, that was first discovered on potted hostas. Some investigating was done into the circumstances that stimulated the development of Fusarium hostae.

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From the link:

When plants were divided in spring, they became stunted with uneven shoot emergence and the leaves yellow and withered. Vascular discolouration was observed in the roots close to the crown, and root rotting away from the crown, with dead areas found in the crown. This is a new disease caused by a new species of Fusarium, F. hostae (reported in Mycologia in 2001) which is similar to the more common Fusarium oxysporum. It was also found recently on bulb crops in Holland.

Wounding is necessary for the disease to develop, and disease severity has been correlated to wound severity. Test showed that the fungus infected best when placed directly onto the wound, and was not easily transferred from infected soil. Thus disease most likely resulted from wounding with infected tools during dividing. F. oxysporum and F. solani also caused similar symptoms, but were less severe.

The disease is worse in pine bark than peat, and worse when dry rather than wet. Severity was worst at 18 – 25°C, least >32°C. Therefore recommendations are to grow in 1:1 pine bark : peat mix, keep well watered, and divide in high temperatures.

Only thiobendazole reduced disease severity as a fungicide dip before planting. Benzimadazoles and strobilurins increased plant weight, but did not reduce disease incidence. Post plant drenches were also investigated, but none were very effective. Currently a combination of dip and drench is being evaluated for control.

References:
B. Wang & S.N.Jeffers(2000).
Fusarium Root and Crown Rot: A Disease of Container-Grown Hostas.
Plant Disease 84:980–988.
Read the Abstract.B. Wang & S.N.Jeffers(2002).
Effects of Cultural Practices and Temperature on Fusarium Root and Crown Rot of Container-Grown Hostas.
Plant Disease 86:225-231.
Read the Abstract.