July 2007. From Cook Islands, pictures of a root rot of Xanthosoma sagittifolium, with the following notes:

The problem occurs only on certain types of heavy loamy soil.

• It does not occur in sandy and in some loam soils.
• Problem occurs in patches in the field.
• Die-back normally starts at 6 months after planting.
• In 2004, a plant pathologist from CABI looked into the problem, but found nothing in terms of fungi or bacteria.
• It is thought to be related to soil nutrition or nematodes.

PestNet members said the following about the symptoms:

The patches certainly suggest a soil related problem, but probably not nutritional, judging from the vigour of adjacent plants in the photos. It would be helpful to see some corms and roots washed free of soil. The lower roots seem to have been killed off. Is there any rotting of the cormels? Rots spreading into the cormels and mother plant could indicate Pythium.

Other alternatives would be nematodes or Rhizoctonia. If nematodes, probably one of the burrowing types, so macerate some roots and leave in water to see what comes out. Check any nematodes microscopically to see if they are parasitic species.

It will not be so easy to recognise Rhizoctonia. Look on the ‘healthy’ roots. Early Rhizoctonia symptoms show up as black streaks on otherwise clean roots and eventually take over killing the whole root. Wash out the roots and plant bases, and have a close look at them before deciding what next to do.