June 2011. Rots in corms of taro from New Caledonia, especially in the lower parts. What could they be? There was no wilting of the plants during the 9 months till maturity.

There were several suggestions (obviously isolations are required to confirm the cause):

• they are similar to those made by Maramiellus stenophyllus (see: www.biosecurite.gov.pf/vegetaux/fiche_on.php)
• those caused by Pythium. See photo taken by Wolfgang Gerlach on page 163 of his book Plant Diseases of Western Samoa (Fig. 157). The caption reads “Pythium rot of taro”. It shows symptoms of Pythium on a corm cut lengthwise, not transverse as yours. There is also a note on symptoms, cause, disease development and control.
• Erwinia soft rots. The symptoms are not unsimilar to E. chrysanthemi/E. carotovora in banana
• Pocket rot (photo, left). In Hawaii, a new Phytophthora is the cause of pocket rot. This pathogen generally infects the corm when taro leaf blight levels (P. colocasiae) are high, and there are few leaves. The corm is weakened, the water mould enters close to the collar, forming a small almost invisible white blemish. It continues to grow and feeds on the corm. When the weather improves and more leaves develop, the corm begins to “fight” back and makes a light brown to red barrrier around the rot. Sometimes the rots become white and soft (cheese-like) or just white and soft. Other micro-organisms continue to expand the rot (Pythium, binucleate Rhizoctonias, and other fungi) but these are not the cause.The causal organism can only be isolated during the early stage. To control the disease, growers must use clean planting material and all diseased corms should be removed from the field. This Phytophthora produces oospores and these can survivie in the field.
• Guava seed (photo, right). This looks like Guava seed. The fungus probably enters the vascular system (i.e. from a root rot) and causes the corm to make a thickening around the vascular system. Like Pocket rot, the thickening is composed of many layers of wound periderm. Thus, growers need to keep the root system healthy. In Hawaii, the growers do not walk close to the taro plants to prevent damage to the roots.

Look for publications on line:

• Jeri Ooka and Janice Uchida, CTAHR on taro diseases.
• Uchida, Silva, and Kadooka Improvements in taro culture and reduction in disease levels.
• Taro Mauka to Makai written by CTAHR staff.

However, New Caledonia said that it was different from M. stenophyllis. It’s like guava seed, with soft rots surrounded by a hard border. It was said that symptoms were similar to those of Pocket rot with cheese-like rots, which do not smell.

A moderator said that in Solomon Islands, Pythium splendens was the major post-harvest rot, with P. myriotylum the cause of wilts in the field. But there are likely differences in species composition between countries. In Palau for instance, P. middletonii was common in roots of taro grown in swamps.

It’s about time for a survey throughout the region to see what Pythium fungi are involved in field and post-harvest rots. And other pathogens too. Identification is a lot easier now with the molecular methods that are available.

Interestingly, in 1983, Pseudomonas (Ralstonia) solanacearum from taro corms in Palau. According to CMI it was “probably Biovar 2)”; this was apparently a new host record. Chris Hayward at UQ commented on the find, and said: “P. solacearum can be an important problem under water-logged or poorly drained conditions, e.g., on bananas in the West Indies and ginger locally (Australia). The bacterium is an excellent survivor in water at least under certain conditions.”

[Editor: Note, there are other interesting books and fact sheets at the French Polynesia website. See book on insects (it would be nice to have an English version for other Pacific islands, and other countries):

http://www.biosecurite.gov.pf/vegetaux/entomologie.php

There are also some more fact sheets:

http://www.biosecurite.gov.pf/vegetaux/recherche_on.php?ssrub=all].