Hirschmanniella miticausa

Only the roots and corms of Colocasia esculenta are attacked by H. miticausa but secondary or referred symptoms are produced on the leaves.The initial foliar symptoms of miti-miti disease are wilting of the older leaves; these leaves become chlorotic, while the new central leaf remains straight instead of bending. Diseased C. esculenta plants die prematurely as a result of corm damage.When cut longitudinally, diseased corms at first show red streaks which radiate from the base of the corm. These streaks later become irregular, 1-10 mm wide, zones of dry brown rot, with the advancing diseased tissues remaining red. The basal portions of severely diseased corms are often completely decayed due to a brown soft rot. The number of cormels is reduced in plants affected by the disease (Mortimer et al., 1981; Bridge et al., 1983).

The disease is, at present, restricted to those areas of the Pacific whereColocasia esculenta is a subsistence crop. This limits the control measuresthat can be recommended, particularly the use of expensive nematicides. The main source of inoculum in new land is planting material infested with H.miticausa. Nematodes can be eliminated from normal planting material (the cormtop and 40 cm of leaf base), without damaging the tissues, by immersing in hotwater at 50°C for 15 minutes (Mortimer et al., 1981). This is not generallyrecommended to C. esculenta growers because of the difficulties in treatment,but it could be used to establish a source of nematode-free planting material.The most practical control measure for small growers is to manually remove allnematodes from planting material. Nematodes rarely occur in the top fewcentimetres of the corm. Trimming the corm back to white, healthy tissueensures that most, if not all, planting material is free of nematodes (Mortimeret al., 1981). Planting corms or cormels, rather than corm tops, increases therisk of spreading nematodes.Where C. esculenta is grown on hillsides, there is a risk of nematodes beingcarried downhill in run-off water. This can be avoided by making new plantingsuphill from old C. esculenta gardens (Mortimer et al., 1981).Such hygiene measures cannot be used in areas where there is intensive andcontinuous production of C. esculenta; for example, in swamp pits in parts ofthe Solomon Islands. The only practical solution in these areas is the use ofresistant cultivars. One resistant C. esculenta cultivar has been identified;this cultivar grows wild and is used only when other foods are scarce. Crossesbetween this resistant cultivar and high yielding cultivars are possible (Patelet al., 1984).For further information, see Jatala and Bridge (1990).

H. miticausa is the causal organism of a corm rot disease of Colocasiaesculenta known as 'miti-miti' in the Solomon Islands. The disease and H.miticausa have been reported from four islands in the Solomon Islands group(Mortimer et al., 1981) and the highlands of Papua New Guinea (Bridge and Page,1984). Miti-miti disease renders the corms of C. esculenta inedible and, whensevere, can destroy almost all consumable corm tissues of the crop. In parts ofthe Solomon Islands, the disease is so devastating that the cultivation of C.esculenta has been almost entirely abandoned, particularly in areas where thecrop was continuously cultivated in swamp pits.