KUMARA¯ DISEASES
• Soft rot is a fungal disease found in the propagating beds and the field. A severe infection can reduce the production of cuttings or even kill the plant. In the field, the disease causes stunted vine growth and if the plant crown is infected, it will kill the plant. Kumara¯ can also be affected by soft rot if their harvest is delayed or crops are exposed to rain and the soil remains wet. This is a key reason kumara¯ are best harvested from dry soil before autumn sets in. Common symptoms can be seen primarily on the tubers at harvest time. Rotting is also frequent after the tubers are stored, particularly if they are kept at a low temperature in the early part of storage (below 10-12°C) when the roots still have unhealed wounds from harvest or handling. As the fungus enters the flesh, it penetrates quickly and causes a rapid soft and wet decay of the whole root.
Control it through harvesting at the right maturity level, careful grading to ensure no damaged tubers go into storage, and good storage conditions. Fungicides applied at the right time can also protect plant and tubers.
• Scurf, or parapara, is a skin fungal disease, especially apparent on lightcoloured kumara¯ and affects all stages: tipu production, in-field production and post-harvest. The disease appears as a brown-black discolouration on the skin of the kumara¯ and can be easily rubbed off. This mostly cosmetic disease has limited early effect on flesh quality of the fresh tuber, but does become more apparent following extended storage and will eventually affect the ability of ku¯mara to produce healthy cuttings. This soil-borne disease is widespread in New Zealand, especially where crops are grown in successive years and is more apparent in wet seasons, on heavy soils with high organic matter levels.
Control is relatively easy: Cuttings should be free of any sign of the disease; or dip kumara¯ prior to tipu production in an appropriate fungicide (seek professional advice). Take cuttings from the tipu which are cut proud rather than pulled from the tubers. Practise crop rotation.