Soil solarisation to eradicate boil smut

Figure 2: AsureQuality staff removing plastic solarisation.
Figure 2: AsureQuality staff
removing plastic solarisation.

In early 2006, industry alerted MAF Biosecurity New Zealand (MAFBNZ) to the potential occurrence of boil smut in a crop of corn/maize in the Gisborne region. Boil smut is caused by the fungus Ustilago maydis. It only attacks corn (Zea mays) and another closely related species not known to occur in New Zealand.

Upon confirmation of the identification, MAFBNZ issued a restricted place notice on the property to prevent further spread of the disease, and started planning for an eradication programme.

Eradication was considered to be feasible in this instance for several reasons:

  • the disease was confined to a portion of a single field
  • the maturity of the galls was such that spore release was unlikely to have occurred
  • technology was available that gave a high probability for success.

In addition, the pattern of disease expression in the field indicated a seed-borne, rather than a wind-borne infection. MAFBNZ informed industry stakeholders and asked that crop scouts report the suspected presence of the disease. Thus far, the disease has not been detected in any other corn-growing area of New Zealand. Surveillance is assisted by the fact that only corn is affected by the disease and symptoms are distinctive (Figure 1).

While the boil smut spores can survive up to eight years in cool dry conditions, they are killed within 15 minutes in water at 52°C. This apparent susceptibility led to MAFBNZ using a process known as soil solarisation to attempt eradication. Soil solarisation was considered the most appropriate method of eradication over such an area, as it minimised the chance of spore dispersal through soil disturbance and was considered to be a more environmentally friendly alternative to fumigation.

This process required the removal of all identified infected corn plants from the property for incineration, and the composting of all remaining plants from the affected area and buffer zone in a compost heap specially designed to prevent any spores escaping. Following removal of the crop, MAFBNZ's contractor AsureQuality secured 200-micron plastic sheeting over the affected area and left it in place over the 2006–2007 spring/summer period (Figure 2). While not all temperature data are available, average temperatures of 34°C (maximum 46°C) and 35°C (maximum 38°C) at depths of 10 and 20 cm respectively, were recorded over late spring and early summer 2007–2008 in the field.

MAFBNZ is confident that solarisation provides the best chance to eradicate boil smut and return to New Zealand's pest free status for this disease.

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Page last updated: 1 November 2010