Incidence of gummosis disease in silky hakea under natural conditions in South Africa

The pathogen Colletotrichum acutatum J.H.Simmonds was developed in South Africa during the 1980s as a mycoherbicide to control invasive silky hakea (Hakea sericea Schrad. & J.C.Wendl.) and was subsequently widely used by a variety of land managers to induce gummosis disease. To determine persistence under natural conditions, disease incidence and severity were monitored annually at five sites in the Western Cape and Eastern Cape provinces, from 2008 until 2017. Disease incidence ranged between 12% and 95% of trees with gummosis in the first year. At four of the sites, high levels of mortality were recorded during the first four years of monitoring. In the year with the highest mortality, between 38 and 64% of trees were dead. No mortality was recorded at the fifth site. Disease incidence and mortality were reduced in the latter years, coinciding with drier conditions. At three of the sites, fires occurred once during the course of monitoring and the disease reappeared one to three years post-fire. It was concluded that although the disease will persist under natural conditions, for maximum benefit the fungus should be actively applied to maintain high levels of incidence and mortality. Depending on site characteristics and weather trends, this should be on an annual, bi- or triennial basis, or following fires or dry years.