Australian researchers have identified two main species of a soil-borne organism that cause forking and cavity spot of carrots in Australia.
Dr Doris Blaesing and Dr Len Tesoriero report that in most cases, Pythium sulcatum causes the symptoms, with P. violae has also been identified in SA.
While P. sulcatum mostly affects carrots, it also causes severe root rot diseases of parsley and coriander.
P. violae is the main cause of cavity spot of carrots in most other countries and has a much wider host range.
Cavity spot caused by P. sulcatum is most severe in summer and autumn-harvested crops. In wet soils, this species also produces mobile spores (zoospores), which infect plant roots and can lead to multiple infection sites on a carrot.
P. violae does not produce mobile spores; it produces spherical swellings that spread with irrigation water. Cavity spot caused by this species is most severe in winter-harvested crops.
The main factors affecting cavity spot development are soil temperature, soil pH and soil moisture. While temperatures can be controlled by site selection and scheduling planting times, other factors can be controlled by crop management approaches. Details are listed at https://ausveg.com.au/articles/investigating-cavity-spot-and-forking-in-carrots/
Researchers at the South Australian Research and Development Institute are developing soil DNA tests for detecting soil populations of P. sulcatum and P. violae (Improving soilborne disease diagnostic capacity for the Australian vegetable industry; VG15009).
Once these have been developed and tested, the next step is to understand the relationship between soil inoculum levels and production factors, both environmental and cultural practices.
This project was funded by Hort Innovation using the vegetable research and development levy and contributions from the Australian Government.