Cavity spot is a serious disease of carrots that has the potential to severely reduce marketable yield.
A survey of Western Australian carrot crops, showed that cavity spot was present in almost half of 200 crops.
World wide, cavity spot is caused by two species of Pythium, P. violae and P. sulcatum.
These are soil-borne fungi that build up on sites that have been repeatedly cropped to carrots. P. sulcatum was identified as the cause of cavity spot in Western Australia.
Other Pythium diseases of carrots include damping off, resulting in low root numbers at harvest, and root dieback, resulting in forked and misshapen carrots.
This report covers research into the causes of cavity spot and related
diseases in carrot production areas in Australia, together with information on
integrated disease control.
- The cause of cavity spot and other Pythium diseases of carrots in Australia Cavity spot disease of carrots is caused by Pythium spp.
- P. violae and P. sulcatum are the most important causes of cavity spot worldwide. P. sulcatum, but not P. violae, causes this disease in Western Australia.
- A survey of Pythium spp. associated with carrot crops in eastern and southern Australia showed that P. sulcatum was the most widespread pathogenic species, occurring in all States and most regions.
- P. violae was recovered from the River Murray basin in Victoria and South Australia. This is the first record of P. violae from carrots in Australia
- The diversity of P. sulcatum isolates, as shown by DNA analysis, suggests that it is a cosmopolitan species that may occur on native Australian Apiaceae.
Cultural methods for controlling cavity spot
Host range and rotation :
The major hosts of Pythium sulcatum are members of the carrot family (Apiaceae).
Grasses (barley, maize, oats, rye and wheat) used for wind protection, and un-related vegetables, are not infected.
There was a significant reduction in the incidence and severity of seedling infection by P. sulcatum when carrots followed broccoli.
Oospores of P. sulcatum are able to survive for at least 21 months in the absence of a host.
Solarisation is a cultural method for controlling soil-borne diseases where soil is heated by solar energy, usually under black plastic.
In the field it is unlikely that temperatures achieved by solarisation will be high enough to reduce the inoculum potential of P. sulcatum, although these temperatures may be sufficient to reduce the inoculum of P. violae.
Chemical methods for controlling cavity spot :
In a field experiment on a badly infested site, cavity spot control was attempted with a number of commercially available chemical and microbial formulations.
Seedling harvests showed that Pythium infection was only reduced in the metalaxyl treatment.
At the final harvest there was no significant reduction in the incidence or severity of cavity spot in any treatment although Amistar® is worthy of further work.
A survey was carried out to determine whether there was evidence of enhanced breakdown of the fungicide metalaxyl on sites where it has been used in the past.
The half-life varied from less than 1 day to 43 days, compared with a published value of 70 days. Enhanced breakdown of metalaxyl appears to be a widespread problem.
Varietal tolerance to cavity spot
Identification of carrot varieties tolerant to cavity spot, that are also suitable for export production, is an important part of integrated disease control.
Many of the most cavity spot tolerant varieties identified did not produce the high root quality demanded by export markets.
We thank carrot growers for allowing access to their properties, especially Luis Gazzola and George Parente for hosting the farm trials.
We thank Robyn Brett (Agriculture Victoria, Knoxfield), Robin Cole (South Australian Research and Development Institute, Adelaide), Sally-Ann Henderson (Agriculture Victoria, Irymple Research Station), Hoong Pung (Serve-Ag Research, Devonport, Tasmania), Rob O'Brien (Queensland Department of Primary Industry, Indooroopilly) and Len Tesoriero (New South Wales Agriculture, Camden) for providing Pythium cultures from the different carrot growing regions of Australia.
We thank Giles Hardy (Murdoch University, Western Australia) and Geoff White (Horticulture Research International, UK) for providing authenticated cultures of P. sulcatum and P. violae.
We thank J. Speijers for statistical advice.
We thank the following companies for the gift of chemical and biological agents and recommendations for their use:
- Crop Care Australasia Pty Ltd for the gift of Amistar® WG
- Novartis Crop Protection Australasia Pty Limited for the gift of Bion® 50 WG
- E-2001 W.A. for the gift of E-2001®
- EM Distributors WA for the gift of EM microbes®
- Lefroy Valley for the gift of Trichoflow™-T
We thank Paul Murphy, Rob Deyl, Tony Shimmin and Rohan Prince for technical assistance in Western Australia and D. Wells for assisting with assessing one of the trials in Victoria.
Staff at Medina Research Station, especially Gavin D'adhemar, are thanked for managing the experiments and assisting with harvest and grading.
We thank the Chemistry Centre (WA) for carrying out metalaxyl analyses on soil samples.
This project was funded by the Department of Agriculture, Western Australia and Horticulture Australia Ltd through the National Vegetable R&D Levy .
The Australian Government provides matched funding for all HAL’s R&D activities.