Agriculture Western Australia, Bag 4, Bentley Delivery Centre, Western Australia 6983

Background and objectives
Carrots are the most important horticultural export from Western Australia. Cavity spot, a soilborne Pythium disease, reduces the marketability of carrots both locally and overseas. The incidence of cavity spot increases with carrot cropping history, and cavity spot is the most serious disease affecting production [1]. Control measures that are being developed include the use of soil fumigants (metham sodium), fungicides (metalaxyl), soil amendments (lime), tolerant varieties, rotational crops and prompt harvest.

Materials and methods
Badly affected sites on commercial properties are being used for control experiments with metham sodium, metalaxyl and lime. A disease nursery is being used to select tolerant varieties for local conditions, for screening rotational crops, and for following disease development throughout the year.

Results and conclusions
Pythium sulcatum causes cavity spot in Western Australia. There is no evidence that P. violae occurs on carrots in the state, or that P. coloratum is widespread. Two experiments on growers' properties have shown that metham sodium (500 l/ha) applied before seeding in autumn or winter, significantly (P<0.05) increases the proportion of marketable carrots without cavity spot by 10 to 15%. On one property with no history of metalaxyl use, metalaxyl at 1.5, 3 or 6 kg a.i./ha, applied in winter 1 week after seeding, significantly (P<0.001) increased marketable yield by 40%, largely from a decrease in the incidence of cavity spot and root forking. On properties with a history of metalaxyl use, metalaxyl at 1.5 or 3 kg a.i./ha applied in autumn or winter at seedling, has not significantly reduced the incidence of cavity spot. There is no evidence of increased tolerance to metalaxyl of P. sulcatum isolates from these properties. Experiments have shown that the addition of hydrated lime or lime sand at a rate sufficient to raise soil pH to 7.2 significantly (P<0.05) reduced the incidence of cavity spot in three successive crops.

Screening commercial varieties suitable for Western Australian conditions started in 1995. Promising varieties include Nerac, Barwon and Navarre. These are being tested on commercial properties to assess their suitability for export production. Field observations and results from continuous cropping in the disease nursery show that the severity of cavity spot increases with carrot age. A harvest predictor has been developed to help growers schedule their harvesting operations at different times of year. A field experiment has shown that plants in the same botanical family as carrots are hosts of P. sulcatum, but plants from other families are not hosts, so could be used in rotation with carrots.

Preliminary recommendations for growers include using lime to raise soil pH, applying metham sodium before planting, using tolerant varieties, harvesting on time or early, and rotating with crops from families other than Apiaceae. Metalaxyl, however, may be useful on some properties.

1. Galati A, McKay AG, 1996. Horticultural Research and Development Corporation Project NV27.