Prospects For Containment And Eradication
if Karnal Bunt is found in Australia

Dr Murray is a plant pathologist with 20 years experience in the diseases of small grains. Dr Brennan is an economist with 20 years experience in analyzing the economic contribution of agricultural research.

Introduction

The finding of Karnal bunt (KB) in the USA South West has rung alarm bells in Australia. KB and its causal fungus, Tilletia indica Mitra, have never been recorded in this country. Industry groups, state and federal departments responsible for agriculture, and the Australian Quarantine Inspection Service (AQIS) are reviewing the current quarantine measures to determine if these need changes to reduce the risk of introduction of KB to Australia.

Although Australia produces only about 16 million metric tonnes of wheat each year, most of this is exported so that the country is one of the largest international traders of wheat. Australia has a reputation for supplying high quality grain with very low contamination of weeds, pests and diseases. This reputation would be endangered if KB were introduced, which could result in losses of about 25% of the present value of production in areas where the disease occurred. Most loss would be due to reduced quality and buyer resistance (Murray et al. 1996b). This paper presents four possible scenarios for the distribution of KB when it is first detected in a country, assuming that the disease breaches the current quarantines measures. We then examine the likelihood that the disease could be contained and eradicated under each scenario, drawing on experiences with maize boil smut (MBS).

Containment of Karnal bunt would require a local quarantine of the affected field following destruction of the infected or infested susceptible crop. This quarantine would involve limiting the movement out of the infested area of any material likely to be contaminated with Karnal bunt teliospores. Such material includes contaminated wheat grain, other produce, machinery and soil.

Eradication would require preventing successful new development of the disease from germinating teliospores until there were no viable spores remaining. Spores of T. indica germinate at different rates and some can remain viable for several years. In the USA, cereal crops will not be grown in quarantined fields for at least five years.

Scenarios for KB introductions

Case 1. KB is found in one or a few fields. This situation could arise from a single introduction event that was detected soon after the introduction. It could potentially arise from a farmer importing a secondhand harvester that was contaminated with KB teliospores.

Case 2. KB is found in a large number of scattered fields. This situation could arise from multiple introductions over a short time that were detected soon after the event. Such an introduction could occur from contaminated fertilizer being widely distributed.

Case 3. KB is found generally distributed in a region or district. This could arise from the earlier introduction of the disease to an area, which was not detected for some time. By that stage, Karnal bunt would have spread from the initially infested fields to most fields in the area.

Case 4. KB is found widely distributed throughout Australia. This situation would occur if the disease was not detected for some time after the initial widespread introduction.

Any introduction of KB is likely to involve a relatively small number of spores to the area contaminated. There is very little information available to predict how quickly these spores would multiply until they reached detectable levels. It is possible that during this time teliospores could spread from the initial sites to a wider area. Thus case 1 introductions could become case 3 before detection, while case 2 could be case 4 at detection. The intensity of surveillance would be expected to be inversely proportional to the size of the infestation at first detection.

Australian experience with maize boil smut (MBS)

MBS is caused by Ustilago maydis. The disease has a similar cycle to Karnal bunt, in that the teliospores lie on or in the soil where they germinate to produce sporidia. These sporidia are able to grow by subdivision, and can infect any above ground part of the maize plant where they produce large sori. Like T. indica, teliospores of U. maydis can survive in soil for several years. They can spread on contaminated seed and plant parts, and in contaminated machinery and equipment. MBS differs from KB in prominence: sori of MBS are much more likely to be seen, and so the disease has a greater likelihood of being detected earlier than has KB.

MBS first occurred in Australia at Bathurst, New South Wales (NSW), in 1911. The affected crop was destroyed and no further maize was grown in the field until 1919, when the disease again occurred. No maize was then grown on the farm until 1929. Small areas of maize were grown from 1929 to 1934, with no MBS being observed. However, the disease occurred in 1935 and the affected crop was destroyed. The district was surveyed in 1936 and 1937 with no MBS being detected. MBS was then found on several district farms in 1938 and again in 1939. All affected crops were destroyed and the farms placed under quarantine to prevent sowing of maize for 25 years. These measures effectively contained MBS and the disease did not occur elsewhere in Australia until 1982 (unpublished report by F. Butler, ca. 1949, held in the Herbarium, NSW Agriculture, Rydalmere; M. Priest, personal communication).

In 1982, MBS was found again, this time on the North Coast of NSW. Surveys showed that the disease was generally established in the district. Despite attempts at containment, the disease has spread slowly to other districts. By 1987, it had crossed the Great Dividing Range to the inland. MBS spread slowly south along the coastal plains to reach the west of Sydney in 1992. Inland, it had spread as far south as Forbes in early 1996 (M. Priest, personal communication). The important irrigation areas in southern NSW so far remain free of MBS.

The first outbreak at Bathurst is a case 1 scenario. MBS was successfully contained but not eradicated. The second outbreak is a case 3 scenario, where despite some attempts at containment the disease continues to spread slowly. Surveillance has been important to maintain area freedom status for maize growing areas beyond the infested region.

Lessons for KB

Case 1. The experience with MBS suggests that a small, isolated outbreak of KB could be effectively contained. The infested area would need to be effectively quarantined with prohibition of wheat growing for several years, perhaps indefinitely. Movement of machinery, soil and other materials from the infested area would need to be controlled. For success, the affected farmers will have to cooperate fully. They will need to be adequately compensated for any losses to secure such cooperation.

Case 2. Containment would be a similar but larger and more expensive operation than case 1. Containment remains feasible, but the greater number of infested sites increases the probability that it would not be successful.

Case 3. If Karnal bunt becomes generally established in an area, prospects for containment to individual farms are small. Other management would need to be used in the infested area. Depending on the location of the area, quarantine actions around the area may prevent or delay spread. This will enable other wheat growing areas to be certified as free of the disease, which will aid marketing of grain from them.

Case 4. Clearly, containment and eradication would not be options. KB would need to be managed by cultural methods and the use of resistant varieties.

The prospects for containment and eradication are summarized in Table 1.

Table 1. Prospects for Containment and Eradication Case Containment Eradicatio n 1 One or a few locations Excellent Moderate 2 Several locations in country Moderate Poor 3 General in a district Poor Unlikely 4 General in country Impossible Impossible

Conclusions

Successful containment and eradication of Karnal bunt require early detection, before the disease has spread widely in a district or throughout the country. Early detection, in turn, requires an intense surveillance program, with only a moderate guarantee of success. However, even broad scale surveillance, such as that currently underway in the USA, can be very costly. We need to examine whether the expense of such surveillance is warranted.

Alerting farmers and grain handlers to the symptoms of KB should be a cost effective way to increase the chance of early detection. We prepared a leaflet (Murray et al. 1996a) to distribute for this purpose. Once the disease is detected, a complete survey can then find the type and extent of outbreak and help choose the appropriate response.

References Cited

Murray, G. M., J. P. Brennan, and R. A. Hare. 1996a. Karnal bunt alert. NSW Agriculture: Wagga Wagga.

Murray, G. M., J. P. Brennan, and R. A. Hare. 1996b. Karnal bunt of wheat: getting closer to Australia? Agricultural Research 9: (in press).