Spongospora subterranea f.sp. subterranea, cause of powdery scab of potato: biology, epidemiology and control

The resting spores of Spongospora subterranea f.sp. subterranea are highly persistent to environmental effects. The following questions are adressed:

Do they need a stimulus for germination?
What is the significant level of primary inoculum?
What is the nature of host resistance?

The importance of powdery scab for Swiss growers has increased seriously despite strict seed certification laws concerning disease levels. A survey of soil samples from potato-producing areas in 1992, using a bioassay with tomato bait plants, showed that infested soil is the main inoculum source. With the introduction of a new potato variety - 'Agria', which is highly susceptible to powdery scab - in 1988 and a constant increase of the area cropped with this variety since then, disease problems may develop rapidly even from soils with initially low infestation. A growing number of other countries are confronted with the same situation, partly because the disease has been neglected in breeding and certification. Control measures are urgently needed.

All the experiments done so far have shown that there is no single control measure which is 100% effective. Therefore only an integrated approach will allow to get control over the disease. This includes cultural measurements like crop rotation, controlled irrigation, seed certification and disease avoidance, then breeding for resistance and chemical control.

To achieve this goal we need additional basic information on the biology of the pathogen and it's interaction with the host. To make breeding more efficient, resistance mechanisms have to be investigated. Interdisciplinary and international collaboration will allow coordination of research in the many disciplines needed for the integrated approach.

The importance of the factors controlling zoospore release by resting spores, zoospore attraction by host and the infection process will be determined using a modified bioassay in combination with an almost resistant and a susceptible potato variety to find the key mechanisms for host resistance based on studies made on the relationship between host susceptibility to tuber infection and the level of root infection by the zoosporangial stage of the pathogen to speed up screening processes in breeding. A PAB against zoosporangia will be produced and evaluated to encrease objectivity of the baitplant-test system.

Previous collaborative research produced a PAB to resting spores of Spongospora subterranea f.sp. subterranea. Initiatives have led to the production of a monoclonal antiserum especially for a highly sensitive detection of the pathogen on tubers and in soils. The development of a standardized test procedure for soils will enable to investigate relationship between inoculum and crop infection level.