Switzerland

Powdery scab situation

Powdery scab was first reported in Switzerland early in the last century but had probably been present already for many years. It has been traditionally a problem of the hilly regions where most seed was produced, with cool and wet springs, favouring the disease. But there has been an increasing incidence of the disease also in the lowland areas in the late 70's and the 80's, partly due to decreasing awareness of the disease, changed cultivation methods, mercury seed dips became banned and the introduction of the highly susceptible cultivar Agria.

In 1991, the disease caused, often for the first time, severe problems, in particular, for growers of cv. Agria. To obtain basic information, in 1992 a survey was made of pathogen and disease occurrence in soil and on tuber samples from 82 selected farms on fields cropped with cv Agria. Information on soil infestation was obtained using a bait-plant bioassay. The results showed that the pathogen is more widespread then previously expected. The pathogen was found in places with no history of powdery scab. Assessment of tuber samples from the fields and, even worse, disease levels at harvest, represented only poorly the actual distribution of the disease:

However, the risk of tuber infection is significant only when soil infestation is high. On the other hand, with insufficient crop rotation and the use of susceptible varieties, disease problems may soon develop from soils with initially low infestation. Ploughing in spring, often following pasture as a pre-crop, was correlated with fields with powdery scab problems.

In this context it is an interesting fact that Wild (1930), working with powdery scab, made her field trials mostly below 500 maS in the Swiss lowlands. Scabbed potatoes were harvested at all sites even where clean seed had been used. Based on her findings Wild stated already at that time that the pathogen was more widespread than she had expected. It may be concluded that the pathogen is endemic in most soils where potato are cropped.

There are no simple and effective control measures available for Swiss potato growers. More than 70% of the Swiss potato area is cropped with varieties which are moderate to highly susceptible to powdery scab (35% highly susceptible). Often rejected lots are fed to the cattle/pigs and the disease may be spread through manure as the resting spores are still infectious after digestion. No chemicals are registered for powdery scab control.

A survey made on 200 farm fields showed similar results of the actual distribution of soil infestation in Switzerland: about one thrird of the soils is infested. Soil infestation turned out to be the most important source of inoculum for the subsequent crop under optimal conditions. on the other hand, infected seed ist the most important source of new infestations. An intergrated conrol strategy has to combine strict seed control for healthy seed and soil infestation testing for infection risk assessment. Different stringency levels are required for basic and certified seed.

Today the importance of Spongospora diseases changes from year to year depending on the weather. Global warming causing drier springs together with an introduced immunology test for the seed inspectors decrease the occurrence the diseases.

Research

Research has concentrated mainly on three topics: the biology, the epidemiology and the control of the pathogen. A rapid and versatile bioassay has been developped (Merz, 1989) which allows to quantify the infectivity of resting spore inoculum or infested soil. The bioassay can be used for soil surveys (Blum und Merz, 1993), studies of the biology of the pathogen (Merz, 1992; Merz, 1993; Merz, 1997) and of chemical control.
Polyclonal (Merz et al., 1996) and monoclonal antiserum (Merz et al., 2005) has been developped against the resting spores to produce detection tools for tubers and soil. Recently a rapid onsite test has been conmmercialized (AgriStrip, Bioreba).
A rapid lab based resistance screening test based on the bioassy above was introduced showing that there is a relationship between zoosporangial root infection and tuber susceptibilty to Spongospora which allows to an early scrrening of potato plantlets for their resistance against powdery scab (Merz et al., 2004).
Further research has been done on (see also reference list)
- the mechanisms of zoospore release of resting spores - as breaking the dormancy of the pathogen could be an effective biocontrol measure: there seems to be no host-specific stimulus.
- molecular markers as necessary tools for investigations on the existence of a sexual stage - an important issue for resistance breeding: molecular work has shown that at the origin of Sss in South America there is a divers population whereas in the rest of the world only a few clones were found (Gau RD,Merz U, Falloon RE, Brunner PC (2013) PLoS ONE 8(6): e67944. doi:10.1371/journal.pone.0067944). Good news for the breeders as long as no new race is imported - quarantine regulations are important!
- Peat ha been identified as the source of Sss contamination in substrate (problem in minituber production) but how Sss and peat came together is still a mystery.

Since Ueli Merz has retired research has stopped. The national research station still screens varieties for their resistance/tolerance/susceptibility to the Spongospora diseases (powdery scab and root galls) in contaminated fields.


Contact address:

Ueli Merz
Retired
(Plant Pathology Group ETHZ)
Erlenstrasse 11
8408 Winterthur
Switzerland

 

e-mail:

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Last update: January, 2025