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The 2nd International Powdery Scab Workshop, Pretoria, South Africa 29th July – 1st August 2014
It may be hard to believe that a hot country like South Africa is suited to a cool wet-loving pathogen like powdery scab of potatoes. Then again, Israel and Australia also do not fit the biological profile of the plasmodiophorid pathogen, Spongospora subterranea subsp. subterranea (Sss). Perhaps it is the adaptability of the pathogen that has attracted a small dedicated band of international plant pathologists to meet and discuss what we know – or more to the point what we don’t know!
I hosted the first European powdery scab workshop in Aberdeen in 2000 and since then there has either been an European or International workshop every two or three years. Such a focus on what may appear a relatively minor pathogen of potatoes could seem outwardly excessive, until you realise just how devastating the disease is and how difficult to control it is. In countries where it is rife (like Scotland!) it has resulted in some growers leaving the potato business. The difficulty in control centres around the fact it is a soil-borne pathogen with an incredibly persistent resting spore or cystosorus. Control centres around avoidance, which with a dwindling land-bank for potato production in the UK can be difficult, and chemical control which is always a challenge with any soil-borne pathogen.
Progress in understanding the pathogen and its biology has been a slow but steady matter. One new factor that was emphasised this time was the ability of the pathogen to substantially affect root growth and thereby limit yield. When cycles of infection occur in the root hairs and surface root cells, function is substantially affected. The pathogen literally punches holes in root hairs allowing nutrients to leak out. In turn this can stimulate attack by other pests and pathogens such as free living nematodes. Both lab and field experiments in New Zealand have demonstrated that heavy infection can reduce root and shoot growth and tuber yield and the nutrient content is substantially affected. There is a view in New Zealand that powdery scab infection is one factor that is restricting further yield increases and is making potatoes less profitable.
In a Tasmanian/South Australia rotational study of commercial crops, soil contamination by Sss, as determined by a soil DNA test, showed that only after 10 years had soil populations fallen to almost undetectable levels. However, a single crop of potatoes was able to raise soil contamination back to the original level found 10 years previously. Three and 5-year rotations did not result in significant reductions in contamination.
Disease avoidance has been the thrust of a soil DNA test developed through Potato Council funding and used commercially in the north of Scotland by SRUC over 5 seasons. In the Scottish climate, where even low levels of pathogen can multiply up substantially before tuber initiation, even detection of trace levels of Sss means a moderate risk of disease in a subsequent crop. Growers are using the test result to avoid growing potatoes in a field, to target resistant varieties in the most contaminated fields and to decide whether the only chemical control op- tion, fluazinam, should be used. One finding from five years of testing is that contamination levels seem to fall substantially when fields are saturated for long periods over winter.
The potential for contamination of peat based substrates used for growing minitubers, the foundation seed for most seed potato production systems, was discussed in detail at the workshop. Powdery scab in mini-tubers resulting from contaminated substrate has been perhaps the most significant biosecurity issue for mini-tuber production around the world. How areas of peat which are continually harvested each season become contaminated is unclear but the cystosori may be spread in dust over long distances.
No-one is in any doubt that durable host resistance is the long term solution to powdery scab. But as a target for breeding it is hardly ever considered. In consequence, many (perhaps most) commercial cultivars are susceptible. One new approach to finding resistance that is being pursued in Australia is somaclonal selection for enhanced resistance to Sss root infection. Normally resistance operates at the tuber level but resistance to root infection would limit build-up of inoculum in the roots. The technique of somaclonal selection has had some success with resistance to Streptomyces spp. , the cause of common scab in potatoes. The need to screen cultivars for both tuber and root infection is now established in some countries. This is needed to understand how much soil contamination may occur through development of root galls, which can even develop on tuberresistant cultivars.
This is only a snapshot of the three days of the workshop. Oh, in case you are still puzzled by why hot countries are afflicted by a cool wet loving pathogen, in these countries potatoes are often grown in the winter time, which is cooler, and irrigation is an essential part of production. Even in summer production powdery scab may develop as water for irrigation may be pumped up from below ground reserves and is often much cooler than the air temperatures. The sudden drop in soil temperature as a result of irrigation can provide the narrow window of just a few hours of suitable conditions for the resting spores to release their swimming spores which then infect potato roots or tubers.
I would like to acknowledge the contributions of both BSPP and the Potato Council towards attendance of this latest workshop.
Stuart Wale SRUC