S.s.s. is the cause of powdery scab of potato (early symptoms on young tuber: cauliflower stage; older tuber with typical powdery scab lesions) and the vector of mop-top furovirus (rod shaped virus particles).
In Europe powdery scab problems were already known since long time when Wallroth in 1842 described the symptoms for the first time. He concluded that the cause must be a fungus, producing spore balls and called the cause Erysibe subterranea. Later on, Brunchhorst (1886) named it Spongospora solani. Lagerheim finally (1891), having studied the earlier papers, proposed the name Spongospora subterranea.
At the end of the 30ies a great part of the known work on the disease was already done. It was only in the 50ies when research started again. Kole (1954) for instance pointed out that the disease was of increasing importance in Holland and research money was therefore available. In the following decades more and more countries started to care about the disease. Additionally in 1969 S. subterranea was identified as the vector of mop-top furovirus (Jones and Harrison) which can cause severe damage in some potato cultivars. These diseases have become more important in parts of Europe over the last decade including the U.K. (Hide, 1981; Wale, 1987), Switzerland (Winter & Winiger, 1983; Merz, 1993), Sweden (Ryden et al., 1989), Finland (Kurppa, 1989), Holland (Turkensteen, pers. comm.), France (Bouchek, pers. comm.) and also other parts of the world including Peru (Torres et al., 1995), Colombia (Sanudo & Jurado, 1990), USA (Mohan, pers comm), Australia (de Boer, 1991), India (Bhattacharyya et al., 1985), Pakistan (Ahmad et al, 1991), New Zealand (Braithwaite et al., 1994), Turkey (Eraslan & Turhan, 1989) and Israel (Tsror et al., 1993).
Life cycle (see also complete illustrated life cycle)
The spongy-like spore balls consists of 500-1000 resting spores (3.5-4.5 um) of which each release a biflagellated primary zoospore (Ledingham, 1935; Kole, 1954) or a ameoba (Massee, 1908; Kunkel, 1915; Piard-Douchez, 1949; Diriwaechter, 1981) depending on the availability of water.
The mechanisms of host infection and further development of the fungus within host is not clear. Some authors claimed that fused amoeba penetrate host cells as plasmodia (Kunkel, 1915; Wild, 1930). Others observed the involvement of zoospores in the infection process (Kole, 1954; Würzer, 1965). Diriwaechter (1981) showed that primary zoospores and amoeba can cause tomato root infection. Karling (1968) summarized the different author's ideas about post infection development: amoeba/plasmodia spread intra/intercellular either activ or passiv in the host tissue. They cause abnormal growth of the infected cells on tubers and potato roots and stolons leading to lesions and galls filled with a powdery mass of spore balls. Ledingham (1935) described for the first time the presence of zoosporangia. Kole and Gielink (1963) showed that the secondary zoospores can cause new zoosporangia and thus spread the disease in a field. Longtime it was assumed that only secondary zoospores can only cause both zoosporangia and resting spores till Diriwaechter and Barbery (1991) proved that this is also the case with primary zoospores.
A sexual cycle is only supposed. Kole and Gielink (1963) postulated that the quadriflagelated zoospores, observed by Kole (1954), originated from the fusion of two gamete-like zoospores and that this zygote will initiate resting spore development. Braselton (1992), counting synaptonemal complexes (SC) in serial ultrathin sections of transitional (sporogenic) plasmodia, considered a haploid chromosome number of 37 which coincides with other Plasmodiophorids. Furthermore, the presence of SCs in many nuclei within multinucleate plasmodia suggests that meiosis occurs.
Epidemiology and Control
The initial inoculum for the disease in a particular field may arise from fungal resting spores that were already present in the soil or imported either with seed or with manure, soil or sludge. We have no information on the level of soil infestation necessary for an outbreak of the disease. Highly infested soils may have not more than 1000 spore balls/g. The resting spores are able to survive in a dormant state in soils for a number of years and are difficult to kill.
According to the textbooks the disease is predominant in wet and poorly drained soils at low temperatures (16-180C). In contrast to that there are reports on heavy infections in sandy soils at moderate temperatures with the extreme in Israel and South Africa, where they observed powdery scab on tubers from fields in the semi-arid zone with soil temperatures at 400C.
The nutritional state of the soil seems to have no influence
on the occurence of the disease. The same is true for the pH where the findings
by several authors - using lime or sulphur to alter the pH - are contradictory
or the amount of chemicals they had to apply for any effect was not economically.
In pH experiments in soil the amount of mobilised zinc through acidification
may also be taken in consideration - zinc has been shown to be toxic to zoospores
(Tomlinson, 1958). In lab experiments no relationship between pH of nutrient
solution and root infection by zoospores was obtained (Merz, 1989).
Direct control Mercuric chloride gave some control effect on powdery scab but this chemical should be avoided because of it's negative environmental impact (Bhattacharyya and Raj, 1986). Recently several chemicals, fluazinam, mancozeb and flusulfamide, showed promising results in field experiments - both as tuber or in-furrow treatments - (Braithwaite et al., 1994; Falloon et al, 1994; Dixon et al, 1994) and fluazinam is now registered in New Zealand for seed and soil treatments. Also in New Zealand the farmers use to treat the seed with formaldehyde.
Field trials with zinc gave some control (Burgess et al., 1992) but the amount of chemical necessary is only economically for small plots where high quality seed has to be produced.
Warm water seed tuber treatment - known to be effective
against diseases caused by viruses - was tested by Mackay and Shipton (1983)
also against powdery scab. They got best results with a 10 min treatment
at 550C two month before planting. As a problem there may be a varying phytotoxic
effect of this relatively high temperature depending on the cultivar.
Indirect control There is no clear indication of any effect of macro and micro nutrients on the occurence of powdery scab. If diseased tubers are fed to the cattle they have to be steamed first as the resting spores are still infectious after digestion and will be spread with manure.
Irrigation increases powdery scab especially during tuber initiation (6-8 weeks after planting). Taylor et al. (1986) showed that starting with irrigation only at tuber initiation or interrupt it from 1 week before till 3 weeks after reduced powdery scab significantly. On the other hand it is possible to control common scab with irrigation.
A good practice of crop rotation helps not to increase powdery scab but there are several reports that even after more than a then years cropping period without potatoes there was a severe attack at harvest time.
As different weeds may be alternative hosts of S.s.s. - at least for its zoosporangial stage (Würzer, 1965; Janke, 1965) - their control may be important. White (1954) found that thornapple (Datura stramonium) is a good catch crop for Spongospora subterranea - but this plant is a poisonous weed. Under Swiss conditions oil seed rape, as main or in-between crop, reduced powdery scab incidence (Winter and Winiger, 1983).
There have been no resistant cultivars available on the marked so far. Powdery scab was not included so far in the main breeding programs. Nothing is known about the resistance mechanisms. There may be a relationship between time of tuber skin production, skin quality and susceptibility. It is a fact that in Switzerland one third of the potato seed produced has medium to high susceptibility to powdery scab. But there must be genetical sources of resistance (Torres and French, 1995). New breeding approaches seem to be successful (Genet et al., 1996) - the New Zealand cultivar Gladiator is highly resistant. The old cultivar `Isola` was known to be powdery scab-save in Switzerland (Munster and Cornu, 1974).
The strict use of certified seed may be the most effective disease control at least were uninfested soil is present. But visual seed testing may be tricky as there is the possibility of tuber infection not visible to the eyes (de Boer et al., 1982). And the raised form of common scab (caused by Streptomyces scabies) is often mistaken for powdery scab and vice versa. The availability of a sensitive monoclonal antibody (BIOREBA AG, Reinach, Switzerland) allows now to use ELISA for routinely identification.
To avoid planting of healthy seed in infested soils may also be a effective control measure. It depends on the availability of reliable soil tests. A bioassay (Merz, 1989; Wale et al, 1993), immunological methods (Harrison et al, 1993; Walsh et al, 1996) or molecular markers (Bulman and Marshall, 1998; Bell et al, 1999) may be promising tools for future integrated management systems.
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Phytopathology Group ETH Zurich / Institute of Plant Sciences / ETH Zurich
Last update: November 1, 2001