Spongospora subterranea f.sp. subterranea (powdery scab)

PotatoPowdery scab is a widespread disease of potato and many descriptions have been published, the most useful being those of Harrison et al. (1997) and Lawrence and McKenzie (1981). Initial symptoms are small (up to 2 mm diameter), purplish-brown pimple-like pustules (swellings), most often at the distal (rose) end of young tubers. These increase in size and rupture the periderm, then they mature as a scab (shallow hollow) filled with a powdery mass of cystosori (sporeballs), which are aggregates of cysts (resting spores) (Harrison et al., 1997).The scabs are usually superficial, smooth in outline, slightly raised and somewhat spongy or corky. Roughly circular in shape, they may reach a diameter of up to 20 mm. If the lesions merge, the composite lesions are large and irregular, and may lead to a dry rot in storage. Wound periderm does not develop in very wet soil, so lesions expand in depth and width forming deep pits, destroying considerable amounts of tuber tissue. Tumours (swellings) may be formed. These can be galls (localized, near spherical outgrowths), cankers (raised, discoloured areas) or warts (small to large, irregular outgrowths). Tuberous outgrowths and extensive warts can result in misshapen and deformed tubers. Large cankers are most common on mature tubers and can result in severe deformities. In storage, these cankers may continue to grow and fresh tumours arise (Karling, 1968; Harrison et al.,1997).Infection on roots and stolons (and also on sprouts that develop in chitting trays) begins as small necrotic spots that develop into milky-white galls, 1-10 mm diameter. Galls turn darker as they mature. Severe root galling can cause wilt and death of young plants (French et al., 1972; Harrison et al., 1997).TomatoDisease on tomato has been described from Pennsylvania in east-central USA. Root galling, with galls ranging in size from minute to 'the size of garden peas', is observed and symptoms are similar to powdery scab of potato roots (USDA, 1960; Karling, 1968).

Host-Plant Resistance

Bitter potatoes of Peru (Solanum x ajanhuirii), Solanum x curtilobum and Solanum x juzepczukii), which are usually only grown at altitudes above 4000 m because of their very high frost tolerance, are also highly resistant to powdery scab under these conditions (French et al., 1972; Franco, 1994).

Some accessions of wild potatoes have been reported to be immune to scab disease, namely, Solanum aracc-papa, S. mamilliferum, S. rybinii, S. curtilobum, S. tuberosum subsp. andigena and S. tuberosum subsp. tuberosum (Karling, 1968). Most commercial S. tuberosum subsp. tuberosum varieties are susceptible to very susceptible, but some are moderately resistant and a few resistant varieties have been selected in many countries. For example, Genet et al. (1995) developed highly resistant varieties in New Zealand. Resistance appears to be under polygenic control and there is little evidence for the existence of pathotypes of the pathogen (Harrison et al., 1997). In Chile, 6 out of 21 varieties screened for resistance had 0-6% scab versus 60% for the most susceptible variety; the three that rated 0 were Huevo los riscos, Industria and Chapeda (Karling, 1968). In six annual field trials in naturally infested soils at high altitudes in Anta, Cuzco, Peru, 467 accessions of both subspecies of S. tuberosum were screened; 20 were highly resistant, including the highly productive andigena varieties Gabriela and Albina. These varieties were therefore recommended for use by local farmers.

Several tuberosum cultivars from Argentina, UK and USA were also resistant (Torres et al., 1995).

Cultural Control

Crop rotation

Rotations of 3 to 10 years are recommended, depending on degree of cultivar resistance, climatic conditions, and soil porosity and drainage (Lawrence and McKenzie, 1981).

Moisture control

Scab can be significantly reduced by measures which reduce excess moisture, especially during tuber set, including improving drainage and withholding irrigation or irrigating, possibly more frequently, with less water (Harrison et al., 1997). Taylor et al. (1986) reported that maximum susceptibility to powdery scab occurs when 50% of the stolons have swollen to at least 5 mm diameter (tuber set) and up to 3-4 weeks after.

Quality seed

Only disease-free tuber seed should be planted and seed production fields should be rogued carefully. If only infected seed is available, it should be chemically treated (Rich, 1983). An alternative to chemical treatment is to treat with hot water at 55°C, but this is likely to result in heat damage to sprouts, depending on the potato variety and the physiological condition of tubers (Harrison et al., 1997).

Soil treatment

The soil should not be applied with manure from animals that have been fed scabby potatoes. The application of sulfur can be useful, but repeated use makes soils too acid to be productive (Lawrence and McKenzie, 1981). Soil solarization was effective in significantly reducing soil populations of the pathogen in the high altitude tropical conditions of Cochabamba, Bolivia (Aguilera et al., 1992).

Economic losses are usually not significant when potatoes are marketed immediately without washing because tubers are generally only affected on the surface which is most often peeled off. At worst, they become unsightly. At high altitudes in the Andes of southern Peru, up to 30% of tuber surfaces were found to be damaged (French et al., 1972). Worldwide, surface lesion areas of 30-50% are not uncommon when crops are subjected to heavy rainfall and low temperatures. A dry rot may develop during storage with high losses, ranging from 30 to 73%, recorded in different locations (Karling, 1968). In the last two decades of the 20th Century, marketing techniques in many countries have involved washing tubers and displaying them in clear plastic-wrapped trays, where scabby potatoes are unacceptable. In some areas of Brazil, the difference in price between washed and unwashed tubers is as much as 50% (Lopes and Nazareno, 1995). Yields may be reduced when cankers or warts develop. In most cases, the sale of seed tubers requires that no, or very few, scabs are visible (Harrison et al., 1997).