Streptomyces scabies & Spongospora subterranea
No symptoms are visible on the aerial parts of the plant such as leaves, stalks or petioles. Usually, first signs of the disease are visible on the surface of young tubers and expand as it matures giving rise to the following symptoms: superficial russet that covers large areas of the skin, reddish-brown raised corky skin, dark-colored, more or less deep holes, and a net-like series of cracks. More than one type of lesion may be present on a single tuber. Other tuber and tap root crops can be affected, like beet, carrot, parsnip and radish. In all cases it results in a reduction of the quality of the tubers and yield losses.
Symptoms are caused by the bacterium Streptomyces scabies, which survives in infected root tissue in the soil. It is spread through water, on infected plant material, and in wind-blown soil. It enters the plant tissues and the tuber mainly through wounds and natural openings. Dry and warm weather during the period of tuber growth increases the infection risk. Since the bacteria need large amounts of oxygen, the probability of infection is higher in loose and well-ventilated soils. The bacteria are most prevalent in dry and alkaline soils. Potato varieties also differ in their susceptibility to S. scabies and some resistant plants have been shown to have fewer, tougher lenticels and a thicker skin.
Treatment of potato plants with compost, compost tea or a combination of both significantly reduces the severity of common scab tuber disease. Bio-fertilizers based on competitive strains of bacteria can increase both yield and the quality of the tuber.
Always consider an integrated approach with preventive measures together with biological treatments if available. Chemical treatment of potato scab is difficult, because it often causes plant injuries. Seed treatments with fluazinam, chlorothalonil and mancozeb showed the lowest percentage of infection.