Septoria lycopersici var. malagutii (annular leaf spot of potato)

Oval, chestnut-brown, necrotic lesions (up to 12 mm in diameter) develop on leaves in the field and in greenhouse inoculation tests. The lesions have raised, unevenly separated, concentric rings on the upper side, with conspicuous, erumpent pycnidia (up to 230 µm diameter) within the central ring. Masses of pycnidiospores may emanate from the lesions forming cirrhi. Lesions often coalesce to form large, necrotic areas which can induce leaf drop (see Pictures). In the advanced stages of the disease, the leaves become brittle, deformed and susceptible to wind damage, and abscission may occur. The combined damage can reduce the effective foliar area by over 60% (Ciccarone and Malaguti, 1968; Jimenez and French, 1970, 1972; Carrera and Orellana, 1978; French, 2001). Stem and petiole lesions are elongate; lesions observed in the field in Peru were dark brown, about 2 mm wide and up to 10 mm long (Torres et al., 1970). In Ecuador, stem lesions in the field were 0.5-1.5 mm wide and 6-15 mm long (Carrera and Orellana, 1978). Differences in lesion size may be due to different varieties being grown under different environmental conditions in these countries.

Planting during the months with lower moisture levels or warmer temperatures is one way of avoiding the disease in areas where potatoes can be grown all year round (a common practice in some tropical countries) except in areas where the altitude is too high to avoid frost (Jimenez and French, 1972). In Ecuador, Carrera and Orellana (1978) tested eight contact fungicides for control of annular leaf spot and found that fentin was the most effective. French (2001) reported that non-systemic fungicides, which are capable of controlling late blight, are effective for control of Septoria provided treatment begins early enough to avoid lesions becoming a constant source of inoculum spread. Carrera and Orellana (1978) observed resistance among 31 cultivars tested, six being considerably more resistant than the others. Three of these cultivars were the commercial varieties Violeta (Solanum tuberosum subsp. andigena), Alpha (S. tuberosum subsp. tuberosum) and Soliman (S. phureja). Jimenez (1970) found that Huayro (Solanum x chaucha) and Chata rosada (S. tuberosum subsp. andigena) varieties developed smaller and fewer lesions than other varieties.

The destruction of up to 60% of foliage has been reported to be caused by annular leaf spot, and considerable yield losses occur in the Andean countries where the disease occurs (Carrera and Orellana, 1978; OEPP/EPPO, 1984).