Ascochyta sorghi (rough leaf spot of sorghum)

On Sorghum spp.The first symptoms of A. sorghi rough leaf spot are small, reddish, discoloured spots (lesions) on sorghum leaves, 2-3 x 2 mm in size. Conspicuous, black, superficial conidiomata of the causative fungus (A. sorghi) were observed to form immediately, often before the discoloration was apparent and usually when the reddish discoloration was barely visible (Luttrell, 1950). Conidiomata form on either the upper or lower surface of the spots (usually on the upper surface). They break off easily, leaving tiny white craters surrounded by a black rim. Spots enlarge until they become broad-elliptic and 8-14 x 4-8 mm. Sometimes they are uniformly dark red to purple. Usually, the centres become tan with a dark red to purple border. Spots coalesce, forming extensive tan blotches outlined by a narrow, dark red border (Luttrell, 1950). Leukel et al. (1944) state that the presence or absence of a coloured border varies with host variety. Conidiomata, appearing on the lesions, may occasionally be observed on apparently healthy green parts of the leaf surface. They are small, black, hard, raised bodies which give the infected areas a characteristically rough feel to the touch. Similar lesions occur on the leaf sheaths and sometimes on the stalks, while abundant conidiomata may develop on the glumes (Tarr, 1962).Entire leaves may die, turn brown and become covered with conidiomata.

Cultural Control and Sanitary Methods

Avoidance of planting in fields cropped to sorghum or Sudan grass is recommended (Tarr, 1962).

Host-Plant Resistance

High levels of resistance to rough leaf spot have been found among sorghum cultivars (Singh and Pavgi, 1977a; Zummo and Broadhead, 1984; Sarwar et al., 1988; Duncan et al., 1990).

A. sorghi has been reported from most countries where sorghum is grown. Crop losses in general are minor (Frederiksen, 1986), but Saccas (1954) has reported losses between 3 and 10% in French Equatorial Africa. The disease first appeared in the USA in 1937 (Tarr, 1962) and there has been no indication since of it being a significant problem there (Luttrell, 1950, Tarr, 1962); Frederiksen and Duncan (1992) classed it as an occasional problem in the USA, but not economically damaging.