Early blight

The early blight is caused by Alternaria solani (Ell & Mart.). Jones & Grout and leaf spots are caused by Phoma exigua Desm. and by Phoma sorghina Doerema, Doren and van Kest. Early blight occurs in all the potato growing areas but is severe in the sub-mountainous regions as well as the plateau areas. Depending upon the severity, early blight/phoma leaf spots may cause significant yield losses. It has been reported to cause losses up to 20% in kharif crop in Ranchi and adjoining plateau region. The incidence and severity of early blight and leaf spots is generally high in the crop receiving imbalanced doses of NPK, particularly low nitrogen.

The symptoms appear just before the initiation of tubers. Initially the symptoms occur on the lower, older leaves in the form of small (1-2 mm), circular to oval, brown spots.  These lesions have the tendency to become large and angular at later stage.  Mature lesions on foliage look dry and papery, and often have the concentric rings, looking like ‘bulls’ eye.  The rings are more prominent in large blotchy spots and give them a target board effect.  The spots are often dry and brittle but some times under conditions of heavy dew or rain, become wet and dark brown. The spots are mostly 3 to 5 mm in diameter but in some cases may be up to 10 mm.  The spots may enlarge and coalesce to form large necrotic areas.  Ultimately the whole leaf dries up.  The spots may also develop on the stem at a late stage of the growth of the plant.

 Leaf spots caused by Phoma exigua Desm. are larger, 1 to 2.5 cm in diameter, with broad alternate light and dark concentric zones.  Affected tubers have grey to greenish black depressed lesions on the surface, upto 3 cm wide.

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