Early blight (Alternaria solani)

Early blight: biology

Early blight is an endemic disease – it exists in virtually every potato field in Canada and can lead to yield losses if not treated. First, infected leaf tissue reduces the crop’s photosynthetic capacity which on its own reduces yield, but then it also erodes the crop’s ability to bulk up its tubers leading to reduced grade and market quality.

The fungus causing early blight survives mainly on crop residue but also in soil, in tubers and on other plants in the potato family. The disease is spread by airborne spores and from leaf lesions, usually starting about flowering time or during wet conditions – either from weather or dew. However, once infection begins, Early blight prefers warm, dry conditions. Early blight is typically more severe when the plants are under mid-summer stresses (e.g. nutrient deficiencies or infection by other diseases) and during alternating wet (dew) and dry periods.

Infections typically start in lower leaves and move up the plant as more spores are produced. Lesions first appear as pinhead-sized dots on older leaves; they expand and develop concentric rings of dead tissue that may give a target board appearance. A yellowish "halo" may also develop around the lesions. It can be difficult to scout for lesions on lower leaves, since these leaves may already be yellowing because they are shaded by the upper canopy. In severe infections, the foliage is completely covered by lesions and dries up.

Lesions may eventually spread to the stems and tubers. On plant stems, brown spots or flecks may develop if the foliar infection is severe. Lesions on tubers are dark brown to black, slightly sunken and round to irregular in shape. They can also be surrounded by a raised violet-blue border while the internal decayed tissue is usually dark brown, dry and corky.