Early Blight

The fungus, Alternaria solani, causes early blight and occurs wherever potatoes are grown. Crop losses can occur if heavy foliar infection takes place around the time of flowering. Disease symptoms first appear as small, brown, pinhead dots on older leaves in the growing crop. These enlarge as concentric rings or "bulls-eye target" causing extensive loss of leaf area and consequent yield reductions. Tuber infection symptoms are seen as dark sunken, generally circular areas surrounded by raised borders that may increase in size during storage. Alternating wet and dry periods are conducive to disease development.

The fungus survives between potato crops in crop residue, soil, infested tubers and on other hosts. Actively growing young leaves and older leaves under high nitrogen regimes do not generally show symptoms. The disease spreads to younger leaves from infected older leaves usually after flowering and intensifies as plants mature. Moist or dew forming conditions favour spore production and germination. Dead or dying potato foliage may be colonized by other Alternaria spp. that may be weakly parasitic or saprophytic.