Tomato Early Blight

The primary pathogen of tomato early blight is the fungus Alternaria solani Sorauer; small-spored species such as Alternaria alternata and Alternaria tomatophila also occur. The disease affects all above-ground plant organs, manifesting on leaves as dark brown spots with distinct concentric zonation and a yellow halo. Elongated spots appear on stems and petioles, leading to cracking and tissue breakage, while fruit infection is most often localized in the peduncle area in the form of sunken, dry, dark spots.

Unlike a number of other tomato diseases, no clear dominant resistance genes used in standard variety coding descriptions have been identified for early blight. In international practice, the codes As (for Alternaria solani) and Aal (for Alternaria alternata f. sp. lycopersici) are used to denote resistance to the pathogen. Due to the polygenic nature of resistance, breeding programs typically assess the level of tolerance or the degree of plant susceptibility using scoring scales rather than the presence of specific genes.