The symptoms of late blight on potato leaves can vary depending on age of lesion and the environmental conditions prior to observation and the tissue infected. Very young lesions are small (2-10 mm) and irregularly shaped, and may be surrounded by a small halo (area of collapsed, but still green tissue bordering the dark necrotic lesion). Lesions continue to grow and become more circular until they are limited by the leaflet margins. They are usually not delimited by the veins and older lesions are typically surrounded by a chlorotic halo.

Necrotic lesions caused by Phytophtora infestans.

Sporulation can be evident on the borders of the lesion, principally on green, but also on necrotic tissue. Sporulation occurs when infected tissue has been exposed to saturated air (100% Relative Humidity) for more than 7 to 8 hrs. The length of time required for sporulation is dependent on temperature host and pathogen (Day and Shattock 1997; Mizubuti and Fry 1998; Flier and Turkensteen 1999), with sporulation occurring later on resistant hosts. Optimal temperature for sporulation is usually regarded to be 15-20 C, but sporulation occurs in reduced amounts between 5 – 15 oC and 20 – 25oC (Mizubuti and Fry 1998). Under optimal conditions sporulation is easily visible as a noticeable fuzzy white growth on lesion margins, and this is more abundant on the lower (abaxial) side of the leaf. Sporulation occurs from lesions on leaflets or on stems. Sporulation does not occur when the immediately preceding conditions have been dry.

Sporulation of P. infestans on abaxial side of potato leaf.

Blight of apice and stem caused by P. infestans.

Potato plants naturally infected by P. infestans in field.

Late blight lesions can be mistaken for several other diseases. Under especially wet conditions, Botrytis cinerea can rot lower leaves of potato plants such that they resemble late blight lesions, however sporulation from B. cinerea is gray. When conditions become quite dry and late blight lesions dry out, they can be mistaken for dried lesions caused by B. cinerea or sometimes by lesions caused by Alternaria solani (early blight). However, early blight lesions are typically zonate with a definite outer margin, whereas active late blight lesions are almost never zonate and typically do not have a definite outer margin. Certain types of frost damage as it occurs in the highland tropics can resemble late blight. When late blight attacks the stem of some potato genotypes it can cause girdling and the leaves wilt above the point of infection. Stem lesions become brittle and the stem frequently breaks at that point. From a distance this can be confused with other classical wilting symptoms. A pungent putrid smell is often associated with heavily infected fields. This is due to rotting of dead tissue and is not a direct consequence of late blight.

Stem and leaf blight caused by P. infestans.

Literature Cited

Day, J. P. and R. C. Shattock (1997). "Aggressiveness and other factors relating to displacement of populations of Phytophthora infestans in England and Wales." European Journal of Plant Pathology 103(4): 379-391.

Flier, W. G. and L. J. Turkensteen (1999). "Foliar aggressiveness of Phytophthora infestans in three potato growing regions in the Netherlands." European Journal of Plant Pathology 105(4): 381-388.

Mizubuti, E. S. G. and W. E. Fry (1998). "Temperature effects on developmental stages of isolates from three clonal lineages of Phytophthora infestans." Phytopathology 88(8): 837-843.