Phytophthora infestans can have devastating effects on a potato or tomato crop, with the potential to completely destroy a field in a matter of days if left unchecked.  Correct and rapid identification of the pathogen is essential to implement control methods.  Initial diagnosis can be done with a hand lens by looking for the symptoms of late blight on affected plants and white sporulation on infected leaves. Click here for an article on other diseases of potatoes and tomatoes with late blight-like symptoms.

Late blight symptoms on potato
Late blight symptoms on tomato

To identify the pathogen place suspect leaves in a moist chamber with a damp towel overnight.  Look for minute white sporangiophores bearing lemon shaped sporangia on the underside of the leaf surface using a hand lens.

Phytophthora infestans reproduces predominately by asexual means and forms sporangia on infected host tissue that either germinate directly to form infection hyphae or at lower temperatures release zoospores that are responsible for additional infections.  Sporangia can be dispersed by wind and rain at local and regional scales (hundreds of meters).  Movement over longer distances is via infected potatoes or tomato transplants.  The pathogen typically survives from season to season as mycelium in infected potato tubers, volunteer potato plants or infected culled potatoes.  These sources of inoculum can contribute to epidemic development on subsequent crops.

Life cycle of Phytophthora infestans. a, Leaf lesion with sporulation; B, Sporangiophore with sporangia; C, Semi-papillate sporangia; D: Sporangia releasing zoospores; E, Infected potato leaf.  Images courtesy of William Fry, Cornell University.