An infection of 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. As such, correct and rapid identification of the pathogen is essential to implement control methods before it gets out of hand. Initial diagnosis can be accomplished by looking for the symptoms of late blight on affected plants and sporulating infected leaves. Click here for an article on other diseases of potatoes and tomatoes with late blight-like symptoms.
To identify the pathogen place suspect leaves in a moist chamber with a damp towel overnight. Look for minute white spores (sporangiophores bearing 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 release zoospores that are responsible for additional infections. Sporangia can be dispersed by wind and rain at local and national 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. Images courtesy of William Fry, Cornell University.