Late blight of Tomato: humid and wet weather perfect conditions!

P. infestans

By Kelly Ivors, Extension Plant Pathologist

Pathogen:  Late blight of tomato is caused by the fungus-like organism Phytophthora infestans.  The pathogen is best known for causing the devastating Irish potato famine of the  1840's, which killed over a million people, and caused another million to leave the country. 

Host crops and plants:  Besides tomatoes,  P. infestans can only infect a few other closely related plants including potato, petunia and related solanaceous weeds such as hairy nightshade.

Host parts affected:  All above-ground portions of the plant.

Symptoms of late blight:  The first symptoms of late blight on tomato leaves  are irregularly shaped, water-soaked lesions, often with a lighter halo or ring around them; these lesions  are typically found on the younger, more succulent leaves in the top portion of the plant canopy. During high humidity, white cottony growth may be visible on underside of the leaf. Spots are visible on both sides of the leaves. As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die (Figure 3). Late blight can also attack tomato fruit in all stages of development. Rotted fruit are typically firm with greasy spots that eventually become leathery and chocolate brown in color; these spots can enlarge to the point of encompassing the entire fruit.

For more information on how to treat late blight of tomato, see http://goo.gl/SzlkN