Guide to Late Blight in Tomatoes

Late blight of tomatoes, caused by the fungal pathogen Phytophthora infestans, is a devastating disease for potato and tomato farmers. The pathogen is famous for the late blight infection that caused the Irish Potato Famine in the 19^th century. Although Europeans first discovered late blight in the 1800's from international trade, the disease still affects farmers today. In this article, we will look at the causes, favored conditions, symptoms and treatment for late blight in tomato plants.

Causes

Late blight is a fungal related pathogen that spreads at almost any time under favorable weather conditions during the growing season. Symptoms of late blight are sudden and severe, and result in plant death. Phytophthora infestans comes in many strains, some ranging from very severe to moderate. The varying levels of strength refer to the damage done by the strain and to how infectious the disease is.

Conditions

Late blight prefers weather conditions that are wet with moderate temperatures. During these weather conditions, late blight infection is more frequent and transmitted from plant to plant quickly. Fortunately, for tomato growers, late blight does not survive during the winter and on growing equipment. Unlike many other pathogens, late blight does not survive in tomato seeds, even from infected plants, the disease also cannot survive on frozen tomatoes or on tomato cages and poles. The fungus needs living tissue to reproduce and survive. When no living tissue is present (i.e. frozen tomatoes or seeds), the fungus simply dies.

Symptoms

Symptoms for late blight in tomatoes include sunken, dark green or brown lesions on the leaves of stems of the plant, states the University of Massachusetts. White fungal growth can be observed on the stem or leaf when the disease develops. Often time's farmers or gardeners will notice a yellow boarder around the legions and this is from another virus, most likely early blight or Septoria leaf spot.

Treatment

Treatment for late blight consists of using late blight resistant varieties of tomatoes, fungicides and preventative measures. Cornell University recommends the use of the lowest labeled rate of protectant fungicide to prevent a serious outbreak of late blight. Fungicides need to be applied regularly during favorable weather conditions of late blight; this ensures that the fungus is killed before infection occurs. Late blight resistant tomato varieties are in limited supply, but include 'Mountain Magic', 'Plum Regal' and 'Legend', states the University of Massachusetts. Because late blight does not survive winters in tomatoes (unless in a greenhouse), there is no extra preventative measures for the following growing year.

Sources:

http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm

http://www.umassvegetable.org/LateBlightAlertforTomatoandPotato.html