Late blight (Phytophtora infestans)

Late blight is caused by an oomycete pathogen, Phytophthora infestans. It is an obligate parasite that only survives in and on living plant tissues.

CULTURAL CONTROLS: One way to manage the disease is to avoid or eliminate sources of inoculum (volunteer potatoes, cull piles, etc.). Avoid planting infected seed potatoes. In fields that are irrigated, it is important to manage water to minimize periods of leaf wetness. It takes about 8 to 10 hours of leaf wetness for late blight spores to germinate and invade plant tissues. Avoid irrigation during rainy weather. Allow the plant canopy to dry completely between irrigations. Avoid excessive irrigation.  

FUNGICIDES: Use a fungicide seed treatment with activity against late blight to limit seed-to-seed pathogen spread at planting. 

Foliar fungicides are recommended when conditions favor disease development. Many late blight fungicides are protectants that stop the spores of the pathogen from germinating and then penetrating plant tissues. They will not cure existing infections in the plant and need to be used preventively (before spores land on plants). It is important to apply these fungicides on a regular and continuing schedule when conditions are favorable to the disease. Short application intervals should be used when disease pressure is high to maintain adequate fungicide coverage throughout the plant canopy.

Curative fungicides can be used at the post-infection stage to inhibit sporulation and spread through the field. These are systemic fungicides that are absorbed into leaf tissues and are translocated to some degree within the plant. They can have an inhibitory effect on P. infestans that has made its way into plant tissues, but are most effective when applied before infection. They will not cure a widespread or severe late blight outbreak.  

SYMPTOMS: Late blight lesions can occur on potato leaves, stems and petioles, and tubers. Leaf lesions are irregularly shaped, brown to gray or black, and usually surrounded by a light green zone. Stem and petiole lesions are brown to black. Under humid conditions, a white moldy growth may be observed on lesions. Tuber lesions are reddish-brown, dry, and granular. They start at the surface and usually do not extend far into the tuber. Late blight symptoms in tubers may be obscured by those of secondary pathogens