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Late blight found on MN tomatoes

Late blight has been identified on tomatoes in Otter Tail County, MN. Growers in that region should take immediate action and scout tomato plantings for symptoms of late blight.

Late blight is caused by the pathogen Phytophthora infestans which can only survive Minnesota's winters if sheltered in a cull pile or a warm compost pile. As a result late blight often needs to be brought into the state on infected transplants, tubers, or on windblown sporangia. Once the pathogen arrives, it needs cool wet weather to thrive and spread.

green and ripe tomatoes with large brown round areas of rot
Firm brown rot on late blight infected tomato fruit.
M. Grabowski, UMN Extension 

How to identify late blight

  • Examine lower leaves and leaves in the center of the canopy where humidity is high
  • Early infections have greasy gray leaf spots and powdery white mold may be visible on the lower leaf surface if humidity is high
  • Spots grow into large, dark brown blotches with a green gray edge
  • Leaf infections spread quickly through the leaflet and into the petiole, resulting in clusters of brown foliage 
  • Stem infections are firm and dark brown with a rounded edge
  • Fruit have large firm, dark brown, circular spots
  • Brown discoloration extends into the fruit and can be seen when fruit is cut in half 
  • In high humidity, thin powdery white fungal growth appears on infected leaves, fruit, and stems
tomato leaf with one large gray spot
Early leaf infection of late blight on tomato
M. Grabowski, UMN Extension 

If late blight is found

Infected plants should be removed and bagged or buried in an active compost pile, disced-under, killed with herbicide or flame-killed. One leaf spot is capable of producing thousands of sporangia that easily spread by wind so it is critical to kill, remove, or bury infected plants as soon as possible. 
the lower surface of a tomato leaf with powdery white coating of sporangia
White powered sugar like dust on lower leaf surface are sporagnia
M. Grabowski, UMN Extension

Protecting plants from late blight

Fungicides are available to protect plants from late blight but are only effective if applied before infection occurs. Growers should consider weather conditions and proximity to known infections when evaluating the need to spray. Late blight thrives in cool (60 to 70 ℉) wet weather. Prolonged, hot dry days can halt the progression of the pathogen. 

Phytophthora infestans is a water mold and not a true fungus. Fungicides specific to water molds must be used and applications repeated according to label instructions. For a current list of fungicides for late blight management visit the Midwest Vegetable Production Guide. Rotate fungicide groups and/or tank mix fungicides to avoid producing fungicide-resistant isolates.

M. Grabowski, UMN Extension educator

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