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Gray mold of tomatoes

Quick facts

  • Gray mold thrives in the high humidity found in high tunnel and greenhouse tomato production.
  • Gray mold is rare in Minnesota field grown tomatoes. 
  • The fungal pathogen infects all above ground plant parts. 
  • It can be highly destructive when environmental conditions favor disease.

How to tell gray mold apart from other issue

Tomato leaf with irregular to V-shaped brown blotches, starting at the margin of the leaf
Gray mold infection on a tomato leaf
Tomato stem end that grey, brown with dead leaves towards the end.
Gray mold infection on tomato stems
Healthy green tomatoes with white rings or halos on the fruit
Small white rings on tomato fruit indicate failed gray mold infections
  • Gray mold occurs in cool temperatures (between 60-75°F) and high humidity (greater than 80%) 
  • In high humidity, fluffy gray spores cover infected plant parts.
  • Leaves have irregular to V-shaped brown blotches, often starting at the margin of the leaf.
  • Die-back symptoms appear as infection progresses from leaves, through petioles, towards the main stem.
  • Brown, oval lesions can girdle the stem.
  • Infected flowers turn brown and die.
  • The spores are light brown-gray on black stalks.
  • Infected fruit develop a pale, soft, watery rot, or have white rings or halos on the fruit, called ghost spots.
  • Fruit symptoms occur on green and red fruit; while on the plant and post-harvest.

Biology

  • Gray mold is caused by the fungus Botrytis cinerea
  • Disease develops in temperatures between 60-75°F (60-70°F optimum). Temperatures greater than 82°F suppress fungal growth and sporulation.
  • Gray mold requires high humidity (greater than 80%).
  • Spores are most likely to germinate when leaves remain wet for 5-8 hours.
  • The fungus survives on numerous weed hosts, on dead plant material, and as hard resting structures (sclerotia) in plant debris and soil.
  • Spores are spread short and long distances by wind and rain, equipment and workers.
  • Infections begin on weak, dying or wounded plant tissue.

Managing Gray Mold in Home Gardens

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Managing Gray Mold on Farms

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Marissa Schuh, Horticultural IPM Extension Educator, Angela Orshinsky, Extension plant pathologist and Michelle Grabowski, Extension educator

Reviewed in 2021

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