Gray mold of tomatoes
Although rare in Minnesota field grown tomatoes, gray mold thrives in the high humidity found in high tunnel tomato production. The fungal pathogen infects all above ground plant parts and can be highly destructive when environmental conditions favor disease.
The fungus Botrytis cinerea.
More than 200 plant genera, primarily broad leafed plants.
Signs and symptoms
- 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 fruit develop a pale, soft, watery rot
- Fruit symptoms occur on green and red fruit; on the plant and post-harvest
- Failed fruit infection results in white rings or halos on the fruit, called ghost spots
- Infected flowers turn brown and die
- In high humidity, fluffy gray spores cover infected plant parts; the spores are light brown-gray on black stalks
- Cool temperature 60-75°F (60-70°F optimum)
- Temperatures greater than 82°F suppress fungal growth and sporulation
- High humidity (greater than 80%)
- Spore germination is optimal with leaf wetness of 5 to 8 hours
Biology and disease cycle
- The fungus survives on numerous weed hosts, as a saprophyte 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
There are no tomato varieties with resistance to gray mold.
- Keep humidity and leaf wetness low by rolling up high tunnel sides, increasing ventilation, and avoiding overhead irrigation
- Space plants well to avoid excess humidity in dense plant canopies
- Remove infected stems, leaves and fruit in a plastic bag
- Clean up leaf debris on the ground
- Prune plants in the afternoon when the morning dew has dried
- Maintain consistent and adequate soil moisture to promote healthy plants and fruit
- Remove all plant debris at the end of the season
Fungicides are available for control of gray mold on tomato; however, they should only be used once cultural practices have been implemented. Fungicide-resistant gray mold has been reported on many crops. Apply fungicide according to label instructions. Rotate fungicide groups and/or tank mix fungicides to avoid producing fungicide-resistant isolates.
For a current list of fungicides for gray mold management see the Midwest Vegetable Production Guide.
Reviewed in 2016