Gray mold of tomatoes
- Gray mold thrives in the high humidity found in high tunnel 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.
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