Gray mold of tomatoes
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- 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; while 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 temperatures of 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. Place 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.
- They should only be used once cultural practices have been implemented.
- Apply fungicide according to label instructions.
- Fungicide-resistant gray mold has been reported on many crops.
- 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