BATON ROUGE, La. – Gray mold, a fungal disease common to tomatoes grown in greenhouses, has been showing up in operations from East Feliciana to Terrebonne parishes in recent months.

“This gray mold, which is caused by Botrytis cinerea, is one of the common fungal diseases of tomatoes grown in greenhouses,” said LSU AgCenter plant diagnostician Raghuwinder Singh.

The pathogen can infect all above-ground plant parts and has a wide host range, including other vegetables and ornamental plants, Singh said.

Prolonged periods of high humidity and cool to moderate temperatures favor disease development.

“Recent episodes of cool, wet weather with overcast conditions may have been a contributing factor in the outbreak of gray mold in Louisiana,” Singh said.

Rene Schmit, LSU AgCenter county agent in St. Charles Parish, hasn’t seen the disease in his area in at least the last six years.

However, Barton Joffrion, the LSU AgCenter county agent in Terrebonne Parish, said the disease has been reported in his area during the past three years.

Botrytis cinerea is an opportunistic pathogen that can rapidly colonize the dead tissue, Singh said.

“This tissue serves as the primary source of infection to susceptible plants,” Singh explained. “Initial leaf symptoms include light brown, water-soaked areas with yellow halo.”

The pathogen produces many spores and white to gray fungal growth on the plant parts. Tiny spores that look like clusters of grapes on branched fungal stalks can be easily seen with a hand lens, Singh said. “These spores readily become airborne and spread by air movement.”

Spore also may be spread on the clothing of greenhouse workers who come in contact with infected tissue. Narrow row spacing in greenhouses is common, but this practice plays an important role in the disease development and spore dispersal, Singh said.

“Dense canopy due to close spacing leads to extended periods of leaf wetness and favors spore germination,” he said adding, “Recirculating air within the greenhouse also aids in the spore dispersal.”

Gray mold can be managed through cultural and chemical practices. Singh said greenhouse growers should avoid sources of inoculum by removing plant debris, sanitize the greenhouse properly before and after the cropping season and scout plants regularly to catch infections at early stages.

Fungicides are effective if applied in a timely fashion. Apply them preventatively during favorable environmental conditions, including cool wet weather with overcast conditions, Singh said.

Growers should alternate fungicides with different modes of action to avoid resistance build up in gray mold.

“Fungicides such as Botran, Decree and Scala are recommended for greenhouse tomato production to manage gray mold,” Singh said.

Consult the LSU AgCenter Plant Disease Management Guide to find more details about the use of fungicides.