Gray mold is a fungal infection found on tomatoes and about 200 other host plants. It is found most often in greenhouses and is a source of postharvest rot for tomatoes in storage. All tomato varieties are susceptible to gray mold. Preventing gray mold is important as there is no known treatment once a plant is infected.
Gray mold can appear on either tomato leaves or on the fruit. On leaves, gray or tan spots are the first symptom. As the mold progresses, there will be a gray, felt-like covering over the leaf. If a gray mold spore lands on tomato fruit, an 1/8-inch ring with a dark spot in the center appears. The fruit's appearance is affected, but the fruit will not rot. On tomato seedlings, symptoms appear at or just below the soil line.
Botrytis cinerea is the single-celled spore that causes gray mold. In greenhouses, gray mold develops when temperatures are consistently below 70 degrees Fahrenheit and humidity exceeds 90 percent. The spore tends to infect plant tissue that is already damaged and rarely affects healthy tissue. Young transplants are more sensitive to gray mold when an excess of nitrogen is also present. Plant foliage that is wet for a prolonged period of time is necessary for spore development. Poor air circulation also contributes to this problem. Mold spores need water covering to completely develop and multiply. Spores are easily carried by wind and can spread mold quickly.
Once a plant is infected, there is no effective treatment. Online resources, such as Cornell University's Vegetable MD Online, offer information on the most recent developments for dealing with gray mold and other plant diseases.
In greenhouses, several precautions can be taken to prevent gray mold development. Keep temperatures above 70 F, and relative humidity below 80 percent. Adequate air circulation helps keep foliage dry and reduces the likelihood that gray mold will develop. Avoid watering plants through overhead systems to keep foliage as dry as possible. When pruning plants, do so close to the stem. Remove and discard any pruned, dead or decaying plant material.
In fields, space plants to permit plenty of air circulation. Make sure that the soil is well drained and nitrogen levels are balanced. Weed frequently to encourage air circulation. Certain fungicides are effective if applied before spores develop; consult your county extension office for the most effective treatment in your area.
Discarding Infected Plants
Infected plants can be plowed under or burned. Allow plants to completely decompose before replanting. Infected plant material should not be composted.