Disaster seemed imminent at the Carson City Community Garden recently. Tomato plants that had been lovingly tended with dreams of tasty fruit started dying rapidly from the ground up. Master Gardener volunteers Jocelyn and Don feared the cause might be the devastating disease late blight, which caused the Irish potato famine. We rushed samples to the Department of Agriculture state plant pathologist, Dr. Wang, hoping that the garden wouldn’t be contaminated for future crops of tomatoes, potatoes, eggplant and peppers.
Fortunately, while the symptoms seemed similar to those of late blight, Dr. Wang found the cause was the tomato russet mite, a tiny insect invisible without a 14x hand lens. Because they are so small, they are rarely noticed until hundreds infest a plant. As they suck the cell contents from leaves, the leaves and stems turn a greasy bronze or russet color. If left uncontrolled, the plant will die. The mites spread rapidly from plant to plant with the wind and, as the population explodes, plants may be defoliated.
Eggs are laid on the undersides of leaves and stems on the lower part of the plant. The mites move up the plant, damaging younger foliage. Gardeners can transport mites from infested plants to uninfested plants on their hands, gloves, clothes and tools.
The University of California, Davis, recommends not planting tomatoes near petunias or any other plants in the nightshade family (potato, eggplant, pepper, nicotiana, tomatillo). Washing plants off and wetting all surfaces, including the undersides of leaves, will raise the humidity around the plant and deter the mites. Insecticidal soap and neem oil treatments also can help. Sulfur dust and wettable sulfur are effective controls; however, as with any pesticide, read the label and wear eye and nose/mouth protection, long pants, a long-sleeved shirt and a washable hat. Be sure to check the label for temperature restrictions on applying sulfur, which can easily burn plants if used improperly. Be careful to bag and seal all infested parts as you remove them from the plants. Do not compost any part of an infested tomato plant.
Thank goodness, we avoided the dreaded late blight. While the mite problem damages plants, it can be resolved with the tactics I mentioned. Or, infested plants can be removed completely and destroyed. However late blight is much harder to control and can be a problem for years.
The next Grow Your Own class, Urban Organic Farming, is from 2:30 to 4:30 p.m. Monday. Call 775-887-2252 for more information.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu.