Jerry Brust, Retired IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Trying to diagnose a grower’s problem from pictures and descriptions can at times be very challenging. It is easy to have blinders on at the first look of a problem. The case in point are pictures from a grower where their greenhouse tomato plants were wilting somewhat and the leaves had darkened dead or dying spots and were curled and drying up (Fig. 1). My line of reasoning was that it could possibly be something such as nutritional deficiencies, a root disease or water stress. Or it could be a foliar disease possibly late blight (but not at this time of year) or botrytis. I sent my opinion on the possible causes to the grower and Educator with the weaselly explanation of “Sometimes what exactly is the cause of a problem we may not be able to figure out, but send a sample and we will see.” Samples were sent and were in good condition, and as unpredictable as can be, the problem was apparent about 2 minutes after examining the sample. The problem much to my chagrin was russet mites. Never saw such a severe case of these mites on tomatoes at this time of year.
Russet mites are extremely small eriophyid mites that are difficult to see for growers or consultants even with 10x magnification (you can see them at 14x magnification). Adult mites are tapered and wedge shaped with just two pairs of legs at their broader head end (Fig. 2). They are usually translucent or yellowish/tan to pink. Eggs are laid on the undersides of leaves, on leaf petioles and on stems on the lower portion of plants. The two nymphal instars usually do not move far from where they hatched and tend to mass on the edges of leaves. When damage to lower plant parts increases, the mites move up to younger foliage. As plants begin to die, mites may aggregate at the highest parts of the plant at which time they can be easily spread by wind or by people as they work in the greenhouse.
The egg to adult period is less than a week in warm weather. Russet mites are most active and abundant during hot, dry weather. This is what fooled me as I was not expecting to see these mites in a tomato greenhouse crop in late winter-early spring in such severe numbers. The source of infestations could come from other solanaceous crops, nightshades, jimson weed or petunias. The mite feeds primarily on plants in the Solanaceae, such as tomatoes, eggplant, pepper, potato and tomatillo.
Adults and nymphs normally feed on the undersides of lower leaves, petioles or stems, which causes a greasy appearance of the plant tissue (Fig. 1). Damaged leaves can turn yellow or dark, curl, become flaccid, wither, and fall from the plant. Mite feeding on fruits can cause longitudinal cracks and bronzing. Damage normally starts at the bottom of plants and moves upward and may be confused with nutritional problems, plant disease or root stress (oh yes!). The mite is usually scouted for by observing the tell-tale damage and then confirming the presence of the mite. My recommendation was that the grower remove via plastic bags the worst damaged (severely infected) plants from the GH and treat the rest with either sulfur or abamectin. The tomato russet mite has become a more frequent problem in greenhouse and even high tunnel tomatoes over the last 5-7 years in the Mid-Atlantic. It will need to be watched for carefully by tomato growers and especially specialists in the coming seasons.
Figure 1. Tomato leaf with a heavy infestation of russet mites.
Figure 2. Magnified (20x) tomato russet mites-tan wedges. Dark dots are tomato trichomes.