Aaack, I have tomato russet mites! Well, the mites are on the tomato plants. Fortunately, I spotted them early before they could do too much damage to the plants.
The mites are too small to see with your naked eye, only .02 mm long. A lens with at least 14X magnification is needed to see them. So identification is generally done based on symptoms rather than sighting. The first sign of an infestation is rather subtle, the stems of the plants will turn from bright green to a russet color as you can see in the photo above. Trouble starts at the base of the plant and works its way up. The leaves at the bottom of the plant will start to die and eventually turn a russet color as well. In hot weather the entire plant can become defoliated fairly quickly as the infestation works its way up the plant. If the plant is not treated it will die.
Only one of my plants, a Blue Beech paste tomato is showing serious signs of infestation right now. The stems on that plant are russeted up to about 2 feet and the bottom leaves are just starting to die as you can see below. You can also see the traces of the treatment I applied in both of the photos.
The treatment I used is wettable sulfur, which happens to be an organic treatment. The recommended rate is 1 to 2 tablespoons of wettable sulfur powder per gallon of water. I used 1 tablespoon of sulfur in a gallon of water and thoroughly sprayed the affected plant, making sure to get the stems and both sides of the leaves. I also sprayed the bases of the neighboring plants. If the infestation continues to spread up the plant then I'll try a 2 tablespoon concentration next week. Never spray with sulfur when the temperature is 90F or above, you will damage the plants. Spray only when there is no wind and be sure to keep the spray off your skin.