Tomato spotted wilt virus (TSWV) causes tomato plants to appear dusty or dirty. Leaves become distorted and stems often have sunken, bronze cankers. Plants may be stunted and fruits and leaves develop yellow rings. The disease is transmitted by western flower thrips, very tiny tubular-shaped insects. Virus-infected plants should be removed and destroyed. See CSU Extension fact sheet 2.949, Recognizing tomato problems for more information about TSWV and the psyllid and early blight concerns discussed below.
Avoid confusing TSWV with Potato/tomato psyllid, the insect cause of "psyllid yellows". This problem results from the plant's reaction to the psyllid's toxic saliva. Psyllid "yellows" causes the top leaves and stems of tomatoes to turn purplish yellow. Psyllids superficially resemble aphids and produce excrement that looks like white sugar or salt sprinkled among the leaves. Abundant, tasteless fruit is produced on psyllid-infested tomato plants. Infested plants are generally best removed.
Early blight causes yellowing and dropping of lower tomato leaves. Usually infested leaves have target-shaped spots of varying shades of brown. Avoid splashing water in the leaf canopy and make sure plants receive sufficient irrigation. Clean up plant debris in the fall to remove over wintering fungal organisms from the garden area.
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Updated Tuesday, July 22, 2014