PSYLLID YELLOWS is a disorder that can occur when potato psyllid nymphs feed on potato plants. The nymphs inject a toxin into the plant when they feed. This results in a range of growth disorders that are collectively described as psyllid yellows. The first symptoms include upward folding of potato leaflets and yellowing, usually starting at the top of the plant. The yellowing can be very bright or almost florescent and fading to white, sometimes with purple on the leaf edges. Severe symptoms affect the entire plant and may also include clusters of small leaves that develop in axillary buds at the top of the plant, which can give the plant a "bushy" appearance. Psyllid yellows can also include misshapen tubers and second growth (heat sprouts and chain tubers) when the disorder is severe. The severity of psyllid yellows is proportional to the number of nymphs on the plant; three or four nymphs can cause some symptoms, but more are needed to produce severe symptoms.
SCOUT FOR PSYLLID NYMPHS: If there have been gaps in your potato psyllid controls, then migrating psyllids can move into your potato crop, and deposit eggs that soon hatch into nymphs. Yellow sticky cards are a great way to monitor the winged adult life stage of potato psyllids as they migrate in and out of potato fields, but they do not help you know if eggs and nymphs are present. Collecting leaf samples is a good way to scout for the juvenile life stages (eggs and nymphs). Collect leaves from 10 plants at 10 locations at the edge of the field. Intensive sampling is recommended because infestations tend to be spotty. Examine the underside of fully expanded leaves taken from the top-middle of the plant. It helps to use a magnifying lens. Potato psyllid nymphs are very distinctive; they are small, flat, and oval. If you find them, you should consider applying an insecticide that targets juvenile life stages.