Warning Signs
Most of the pathogens described in this guide are very difficult to identify from symptoms alone. A few general symptoms, however, can be cause for concern.
Mosaic – An angular, sharply bordered pattern of light and dark areas on leaves, typically divided by small leaf veins. Sometimes best seen when leaf tissue is held up to the light and viewed from beneath.
Stripe or streak – In monocotyledonous plants such as grasses or lilies, mosaic areas tend to become elongated, in a symptom called streaking or striping. This symptom is also sometimes best seen from the underside of the leaf.
Mottle – A variegated pattern with rounded, diffuse boundaries between light and dark areas. This can be a very difficult symptom to see clearly.
Ringspots – Circular patterns, often consisting of a light-colored ring around darker leaf tissue. Ringspots can be yellow (chlorotic), red, or purple. They can even be brown and dead. They often form a target-like bullseye pattern.
Chlorosis – Fading due to lack of chlorophyll. This fading can be general throughout the leaf, around leaf edges, or can form spots, streaks, ringspots, or other patterns. Chlorotic areas can be pale green, yellow, reddish, or white depending on what pigments besides chlorophyll are present in the plant.
Flower breaking – Loss of flower pigment, causing small light or dark streaks on petals.
Veinbanding, vein yellowing, or vein clearing – Yellowing (chlorosis) or translucent clearing in the tissue around leaf veins causing a banded appearance.
Dwarfing or stunting – A general reduction in growth, sometimes apparent only when compared with healthy plants of the same variety.
Shoestringing, leaf narrowing, or blistering – A striking narrowing of the newest foliage of the plant, sometimes accompanied by a dark green blistering or bubbling of leaf tissue.
Line pattern – Also known as oak-leaf pattern. A green-yellow patern formed by single or multiple irregular lines or bands of the leaf tissue.
Necrotic spotting or flecking – Small areas of dead tissue in the leaves.
Stem necrosis – Small black areas on stems.
Cankerous tumors – Generally in woody plants, swellings in stems and branches. These swellings sometimes have a roughened or cracked surface.