These diseases are caused by viruses that live in plant cells. Here they alter the cell’s internal processes causing the cell to change how it works and performs. This produces a wide range of different symptoms, depending on which virus is involved and which plant it has infected. These include discolouration, distortion, stunted growth and/or poor or irregular growth.
Some symptoms can be very hard to distinguish from other problems, such as some plant disorders, including mineral deficiencies, other diseases or even weedkiller damage.
While virus infections can cause severely distorted growth, they rarely kill their host plant. However, an infected plant becomes a source of infection for other plants.
Recent new viral infections to affect garden plants include canna yellow mottle virus (CaYMV) and canna yellow streak virus (CaYSV), hellebore net necrosis virus (HeNNV) and impatiens necrotic spot virus (INSV).
Not all plant viruses are bad and affect plant growth. Colour breaks in the flowers of some plants, tulips being a good example, originally came about through viral infections.
Leaves Yellow or pale green discolouration in various patterns, such as streaks, splashes, mottles, mosaic patterns, spots or ringspots. There may also be areas of brown, dead tissue – necrosis.
Leaf growth may also be stunted or distorted, often with a very reduced leaf size or leaf area, and including leaf crinkling, leaf rolling or leaves bent downwards.
Flowers Small, misshapen or distorted flowers. There may also be white streaks where colour is totally missing or patches of other colours.
Fruit Similar to flowers, affected fruit may be distorted, malformed or irregularly shaped, and with irregular colour patterns.
Shoots A proliferation of congested tufts of stunted stems – called a witches’ broom.
Viruses are usually named after the first plant they were originally discovered on and the symptoms they produce, then with a shortened letter version, such as tomato spotted wilt virus (TSWV), cauliflower mosaic virus (CaMV), tobacco mosaic virus (TMV), which attacks members of the Solanaceae family including tomatoes.