Plant leaves can change colour for a variety of reasons. Nutrient deficiences or excesses can affect a loss of green colour (yellowing) or developmental absence of green colouring (chlorosis), and these conditions can be interveinal or involve the entire leaf. Occasionally, spontaneous mutations leading to variegation occur in leaf tissue, but these instances are rare. Insects with piercing/sucking mouthparts, such as aphids and leafhoppers (common plant pests), often carry plant viruses in their saliva. Susceptible plants can thus become infected by such viruses quite easily. Symptoms of virus infection varies considerably from species to species, but variegation is common, as is distortion of the leaf margins. At this time of year, when plants are slowing down, virus symptoms may be more prominent. Normally, these kinds of problems (nutritional and viral) are not serious, and disappear over time, with good culture.
