Viruses are a threat that many growers remain unaware of. They are usually spread by insect feeding, amd can wreak devastation in a clone garden. Viruses often sit latent until ideal conditions present themselves, at which point they can produce symptoms ranging from slight streaking to induced nutrient deficiency or full plant collapse.
In Australia the main vector of viruses are thrips. If your mother plants have ever had thrips problems in the past they will likely to be carrying a virus which will be passed down to any clonal generation. Thrips will usually be found hiding in the growing tip of the plant; they are long and slender and can be seen with the naked eye if you look close enough .
If you have any flowering male plants in the vicinity this is where you will find the majority of the thrips, as they enjoy feeding on the pollen. Thrips produce a sticky honeydew on the leaf surface which often turns black from a mold that grows on it, as well as leaving feeding marks on it.