Smudges are defined as dark spots that are often created on the foliage following the fall of honeydew produced by the aphids. These spots are caused by the proliferation of microscopic fungi, which feed on the sugar contained in the honeydew; the fumaggini are certainly harmful from an aesthetic point of view, since they smear leaves and fruits; if they proliferate excessively they are also harmful for the whole plant, because they inhibit the photosynthesis of the leaves covered by the fungi. With the passage of time the smoke stains can expand a lot, weakening the plant.
Certainly prevention against aphids solves this problem at the root; once the aphids are spread on the plant the fumaggini tend to develop with great probability.
Traditionally the fight against this type of fungus is carried out with the use of products based on sulfur or copper, which inhibit the development of fungi; generally the fumaggini tend to stop developing after a couple of treatments based on sulfur, to be carried out far from the blooms, to avoid disturbing bees and other useful insects.
If the problem occurs on a houseplant, or in any case in small areas, it is also possible to remove the maize mechanically, with the aid of a damp microfibre cloth; however we will have to pay attention to completely remove the fungus, which otherwise will tend to develop rapidly.
Certainly a constant control of the presence of aphids on the plants solves the problem of the fumaggina at the root; in the case of infestation by aphids, the removal of the honeydew from the foliage from them avoids the development of fungi.