Fugal Diseases
Stem Blight
This is really the only potentially devastating fungal disease for brugmansias in cultivation and can be hard to recognise. It causes dry rot on the stems and leaf stalks. Infected plants lack vigour, even though the root system seems to be normal. First signs can be the yellowing of leaves or black-brown spots particularly on their nodes. New plant cuttings can be washed and cleaned to reduce the threat of introducing stem blight to a healthy collection.
Leaf Spot
Leaf spot is normally encountered in cool damp weather conditions. Symptoms include small brown spots several millimetres in diametre with a halo of yellow leaf tissue around them. General copper based garden fungicides are effective.
Botrytis
Botrytis cinerea or grey mould can be a problem for over wintering plants in poorly ventilated basements or greenhouses. The fungus grows like a dense grey carpet on any dead tissue, it produces millions of spores which eventually diffuse though the entire area. Effective preventative measures are the removal of all dead plant material and withering flowers together with sufficient ventilation.
Phythium and Phytophthora
Responsible for root and stem rot. In the overwintering phase, the plants are under stress which makes them susceptible to pathogenis fungi. Over-wintering and an excessively wet root ball combined with cool temperatures favour the infection. Prevent by not over watering in winter keeping the root ball only slightly moist.
Pests
Red Spider Mite
The first signs are minute white dots on the leaves, looking rather like a fine dusting of flour. The mites can be seen on the underside, often with the whitish frass of the egg shells from which they have hatched. The adults are 0.5mm long and of pale colour with two darker patches about halfway across their legs. They become blood red in cool conditions, hence the name. They favour warm conditions and low humidity and can multiply extremely rapidly. As infestations become heavier, fine webbing can be seen between the leaf stalks. Heavy infestations cause extensive discolouration and drying of the leaves which become silvery. Eventually the mites become so crowded they migrate to other leaves by gathering at the tips before producing a shrimp coloured ball which falls to adjacent leaves. Infestations can be prevented by daily hosing of leaf undersides. Biological controls are extremely effective. The predatory mite Phytoseiulus persimilis, which specialises in feeding on red spider mite and broad spider mite is now widely available, know in the industry simply as 'Persimilis'. It is supplied in vermiculite which can be scattered around the base of the plant. Other biological controls include a group of minute black ladybird beetles. Also, a garden centre chemical pesticide spray can be effective at reducing red spider mite. Rotating between different chemicals can reduce development to resistance.
Broad Mite
These mites are smaller than than red spider mite. They attack the youngest leaves, youngest stems and young flower buds. Light infestations can be detected as a slightly dusty appearance to young leaves which expand abnormally slowly. As the infestation grows the leaves fail to reach maximum size and flower buds will drop off. Cucumeris is supplied in a granular medium such as vermiculite and can be sprinkled over the canopy and base as a preventative measure.
Aphids
Aphids have a great many species of various colours. Light infestations cause some leaf distortion while heavy infestations weaken young shoots. In Brugmansia they can be a particular problem in glasshouses and during winter storage. Outbreaks can be controlled by pest oils and soaps or by insecticides available at garden centres.
Shield Bugs
These can do enormous damage to Brugmansia plants, particularly in country gardens. Young leaves are deformed and crippled and appear to be either torn to pieces or full of small and larger holes surrounded by a yellowish edge. Shield bugs are active between June and September in the Northern hemisphere. Pyrethrum, Imidacloprid and Thiacloprid are effective.
Whitefly
Whiteflies resemble tiny white moths about 1.5mm long and are found on the underside of the leaf. Heavy infestations result in leaf distortion and severely weaken the plant. Whitesflies have developed a resistance to a variety of chemical insecticides but the systemtic Imidacloprid is generally effective as a spray.
Mealybugs
On brugmansias, they are typically found on the leaf undersides nessled up against the mail vein or leaf axils. Heavy infestations cause leaf deformations and black soot as a result of the copious amounts of honeydew deposits. Root ball should be inspected, as a secure measure of removing them the plant can be cut and re-rooted and the original rootball destroyed.
Thrips
Thrips and their wingless nymphs puncture and suck out the contents of cells on the leaf undersides. Flower Thrips also feed on the flowers. The adult insects are generally about a millimetre in length. Particularly good conditions for Thrips are are dry and warm weather. Infected leaves have a yellow or silver-grey appearance from the pinprick lesions. High infestations may cause die off and withering of the plant parts. Similar looking damage may be caused by spider mites. Thrips can be treated successfully with systemic insecticides like Imidacloprid and Thiacloprid.
Caterpillars
Caterpillars are a more serious pest on younger plants. Individual caterpillars can be picked off and moved! They are generally found on the underside of the leaf hiding!
Other Pests
Other pests includes Beetles (Australia, Asia and S.America only), Earwigs (See European Earwig), Grasshoppers (Australia only) and Fungus gnats (sign of over-watering).
This information has been extrapolated from reference:
Hay.A, Gottschalk.M, Holguin.A, 2012. Huandaj - Brugmansia. Royal Botanic Gardens, Kew, Richmond, Surrey, UK.