One of the main scourges of ornamental plantations is Armillariella mellea, a parasite fungus that can cause serious damage in old plantations, where there is a high proportion of weak and damaged trees. It is less common in small gardens, except in humid soils or in the proximity of recently felled areas. Fire blight, which appeared in Great Britain in 1957, but is almost unknown in Spain, is a terrible disease of Rosaceae (Pyrus, Cotoneaster, Sorbus, etc.), often causing death.
Amongst the insects, aphids, which bite the young shoots and leaves, and mealybugs cause important damage to older trees when they are found in large numbers; they produce a sometimes abundant molasses in which a powdery blackish fungus can develop (bold) which diminishes the decorative value of the trees. Elms, especially in the Midday period, are sometimes attacked by an insect whose larvae devour the parenchyma of the leaves, leaving only the nerves to subsist. The hobbyist, fortunately, has a very extensive range of fungicide or insecticide treatments that can be used when circumstances demand it and after having received advice from competent people, such as the agents of the Agricultural Extension Service for example.
The size of the trees sometimes precludes the application of treatments that are necessary.
Other alterations such as deficiency diseases, due to soil deficiencies, chlorosis caused by excess lime, cold attacks or sunburn, may give the appearance of parasitic diseases; having to decide on the treatment to be carried out, it is first advisable to determine the nature of the damage. Apart from the Agricultural Extension Service, qualified officials of the Agricultural Research Service, of agricultural institutes, of the horticultural services of large cities and technicians of the main nurseries can help amateurs to determine the diseases and parasites and advise them in the choice of treatments. One of the best ways to keep the garden in good condition is hygiene, by removing dead, sick or damaged branches and treating and healing wounds. With these cares the trees are able to resist in the best conditions to the attacks of the parasites.