Diseases in plants are caused by fungi, bacteria, and viruses. Fungi are the most damaging. They are known as pathogens, and most are saprophytes, i.e. living on dead matter, using it as a spring board for their attack on healthy plants. These parasitic varieties are highly specialised, the rusts and the mildews being the most well known. The fungi are able to survive the most adverse conditions because of tough chitin containing cells in strands of mycelium. The best time to attack them is in winter. Most reproduce by spores which are airborne and travel in the water or air. They contain the most serious risks for trees, and may wipe out whole varieties in a country.

Bacteria are less important. They are single celled and reproduce by meiosis. These along with viruses are often carried by insects, and cause annoying damage to the appearance of the plant. They are all controlled by a variety of fungicides, but the best measure is to burn all infected plants, use sterile soil, and maintain vigourous hygiene.

Although the diseases cause less concern than pests, they are difficult to control as they reproduce by millions of spores, which can get everywhere, even in the most sterile conditions. They are of particular concern to those who are growing ornamental crops, especially bulbs and cuttings, they are, although successful in treating them, must understand that fungi are an important part in helping plants to grow healthy and strong and often help to provide nutrients to the plant. Growers must learn to accept this, if they wish to gain better capital returns. There is also concern among scientists that new serious strains of bacterias and fungis may be released into the wild via import and export of plants. The best case is that of Dutch Elm Disease which wiped out 20 million Elms in Britain. Stringent border controls must be maintained as for pests, if we wish to keep this island from another disaster of this sort.