The term “disease” as used in reference to plants has quite a broad meaning, which describes any sustained and progressive weakening of the plant’s structure or functions of its parts. There are two general groups of causative factors: non-living (abiotic) which includes environmental factors such as lack of a certain nutrient in the soil or being subject to salt spray in a coastal location which impedes normal leaf functions. These factors and others can affect a plant by placing it under abiotic stress, manifested by a range of conditions such as stunted growth or deformed leaves or other organs. The second group consists of living biotic agents (pathogens), including fungi, viruses, bacteria, plant parasites and others. Pathogenic plant diseases are varied and widespread and can be devastating in certain instances. But because the pathogens have in many cases coevolved with certain species or groups of species, individual plant diseases may be confined to a limited number of species; in certain cases they are host specific. For example, the fungus-like blight seriously affecting potatoes and tomatoes, is not a general threat to other garden plants. By contrast, insect pests often affect a wide range of species, the extreme example being locusts in Africa, which when a severe outbreak occurs, all types of vegetation in the path of the locusts are decimated.
The best management measures to take against the threat of plant disease is to maintain a healthy garden. A tree or shrub suffering from abiotic stress conditions is more likely to suffer from and succumb to a pathogenic attack because of lowered resistance. On the other hand, a healthy vigorous plant may be able to overcome the pathogenic infection on its own or with proper treatment and recover fully. In most cases where a healthy, well cared-for plant exhibits disease symptoms, the malady will be easier to diagnose because abiotic factors are presumed not to be at play. Unfortunately, it is not always that easy because symptoms are often not clear-cut. Visible evidence of a disease is designated as either a symptom or a sign. Abnormal performance such as dieback, leaf discoloring, wilting or areas of wood decay on the trunk or branches are considered symptoms. Clear evidence of a causal agent such as fungal growth on any part of the plant or the presence of a colonizing parasitic plant are referred to as signs.
Keeping a watchful eye on garden plants and reading the signs that something is abnormal is important. In so doing, take action to prune off any dead branches, reduce the epiphytic load of plants like Spanish moss which can impair normal leaf functions and cut off any parasitic plants like Mistletoe. An estimated 85% of plant diseases are caused by fungi or fungi-like organisms. If fungus growth is a problem, it may be exacerbated by high humidity below the garden canopy. Thinning of branches to allow more light to enter will reduce the humidity level and discourage fungal growth. Removal of dead plants and dead wood is also recommended.
It is beyond the scope of this blog to discuss individual plant disease diagnosis and treatment. Chemical treatments are available for most maladies once a diagnosis has been made.