As previously mentioned, the two main categories of agents capable of causing plant disease are abiotic and biotic, nonliving and living factors, respectively. In natural settings, it is not uncommon for plants to be affected by both abiotic and biotic problems and it is often difficult to determine which came first. However, in many cases, plants which are initially stressed by abiotic factors will be weakened and therefore predisposed to biotic problems. For example, rhododendrons whose roots have been weakened by drought stress are more susceptible to the fungal root rot caused by Armillaria than their healthy counterparts.

Biotic Agents
Unlike abiotic agents, biotic agents are able to spread from plant to plant. This is an important attribute since the number of diseased plants can increase over time as a direct result of the growth, multiplication, and movement of the causal agent.

Fungi
The majority of plant diseases are caused by fungi. There are well over 100,000 different species of fungi and only a surprisingly small portion of them are capable of causing plant diseases. Fungi are similar to plants but lack chlorophyll and the conductive or vascular tissues that are found in ferns and seed plants. Fungi are small, usually microscopic organisms, that consist of a mass of filaments or threadlike strands called hyphae. The primary means of reproduction and spread of fungi are by spores. Many fungi produce more than one type of spore during their life span and this often influences how diseases are spread.

The primary ways that fungi infect plants is through direct penetration of tissues although they also infect through natural openings such as stomates, hydathodes, and lenticels, or through wounds. In most cases, fungi require free water on plant surfaces in order to infect. Because of this environmental criterion, fungal diseases are frequently more common after periods of wet weather or when overhead irrigation is used. Fungi are primarily spread by wind, splashing water (from rain or irrigation), insects, and through cultural practices (e.g., on pruning shears, on pots, or in contaminated soil).

Bacteria
Bacteria are very different than fungi and are single-celled microorganisms that do not have an organized nucleus. As with fungi, only a small percentage of bacteria found in nature are capable of invading living plants and causing plant disease. Bacteria have cell walls and most plant pathogenic bacteria are rod-shaped. Bacteria reproduce primarily by cell division. This can occur in a short period of time and their initial presence or growth within a plant is usually not visible.

Unlike fungi, bacteria cannot invade plant tissues that are intact and healthy. As a consequence, most infections occur through wounds. Bacteria also infect through wounds made by insects during their feeding activities and through natural openings in a plant such as nectaries or stomates. Bacteria are spread from plant to plant by splashing water (from rain or irrigation), by insects, and through a variety of cultural practices (e.g., as contaminants on pruning shears, in plant or soil debris in pots). Bacteria can also be transmitted by seeds from infected plants.

Phytoplasmas
Phytoplasmas are a relatively new type of disease agent which are closely related to bacteria but they lack a rigid cell wall. These organisms used to be called mycoplasma-like organisms or MLO's. As with bacteria, phytoplasmas have no organized nucleus and are microscopic and unicellular. They can be irregular and amoeba-like or spiral in shape. Phytoplasmas are tissue-specific and only live in the phloem or the nutrient-transport system of their plant hosts. Most phytoplasmas are incapable of living outside of their plant host or insect vectors.

Since phytoplasmas can't survive as free-living microorganisms, they are incapable of infecting plants without outside assistance, through insect vectors or mechanical means of transmission. As a consequence, the primary way that phytoplasmas are spread is through the activities of phloem-feeding insects such as leafhoppers. Phytoplasmas can also be spread mechanically by grafting infected plant parts onto healthy plants.

Viruses and Viroids
Viruses are unique plant pathogens since they consist of nucleic acid and a protein coat and have no cellular structures. Additionally, viruses are unable to replicate or reproduce without the aid of the components of the plant host cell. Viroids are even more simplistic than viruses since they lack a protein coat and only consist of nucleic acid.

Because of the nature of these disease agents, wounds are necessary in order for viruses and viroids to infect. Therefore, the primary means of spread is through the feeding activities of a number of insects, predominantly aphids, whiteflies, and leafhoppers. Viruses can also be spread by nematodes and in infected pollen. Human activities are also very important for spread of these disease agents. Included among these are grafting and mechanical transmission associated with the handling of infected plant material.

Nematodes
Nematodes are tiny, translucent roundworms, oftentimes just barely visible to the naked eye. As with fungi and bacteria, only a small portion of nematodes found in nature are parasitic to plants. Most nematodes have three life stages: egg, larva, and adult. The latter two stages are the most damaging to plants. Plant parasitic nematodes are obligate pathogens and have developed specialized structures called stylets that allow them to pierce plant cells and extract cell contents.

Nematodes can infect plants through direct penetration. This usually occurs at the tip of a root. They also infect through wounds, through natural openings, and through the activities of vectors such as some insects. Nematodes are also spread by infected plant material and by contaminated soil and plant debris.

Abiotic Agents
Plant health problems attributed to abiotic agents can also be referred to as disorders rather than diseases. Both terms are used to describe the same types of abnormalities in a plant although disorder usually implies the causal factor is nonliving whereas disease usually implies the causal factor is a living agent. Abiotic disease agents can be categorized as being environmental or cultural. These types of agents are often overlooked as probable causes of plant health problems because they are very difficult to identify since they cannot be cultured or viewed microscopically. As a consequence, the ability to pinpoint the causal factor requires close review and examination of the environmental and cultural history of the plant in question.

Environmental Factors
Many types of environmental factors cause plant diseases. Among these are unusual precipitation patterns resulting in drought or waterlogged soils, limited snow cover, excessive winds, lightning, hail, late spring or early autumn frosts, and extreme temperature fluctuations, especially during the winter. Air pollution is another important factor. Among the pollutants encountered in Connecticut are ozone, hydrogen fluoride, sulfur dioxide, ethylene, and peroxyacyl nitrate. Some of these compounds are more problematic in glasshouses and others primarily occur outdoors.

Environmental disease agents result in a wide variety of symptoms. For example, drought or dry soil conditions result in root damage and death. Nonwoody feeder roots, usually located in the top 15 inches of soil, are particularly sensitive and are the first ones affected. Without moisture, these roots shrivel and die. When these roots become nonfunctional, a water deficit develops since the roots cannot provide water to the top of the plant. Symptoms of drought vary with the plant species and the severity of the water deficit but are often not evident until sometime after the event has occurred - even as much as one or two years later! Symptoms include loss of turgor in needles and leaves, drooping, wilting, yellowing, premature leaf or needle drop, bark cracks, and twig and branch dieback. Leaves on deciduous trees often develop marginal scorch and interveinal necrosis whereas needles on evergreens turn brown. Drought-stressed trees and shrubs can also exhibit general thinning of the canopy, poor growth, and stunting. In extreme cases, drought can result in plant death.

Cultural Factors
Cultural factors associated with plant health problems are quite diverse. Among the common factors are site and soil attributes (e.g., pH, organic matter, drainage, soil type), planting practices (e.g., preparation of the rootball and planting hole, planting too deep or too shallow), plant hardiness, construction activities resulting in soil compaction or severing of roots, and mechanical injuries from lawn mowers and string trimmers.

Other types of problems result from incorrect or improperly timed pruning, incorrect mulching, fertilizing practices (e.g., incorrect timing, inappropriate applications resulting in toxicities or deficiencies), and watering practices (e.g., time of day, too much or too little, frequent, shallow watering).

Another type of cultural agent that can result in plant damage and death involves chemicals. Included in this category are deicing salts and misapplied pesticides, particularly herbicides. For example, when misapplied, herbicides can result in damage which varies with the particular compound and plant species. Symptoms can develop several days to weeks after exposure or, in some cases, not until the following spring. Symptoms include chlorosis, necrotic spotting, marginal scorch, twisting, growth abnormalities, leaf/needle drop, dieback, general decline, and plant death.

As with environmental factors, cultural factors can affect plant health in many ways and result in a wide range of symptoms. For example, when mulches are applied too close to the base of a plant and too thick, they can result in root and crown rots and asphyxiation of roots, respectively. This can cause plant decline and death.