Diseases that infect underground plant parts are prevalent in both herbaceous and woody ornamental plants. They can be caused by fungi, bacteria, or soil-borne nematodes. Infections that lead to the disintegration of underground tissues are difficult to manage because they are not visible. The symptoms are thus typically expressed late in the stage of extensive infestation when the disease has progressed beyond control. Lastly, management is difficult because soil treatments are ineffective. Some of the more severe pathogens are those that can persist for long periods without a living host. These survive in a dormant state or as resistant structures capable of surviving in the environment until they meet their next susceptible host.
Rhizoctonia and Phytopthora are root rots. These two fungi can attack the root systems of many different plants. Rhizoctonia prefers drier soil conditions, but in the greenhouse, it can produce an aerial web blight that infects foliage under high humidity. Below ground, Rhizoctonia may infect roots and stems and produce reddish cankers. Phytopthora is a water mold fungus that can be active in conditions of free moisture or flooding. Both fungi can produce damping-off disease, a name given to conditions where tender seedlings die from root and stem soil line infections. Bulb rots occur on plants with underground storage organs. Because these organs are often sites for storage reserves and are high in carbohydrates, they are especially susceptible to underground rots. Often the initiation of a bulb rot is due to a wound created by an insect or by mechanical means. This opportunity lowers the plant’s defense and allows for colonization by bacteria or fungi.
Bacteria can only enter the host tissue through wounds, but bacterial soft rots can be very destructive. These may originate in the leaves where bacteria enter and migrate to the bulb or rhizome or directly at the storage organ. Often a foul smell accompanies a bacterial soft rot infection.
Crown and collar rots occur at the soil line where the plant emerges. In this zone, the plants must be able to withstand the freeze-and-thaw cycle as well as the mechanical abrasion of soil particles. When wounds are produced, fungi and bacteria can invade the host tissue. A crown rot is typically associated with herbaceous plants. The tissue may turn brown to black in the localized area around the soil line. The discoloration may migrate upward and downward around the outside of the tissue from the point of infection. Eventually, when the pathogen has almost completely encircled the stem, the plant will begin to show wilting and dieback symptoms. Collar rots are typically associated with the woody stems of trees and shrubs at the soil line. Poorly drained soils and nutrient deficient soils can lead to plants being more susceptible to collar rots. As the pathogen enters a host tissue at the soil line, it may grow down into the roots where it can cause extensive damage. Control of these types of diseases is very difficult.