Ornamental plant diseases can immensely change the plant's physical quality. In most cases, these diseases directly affect the plant's health and survival. Diseases can also affect the appearance of plants, making them unattractive to look at. Identifying diseases of ornamental plants is important in order to suppress, cure and prevent these diseases effectively.
Foliar diseases are the most common among plant diseases. They appear as leaf spots and scars. The two types of foliar diseases are brown spot (caused by the fungus Septoria glycines) and frogeye leaf spot) caused by the fungus Cercospora sojina). Foliar pathogens are normally active with high amounts of moisture and prolonged leaf wetness.
Irrigation early in the morning will help wash off leaf-bound dews filled with nutrients that encourage foliar fungal growth. Planting ornamental tropical plants in places where they can dry easily will help prevent foliar diseases. Watering earlier in the day will give enough time for the sun to dry the water. Avoid placing plants too close to water sprinklers to prevent foliar disease development.
Grayish or white powdery mildew is often a sign of fungal diseases. The powdery mildew coating itself is the fungus that grows on the surface of leaves. The leaves look as though someone has sprinkled talcum powder or powdered sugar on them.
Closely related species of fungi that belong to several genera in the family Erysiphaceae are the primary causes of powdery mildews. They are fairly host specific, so the fungus species that infects one type of plant may not affect another type of plant. Powdery mildew fungi attack only close connected plants; therefore, enough spacing of plants can discourage mildew transfers. High humidity encourages powdery mildew fungi; therefore, you may find them more often in shaded areas.
Crown and Root Rots
Almost all ornamental plants have diseases related to crown and root rot. High moisture and poor soil conditions, like in the case of clay soil, can cause the crowns and roots to rot. Freezing, thawing and injury caused by insects may also lead to crown damage.
Several species of soil borne pathogens in the genus Phytophthora cause crown and root rot diseases of most ornamental trees and shrubs. Plants can develop Phytophthora rot if soil around the base of the plant remains wet for prolonged periods, or when planted too deeply. The leaves of plants affected by crown and root rot appear drought stressed. The plants often wilt and die rapidly with the first warm weather of the season. Leaves may turn dull green, yellow, red or purplish.
Avoid planting in wet zones, such as places with standing water, or close to drainage areas. Do not overwater in order to avoid crown and root rots.
Viral and Phytoplasma Diseases
Pathogens that cause viral and phytoplasma diseases are not visible to the naked eye. They propagate through vegetation where the systemic infections usually occur. These pathogens move with the plant sap, which is why they are hard to eradicate once they infect plants.
Plants with viral and phytoplasma diseases appear streaked, spotted or mottled (mosaic). Flowers appear distorted, crinkled and streaked with contrasting white and yellow on normally dark colored flowers.
Often, the plant's seed may also become infected; therefore, avoiding planting these seeds will prevent spreading of the virus. Eliminating the source of the virus is the best way to prevent transfers of these diseases to other plants, such as vectors or insects that bore into plants that carry these pathogens.