HORTICULTURE

Impacts of Plant Health Problems

Throughout history, plant diseases have had a large impact on humans and the environment. Plant diseases have impacted:

• food

• fiber

• ornamental plants 

• natural areas/ecosystems

Impacts of Plant Health Problems: Food

• Food

• 1840’s Great Famine in Ireland caused by late potato blight disease (fungal disease caused by Phytophthora infestans)

  • Result ~1 million deaths and ~1million emigrating from Ireland

• 1860”s coffee rust in Ceylon (now Sri Lanka)

  • Affected coffee imports to England, country switch to tea drinkers

Impacts of Plant Health Problems: Fiber

• Fiber

  • Boll weevil infestations around 1915 devastated cotton crops, led to migration of many sharecroppers north

Impacts of Plant Health Problems

• Ornamental plants 

  • Fusarium wilt is a devastating disease for queen palms

  • Introduction of Emerald Ash borer led to the devastation of many ash species including many ornamental varieties

Impacts of Plant Health Problems: Natural areas/Ecosystems

Natural areas/ecosystems

• Chestnut and American Elm trees were once commonplace in N. America

  • Introduction of the nonnative fungus Cryphonectria parasitica, infected chestnut trees with chestnut blight fungus and the tree was almost completely eliminated

  • American chestnut was a keystone species in the eastern hardwood forests, and its demise has altered forest ecosystems by reducing species diversity, reducing availability of hard mast, and changed soil and litter dynamics.

• The American elm met a similar fate with the introduction of Dutch elm disease caused by the fungus Ophiostoma ulmi

Plant Health Problems

Plants are subject to a wide variety of diseases and disorders that impact them in the following ways.

Diseases and disorders

• can affect the aesthetics of the plant

• can result in plant disfigurement

• can cause crop loss due to reductions in yield and quality

• or even plant death

Plant Disease Definition

DEFINITION: A plant disease is any condition caused by living or non-living agents that interferes with its normal growth and development.

• Note: plant health problems attributed to non-living agents are more commonly referred to as “disorders”

Causal Agents

Causal agents are living or nonliving 

• Biotic agents (also referred to as plant pathogens) are living factors that cause disease

• Include fungi, bacteria, phytoplasmas, nematodes, viruses or viroids

• Abiotic agents- are non-living factors that cause disease

• Include environmental factors and cultural factors

Biotic Agents: Fungi

Fungi- cause the majority of plant diseases 

Description: Small, usually microscopic, consisting of a mass of filaments or threadlike strands called hyphae. Similar to plants but lack chlorophyll and vascular tissues  Reproduce primarily by spores. 

Infection: Primarily through direct penetration of plant tissues or through natural openings or wounds. Usually requires free water on plant surfaces in order to infect so fungal diseases are frequently more common after periods of wet weather or when overhead irrigation is used. 

Spread: Primarily by wind, splashing water (from rain or irrigation), insects, and through cultural practices (e.g., on pruning shears, on pots, or in contaminated soil).

Biotic Agents: Bacteria

Description: Single-celled microorganisms with cell walls that do not have an organized nucleus. Most plant pathogenic bacteria are rod-shaped. Bacteria reproduce primarily by cell division. Thier initial presence or growth within a plant is usually not visible.

Infection: Primarily through wounds or sometimes natural openings. Unlike fungi, bacteria cannot penetrate healthy tissue. 

Spread: Plant to plant by splashing water (from rain or irrigation), by insects, and through a variety of cultural practices (e.g. on pruning shears or in plant or soil debris in pots). Bacteria can also be transmitted by seeds from infected plants.

Biotic Agents: Phytoplasmas

Description: A relatively new type of disease agent. Formerly called mycoplasma-like organisms or MLO’s. Closely related to bacteria but they lack a rigid cell wall. Phytoplasmas have no organized nucleus and are microscopic and unicellular. They can be irregular and amoeba-like or spiral in shape.

Infection: 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 so transmission occurs via phloem-feeding insects like leafhoppers

Spread: Insect vectors and mechanically by grafting infected plant parts onto healthy plants.

Biotic Agents: Viruses and Viroids

Description: Acellular agents consisting of protein (viruses) and nucleic acids (both viruses and viroids). Require a host to reproduce.

Infection: Because of the nature of these disease agents, wounds are necessary in order for viruses and viroids to infect

Spread: Primarily by the feeding activities of a number of insects, predominantly aphids, whiteflies, and leafhoppers. Also by nematodes and infected pollen. Human activities like grafting and mechanical transmission associated with the handling of infected plant material can also contribute.

Biotic Agents: Nematodes

Description: Tiny, translucent roundworms, just barely visible to the naked eye. Like fungi and bacteria, only a small portion of nematodes are parasitic to plants. These obligate parasites have specialized plant-piercing structures called stylets which they use to extract cellular contents. Of its three life stages: egg, larva, and adult, the latter two are most damaging to plants. 

Infection: Nematodes can infect plants through direct penetration usually at the tips of roots. They also infect through wounds and natural openings, and through the activities of vectors such as some insects.

Spread: Insect vectors, infected plant material, and by contaminated soil and plant debris.

Abiotic Agents

Abiotic disease agents can be categorized as being cultural or environmental. 

Cultural factors are very diverse in nature but imply human cause

• Examples include altering physical and chemical properties of soil, causing damage with trimmers, mowers or construction activities, improper planting incorrect timing or application of water and fertilizer, improper pruning, etc. 

Environmental factors are attributed to natural conditions

• Examples include precipitation patterns, air quality (and human caused pollution), wind, lightning, hail, snow cover, etc. 

Abiotic Agents

Abiotic agents are often overlooked as probable causes of plant health problems because they are very difficult to identify 

• Cannot be cultured or viewed microscopically. Requires close review and examination of the cultural and environmental history of the plant in question.

Distinguishing Biotic from Abiotic Causal Agents

Disease Triangle

Each side of the triangle represents a factor that influences the presence of a disease. 

All 3 factors must be present for a disease to occur

Disease Triangle for Crabapple Scab

disease causing fungus Venturia inaequalis

Crabapple tree

Cool, wet spring conditions

Managing Plant Health Problems

Identifying plant health problems starts with identifying symptoms- internal or external reactions to disease

• It is not uncommon for a diseased plant to exhibit more than one symptom

  • Ex: Tomato plants experiencing a disease called Septoria first develop leaf spot approximately 1⁄4 inch in diameter with dark margins and tan centers scattered over the surface of the leaf. However, as the disease spreads, the leaves quickly develop a blighted appearance as the leaves turn completely brown and shrivel.

Common Symptoms of Plant Disease

• Leaf spot- dead, discolored or injured areas of tissue which usually have distinct margins; spots often appear on leaves or fruit

Common Symptoms of Plant Disease

• Blight- rapid yellowing, browning, collapse, and death of leaves, shoots, stems, flowers, or the entire plant

Common Symptoms of Plant Disease

• Chlorosis- yellowing of leaves and stems which are normally green

Common Symptoms of Plant Disease

• Necrosis- browning or blackening of areas on a plant indicative of the death of plant cells; can occur on leaves, stems, roots, leaf margins, veins, etc. Unlike chlorosis, necrosis is irreversible.

Common Symptoms of Plant Disease

• Wilt- loss of turgor or drooping of leaves, shoots, or the entire plant due to lack of water

Common Symptoms of Plant Disease

• Distortion- twisting or other abnormal traits of leaves, stems, and shoots

Common Symptoms of Plant Disease

• Mosaic- uneven pattern of yellow, light green or dark green, usually on leaves

Common Symptoms of Plant Disease

• Canker- dead area on a stem or branch; can be sunken, swollen, or discolored and are usually distinguished from adjacent healthy tissues by color

Common Symptoms of Plant Disease

• Rot- breakdown and decay of plant tissue, often used to describe conditions in roots and fruit

Common Symptoms of Plant Disease

• Dieback- death of the tips of leaves, shoots, and stems; failure of branches to develop, especially in the spring

Common Symptoms of Plant Disease

• Witches’ broom- abnormal proliferation of shoots from the same point on a plant resulting in a bushy, broom-like appearance

Common Symptoms of Plant Disease

• Gall- a swelling or abnormal growth of plant tissues; can develop on leaves, stems, and roots

Common Symptoms of Plant Disease

• Stunt- abnormally small sized plant parts due to the failure of those plant parts to grow to full size; often used to describe an entire plant