A plant disease is any abnormal condition that alters the appearance or function of a plant. It is a physiological process that affects some or all plant functions. Disease may also reduce the yield and quality of the harvested product. Unlike plant injuries that occur instantly, disease is a process of change that occurs over time.
Abiotic diseases, or non-infectious diseases are caused by conditions that are external to the plant such as nutritional deficiencies, drought, soil compaction, salt injury, ice, or sun scorch. These are non-living factors and therefore cannot spread from plant to plant. However, they are common and should be considered when assessing plant health.
Biotic diseases, or infectious diseases are caused by living organisms. When they infect plants and cause disease, they are called pathogens. Pathogens can spread from plant to plant and may infect any type of plant tissue including the leaves, stems, crown, shoots, roots, tubers, fruits, seeds, and vascular tissue.
Visible effects of disease on plants are called symptoms. Any detectable changes in colour, shape, and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom.
Signs of plant disease are physical evidence of the pathogen, for example, fungal fruiting bodies, bacterial ooze, or nematode cysts. Signs also can help with plant disease identification.
Types of Plant Pathogens
The plant invaded by a pathogen and serving as its food source is referred to as a host. Plant pathogens are very similar to those that cause disease in humans and animals.
A. Fungi and Fungal-like Organisms (FLO)
Fungi and FLOs are collectively the largest cause of disease in plants. These organisms are unable to manufacture their food, lack chlorophyll, have filamentous growth and some may reproduce through spores. Some fungi and FLOs are also able to overwinter on plant debris or in the soil, making disease management extra challenging.
LEFT: fungal-like organisms, Phytophthora infestans of potato; RIGHT: fungus, Fusarium causing wilt in tomato vines.
A famous and historical example of fungal disease incidence was the Irish potato famine that occurred in 1847. Potatoes infected with Phytophthora late blight caused the vines to die prematurely and tubers to rot in the ground and storage. Because of this disease, approximately 2 million people either starved to death or immigrated to the United States.
B. Bacteria
Bacteria are single-celled microscopic organisms with cell walls that reproduce by binary fission (one cell split into two). Introduction to the plant must occur through natural openings or wounds in the plant. Bacteria overwinter primarily in soil and in or on plant material that does not decompose. Some survive in insect vectors.
Bacterial lesions are often blocky or angular and later have yellow halos around them. Left: bacterial blight of geranium caused by Xanthomonas hortorum pv. Pelargonii. Right: Citrus canker is a devastating bacterial disease that has no cure. Eradication is the only method to prevent the spread to other trees. Vectors play a crucial role in dispersal.
C. Phytoplasmas
Phytoplasmas are microscopic, bacteria-like organisms that lack cell walls and thus appear filamentous.
D. Viruses and Viroids
Viruses are intercellular (live inside the cell) nucleic acid particles with a protein coat that infect other living organisms and replicate inside the infected hosts. Viroids are virus-like particles but lack a protein coat. Viruses and viroids are primarily transmitted by vectors including insects, nematodes, and fungi, which introduce the virus or viroid during feeding. Viruses and viroids also can be transmitted through seed, vegetative propagation and pruning.
E. Nematodes
Nematodes are microscopic worm-like animals. The majority of nematodes are soil-dwelling animals and move with soil. However, some are transmitted through insects and infect above-ground plant parts.
F. Parasitic Higher Plants
These plants contain chlorophyll; however, they are unable to produce their food. They parasitise other plants to obtain nutrients and water. Examples include mistletoe and dodder.
Disease Triangle
Three components are necessary for disease to occur in any plant system:
- A susceptible host plant.
- A virulent pathogen.
- A favourable environment.
LEFT: Disease (shaded region) will occur if a susceptible host plant is in intimate association with a virulent plant pathogen under favourable environmental conditions. This concept is represented by the shaded portion of the diagram above. When there is a high degree of overlap (as the shaded area becomes larger), there will be a moderate to high amount of disease. RIGHT: Variables within each component of the disease triangle may affect the presence of disease. This diagram represents a system in which the host is displaying disease resistance even in intimate association with the pathogen under favourable environmental conditions.
Only when all three of these components are present at the same time, does disease occur.
Within each of these components – host, pathogen, and environment – numerous variables affect disease incidence and severity. Disease incidence is the amount of plant units that are diseased in an area. This helps predict the spread of disease. Disease severity is the amount of tissue affected by a diseased plant.
Disease Cycles
For disease to develop, a pathogen must be present and successfully invade plant host tissues and cells. The chain of events involved in disease development includes inoculation, penetration, infection, incubation, reproduction, and survival.
A. Inoculation
This describes the introduction of the plant pathogen to the host. Different pathogen groups employ different inoculation methods that are equipped with various specialised mechanisms that aid in the inoculation process. For example: some fungal pathogens release spores into the air, and the spores are then spread with the aid of air currents.
B Penetration
Wound sites and natural openings such as stomata and hydathodes, facilitate the entrance of some plant pathogens; others have evolved unique mechanisms for direct penetration. Fungi and nematodes can actively penetrate host tissues and cells if environmental conditions, such as moisture and temperature, are favourable for the penetration process.
C. Infection
This occurs when the pathogen invades the plant tissue and establishes a parasitic relationship between itself and the plant. Viruses, bacteria and phytoplasmas are not able to actively penetrate or enter plant host tissues; therefore, they must rely on other methods to infect plant tissues and cells. Associations with insect vectors have been established by the pathogens to aid inoculation and dispersal.
D. Incubation
Once inside the plant, pathogens may undergo an incubation period and remain latent for some time before initiating disease.
E. Reproduction
Plant pathogens can reproduce sexually and asexually. It is dependent on the pathogen.
F. Survival
Plant pathogens have evolved so they can survive prolonged periods of unfavourable weather conditions. For example: Diplodia tip blight, affecting pine, is a fungal disease which reduces the amount of UV light penetrating and prevents cell death. In addition, some nematode species will lay their eggs within a cuticle casing. The casing is very hard and prevents other microbes and chemicals from penetrating killing the eggs before hatching.
If any step is disturbed in the cycle, the disease will be less severe or fail to develop. Knowing and understanding the disease cycle for a particular disease is very helpful in managing the disease. There are two types of disease cycles – monocyclic and poly-cyclic:
- Monocyclic pathogen diseases: these pathogens complete their life cycles once in a season. Example: Dutch elm disease.
- Polycyclic pathogen diseases: the pathogens complete their life cycles multiple times during a season resulting in secondary infections as the growing season progresses. Example: apple powdery mildew and black spot on rose.
Stages in the development of a disease cycle.