Plant Pathology

1. Plant Pathology

2. Plant Diseases • Plant diseases can be divided into two categories: infectious (caused by pathogens) and non-infectious (commonly called disorders) • Infectious diseases can be transmitted from plant to plant and produce signs resulting from the interactions of the pathogen, host and environment • There are three main types of infectious plant diseases: bacterial, fungal and viral • Non-infectious diseases (disorders) are caused by a range of non-living agents such as nutritional imbalances, environmental extremes, chemical toxicities, mechanical injuries and genetic problems • Disorders may produce signs that are similar to infectious diseases, and may also make plants more susceptible to infectious diseases

3. Infectious diseases in plants • Affect some or all plant functions • Disease may reduce yield and quality of harvested product • Disease is a process or a change that occurs over time • It does not occur instantly like injury

4. Symptoms and Signs of Plant Disease • Visible effects of disease on plants are called symptoms • Any detectable changes in color, shape, and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom • Signsof 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

5. Symptoms • Blight – A rapid discoloration and death of twigs, foliage, or flowers • Canker – Dead area on bark or stem, often sunken or raised • Chlorosis– yellowing (chlorosisis so generic that without additional details diagnosis is impossible) • Decline – Progressive decrease in plant vigor • Dieback – Progressive death of shoot, branch or root starting at the tip • Distortion  – malformed plant tissue • Gall or gall-like – Abnormal localized swelling or enlargement of plant part.  It could be caused by insects, mites, diseases, or abiotic disorders • Gummosis – Exudation of gum or sap • Leaf distortion – The leaf could be twisted, cupped, rolled, or otherwise deformed • Leaf scorch – Burning along the leaf margin and into the leaf from the margin • Leaf spot – A spot or lesion on the leaf • Mosaic – varying patterns of light and dark plant tissue • Necrosis – dead tissue (necrotic areas are also so generic that without additional details diagnosis is impossible) • Stunting – lack of growth • Wilt – General wilting of the plant or plant part • Witches broom – Abnormal broom-like growth of many weak shoots

6. Symptoms • Even though a plant has symptoms on a specific part, it does not necessarily mean the damaged tissue contains the organism causing the symptoms • For example, a root rot can cause chlorosis and wilting of stems and leaves, BUT the disease causal organism is in the roots • It is imperative to examine as much of the plant as possible to determine exactly where the problem is originating

7. Signs • Signsare the actual organisms causing the disease.  • Conks –  woody reproductive structures of fungi • Fruiting bodies – reproductive structures of fungi; could be in the form of mushrooms, puffballs, pycnidia, rusts, or conks • Mildew– whitish growth produced by fungi composed of mycelium • Mushrooms –  fleshy reproductive structures of fungi • Mycelium – thread-like vegetative growth of fungi • Slime Flux or Ooze – A bacterial discharge that oozes out of the plant tissues, may be gooey or a dried mass. • Spore masses – masses of spores, the “seeds” of a fungus.

8. What causes plant disease? • Infectious plant diseases are caused by living organisms that attack and obtain their nutrition from the plant they infect • The parasitic organism that causes a disease is a pathogen • Numerous fungi, bacteria, viruses, and nematodes are pathogens of major commodity crops and vegetable crops • The plant invaded by the pathogen and serving as its food source is referred to as a host

9. Types of pathogens Fungi Bacteria Viruses Nematodes

10. Role of the environment • A favorable environment is critically important for disease development • Even the most susceptible plants exposed to huge amounts of a pathogen will not develop disease unless environmental conditions are favorable

11. The Disease Triangle

12. Fungi • Fungi reproduce and spread by producing spores • When a spore comes in contact with a susceptible plant, it will germinate and enter the host if the proper environmental conditions are present • Hyphae develop from the germinated spore and begin to extract nutrients from host plant cells • The hyphae secrete enzymes to aid in the breakdown of organic materials that are ultimately absorbed through their cell walls

13. Fungi • Damage plants by killing cells and/or causing plant stress • Can infect plants through natural openings (e.g. stomata), via mechanical pressure or through wounds • Can survive in a wide range of habitats, including living and dead plants, and soils • Fungi can be transmitted by: • water, wind and air • soil • seed • insects and other animals • human activities such as chipping, transplanting, tractor passes and movement of soil

14. How fungi penetrate plant cells

15. Bacteria • Extremely small organisms requiring microscope to be seen • Can rapidly multiply under favorable conditions • Cells clump together in masses called colonies • Obtain food from dead or decaying organic matter or living tissue

16. Bacteria • They are usually carried in infected plant material, but can survive in soil and on the surface of plants, even in dry conditions, for long periods of time. • Bacteria can infect plants through wounds and natural plant openings such as stomata • Unlike fungi, bacteria must find a natural opening for entry • Bacteria can be transmitted by: • water (rain or irrigation water) • seed • vegetative propagation material • insects • human activities such as chipping, transplanting, harvesting, tractor passes and movement of soil • Bacterial pathogens are dependent upon water • Conditions must be very wet and/or humid for them to cause significant and widespread damage

17. Bacteria • Bacteria move between plant cells and secrete substances that degrade plant cell walls so the contents can be utilized • Some produce enzymes that break down plant tissue, creating soft rots or water-soaking • Like the fungi, bacteria cause symptoms such as leaf blights and spots, galls, cankers, wilts and stem rots • Bacterial leaf spots appear different from fungal leaf spots due to their intercellular movement • Veins often limit the development of a lesion, so they appear angular or irregular, not round Comparison of Fungal and Bacterial Leaf Spots

18. Viruses • Viruses can multiply only in living cells • Some viruses are often systemic; others leave some plant tissue virus-free • Viruses cause mottling, spots, and mosaic-like patterns, crinkling, and other malformations on leaves and fruits, and may stunt plants.  • There is no cure for infected plants, so control depends on prevention • Viruses are named after the first plant on which they were studied and the most obvious sign of the disease on that plant • Viruses can be transmitted by: • living vectors such as insects, mites, nematodes, fungi, humans • mechanical vectors such as tools • leaf-to-leaf contact, seed and pollen

19. Also….nematodes • Round, slender, threadlike worms • Some are parasites on animals, insects, fungi, other nematodes, and plants • Plant-parasitic nematodes have a stylet • Most live in the soil and feed in or on plant roots

20. Disease Cycle • Fungal, bacterial and viral diseases follow a cycle of the infective agent invading and infecting an appropriate host • dependent on the interaction between host, pathogen and environment • The infective agent grows and reproduces in the host, which develops signs/symptoms of disease • Specific fungi, bacterium or viruses have one or a number of methods for spreading • air, water, soil, insects, seed, cuttings and humans. • Some disease organisms need to survive in a growing host plant, but others may be able to survive for a time outside a host plant in dead host plant debris; in water, soil and/or seeds; or as spores.

21. Disorders in plants • Conditions that promote disorders in plants include: • unfavorable temperatures • unfavorable soil moisture • unfavorable atmospheric moisture • unfavorable soil structure • incorrect soil pH • nutrient deficiencies and toxicities • harmful substances • mechanical injuries • genetic abnormalities • Disorders can predispose plants to attack by pathogens

22. Disorder or disease? • Look at the whole plant • Check for damage to roots, foliage and flowers • Note the distribution of signs within the plant—for example, is only the new growth affected, or only one stem? • Note the pattern of affected plants within the crop • Note the changes to plants over time—the signs produced by plant diseases often vary with weather conditions or age of plants

23. Diagnostics

24. Plant Disease Management Cultural management Mechanical management Clean up or prune out infested plant materials to reduce the source of inoculum Rotate crops when possible to starve pathogens Apply mulch in gardens to create a splash barrier Use soil solarization to reduce soil pathogens Rogue host plants • Well-adapted varieties • Disease resistant varieties • Plant certified seed or seed pieces • Place plants in the appropriate environment for optimum growth (minimize stress on the plant) • Apply fertilizer and water according to plant needs • Prune correctly, as needed, and at the correct time of year

25. Summary • Understanding the difference between a sign and a symptom is key in identifying a plant disease • A plant disease cannot develop if a susceptible host, pathogen, and favorable environment do not occur simultaneously • The major plant pathogens responsible for infectious disease development in plants are fungi, bacteria, viruses, and nematodes • The disease cycle describes the interaction of the pathogen with the host