Plant Disease Basics
Posted: August 21, 2012
For a printable PDF of this document, please click here.
Why Are Plant Diseases Important?
Usually, farmers are worried about the financial losses that plant diseases can cause because of reduced quantity and/or quality of the product. For example, in 1970 U.S. farmers lost $1 billion to an epidemic of southern corn leaf blight. Disease epidemics can also threaten entire plant species. Historical examples of destructive plant disease epidemics include American chestnut blight and Dutch elm disease (Agrios 1997).
What Causes Plant Disease?
Pathogens like bacteria, fungi, nematodes, viruses, and phytoplasmas, as well as abiotic problems, can all cause plant diseases.
Abiotic problems are caused by adverse extremes in the environment, such as nutrient deficiency, prolonged water stress, and air pollution.
Bacteria are single-celled organisms with no nucleus. Most bacteria associated with plants are saprophytic (feed on dead organic debris) and do no harm to plants. But a few—around 100 species—can cause plant diseases (Jackson 2009). Under favorable conditions, they reproduce very quickly, some doubling their population in just 9.8 minutes (Eagon 1972). Think of the pink goo that grows on rice left in the refrigerator for too long. These are bacteria. Bacteria can cause blights, leaf spots, fruit rots, vascular wilts, and galls. They typically enter the plant through natural openings, such as hydathodes and lenticels, or wounds created by wind-swept sand, hail, heavy rain, and/or mechanical damage. They can be spread by infected seed or from plant to plant by water splashing, insects, and humans.
soil or plant cell walls. Most consume dead plant material, but some
can use enzymes to dissolve and digest living plant tissue. Drawing
courtesy of Lenore Gray.
Fungi usually grow as threads or strands called hyphae. A single hypha can be only a few inches long or miles long. Mushrooms are one type of fruiting body that some fungi form to reproduce. Fungi reproduce via the production of spores. These spores may be spread long distances by air or water, or they may be soilborne. Certain fungi are also able to produce structures that enable them to survive in the soil for many years. They can cause root and stem rots, shoot and leaf blights, leaf spots, cankers, vascular wilts, and postharvest storage rots (Agrios 1997).
Nematodes are tiny microscopic round worms that generally live most of their lives in the soil. Plantpathogenic nematodes comprise only a very small subset of all the nematode species. These nematodes have a needlelike mouthpart called a stylet used to pierce the plant cells and feed on the cell contents. Feeding either kills the plant cells, leading to the development of lesions, or causes galls to develop on the roots, reducing the ability of the plant to take up water and nutrients.
nematode affects lettuce (pictured above), tomatoes, carrots (pictured on p. 1) and many other crops. Photo courtesy of B. Gugino.
Phytoplasmas are specialized bacteria without a cell wall that depend on the host plant to survive. Typically transmitted by insects during feeding, they live in the plant sap and clog up the vascular system, reducing the ability of the plant to move water or nutrients. A typical symptom is phyllody, the production of leaflike structures instead of flowers, or a witch’s broom or bushy appearance.
Virus particles are pieces of RNA or DNA with a protein coat. They multiply by inducing host cells to form more virus particles. Viruses are spread through vegetative propagation (cuttings, grafting, etc.), seed, insects (most common), or nematodes. Typical symptoms include stunting, mosaic, or ring spot patterns on leaves and fruit.
How Do Pathogens Cause Disease?
Pathogens use a number of different strategies to enter their host and capture the plant’s energy for their own use. Some enter through natural openings or wounds created by mechanical damage or from severe weather events (i.e., hail, wind-swept sand). Others, such as some fungi or nematodes, can directly penetrate through the plant tissue, while viruses rely primarily on insects for transmission. Once inside the plant, some pathogens use enzymes that break down the tissue of the plant just like the enzymes in our stomachs break down food. Others have toxins that actually kill the plant tissue before the enzymes break it down. Still others secrete hormones that change the way the plant grows. For example, the hormones may tell the plant to move more nutrients to the area where the pathogen is lodged. All viruses and a few bacteria force the plant to produce pathogen gene products. Since the plant is putting most of its energy into making more viruses, the cells starve (Miles and Brown 2007).
Agrios, G. N. Plant Pathology. Vol. 4. San Diego: Academic Press, 1997.
Eagon, R.G., “Pseudomonas natriegens, a marine bacterium with a generation time of less than 10 minutes.” Journal of Bacteriology 83, no. 4 (1972): 736–37.
Jackson, R. W. Plant-Pathogenic Bacteria: Genomics and Molecular Biology. Norfolk, U.K.: Caister Academic Press, 2009.
Miles, A., and M. Brown. Teaching Organic Farming and Gardening: Resources for Instructors. Santa Cruz: University of California Farm and Garden, 2007.
This fact sheet was adapted from A. Miles and M. Brown, Teaching Organic Farming and Gardening (Santa Cruz: University of California Farm and Garden, 2007).
Prepared by S. Tianna DuPont, sustainable agriculture educator, Penn State Extension, and Beth Gugino, Penn State Department of Plant Pathology. Reviewed by Emelie Swackhamer, Penn State Extension.
This publication was supported in part by funding from the Beginning Farmer and Rancher Development Program of the National Institute of Food and Agriculture, USDA, Grant #2009-49400-05869.
Penn State College of Agricultural Sciences research and extension programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture.
This publication is available from the Publications Distribution Center, The Pennsylvania State University, 112 Agricultural Administration Building, University Park, PA 16802. For information telephone 814-865-6713.
Where trade names appear, no discrimination is intended, and no endorsement by Penn State Cooperative Extension is implied.
This publication is available in alternative media on request.
The Pennsylvania State University is committed to the policy that all persons shall have equal access to programs, facilities, admission, and employment without regard to personal characteristics not related to ability, performance, or qualifications as determined by University policy or by state or federal authorities. It is the policy of the University to maintain an academic and work environment free of discrimination, including harassment. The Pennsylvania State University prohibits discrimination and harassment against any person because of age, ancestry, color, disability or handicap, genetic information, national origin, race, religious creed, sex, sexual orientation, gender identity, or veteran status and retaliation due to the reporting of discrimination or harassment. Discrimination, harassment, or retaliation against faculty, staff, or students will not be tolerated at The Pennsylvania State University. Direct all inquiries regarding the nondiscrimination policy to the Affirmative Action Director, The Pennsylvania State University, 328 Boucke Building, University Park, PA 16802-5901; Tel 814-865-4700/V, 814-863-0471/TTY.
Produced by Ag Communications and Marketing
© The Pennsylvania State University 2012
CODE # EE0036 5M6/12printer5131