Someone who has a plant problem asks two questions: What is the problem, and how do I correct it? These are hard questions for beginners and experts alike. Diagnosis is the process of determining the cause of a problem. It can be a long or short process depending on one’s ability and the nature of the problem. Once the cause is known, an appropriate control strategy can be developed.
Plant pathologists take many different approaches to diagnosing plant disease problems. The first step is to decide whether the problem is a plant disease. The broadest definition of plant disease includes anything that adversely affects plant health. This definition can include such factors as nutrient deficiencies, lawnmower damage, air pollution, and pathogens. A stricter definition usually includes a persistent irritation resulting in plant damage. This excludes mechanical damage such as lawnmower injury to trees or natural events such as hail or lightning. A very strict definition includes only those (living) things that replicate themselves and spread to adjacent plants. This includes such biological organisms as nematodes, fungi, bacteria, and viruses. Plants damaged by macroscopic organisms, such as deer, rodents, and birds usually are not considered to be diseased.
Many novices use the picture-book method of diagnosis: looking at textbook pictures of problems and attempting to match the problem with the picture. “The Ortho Problem Solver” and the APS Plant Disease Compendia series are examples of texts that have many useful, high-quality color pictures. Although this method is useful for simple and common problems, it is usually inefficient and inaccurate for more complex or difficult problems.
Another simple technique, used by “The Ortho Problem Solver,” is the checklist. Through a series of 70 questions, a person builds a case history of the problem. The questions include the kind, age, and size of the plant. The plant’s location, location of the property, and relationship to other plants also are part of the checklist. Information on the recent weather and soil conditions, soil coverings, and recent care also are needed. Describing the overall condition of the plant is very important.
Symptoms and signs are used to diagnose the condition of a plant. Symptoms are the physical characteristics of disease expressed by the plant. Symptoms can include wilt, galls, cankers, rots, necrosis, chlorosis, and general decline. Definitions of these can be found in the glossary. Signs are physical evidence of the pathogen causing the disease. Signs can include fungal fruiting bodies (such as mushrooms or pycnidia), mycelia, bacterial slime, presence of nematodes or insects, or the presence of insect holes accompanied by sawdust or frass. Again, these terms are defined in the glossary.
The checklist approach is a good guide for the types of questions to ask, but not all the questions are necessary. Some pertain only to certain situations or plants. Other approaches to disease diagnosis attempt to narrow the possibilities with each question and are like synoptic or dichotomous keys.
The diagnostic keys differ depending on the author. Some keys start by identifying the part of the plant that is affected. Is it the whole plant, or just the leaves, branches, stems, or roots? Some refine the first questions to stages of the plant; for example, are the seedlings, flowers, fruit, or seed affected? Some diseases affect only certain portions of a plant. Many other references start out with the plant itself: is it a cherry tree, foliage plant, or zinnia? Only a limited number of diseases attack a given plant species.
There is no one key set of questions or techniques for diagnosing plant diseases. Experience and practice are the best teachers. It is easier to diagnose plant problems by making a personal, on-site inspection. Subtle influences of the site, plant environment, and possible management practices can be seen that may have been overlooked by the grower. Difficulties arise when the diagnostician is presented only a portion of the plant because that portion may or may not indicate the real problem. The worst situation is a request for a diagnosis by phone, because misunderstandings and an inaccurate diagnosis can easily occur. However, sometimes this is the only contact someone may have with a diagnostician.
The Systematic Approach
To organize your approach to a plant disease problem, the “Systematic approach to diagnosing plant damage” by J. L. Green, O. Maloy, and J. Capizzi is useful and is presented briefly here. It consists of a detailed handout, a study guide, and a color slide set. It is designed as a Master Gardener seminar and is useful to hear or to present yourself.
The approach involves defining the real problem and distinguishing between living and nonliving causes of plant damage by looking for patterns, determining the development of the damage, and building a case history of the problem. With these steps, it is usually easy to narrow the possibilities and to turn to appropriate reference materials including textbooks, herbarium samples, and knowledgeable specialists.
Define the Real Problem
Identify the plant and what it should look like at this time of year. A grower or gardener may mistake a normal stage of development for a diseased plant. Describe the abnormality in terms of symptoms and signs. Although a plant may exhibit symptoms of wilting, the real problem may be due to rotted roots, a girdled trunk, or lack of water. Determine what part(s) of the plant is/are affected. The rest of the procedure involves distinguishing between living and nonliving factors.
Look for Uniform or Nonuniform Patterns
Uniform damage is indicative of nonliving factors. Damage may occur on many plant species in the same area, on all the plants in a particular row or block, on all the leaves or shoots on one side of the plant, or on the same-age portion of each leaf or needle. Nonuniform damage to plants is indicative of living factors such as pathogens or insects. This damage shows up as scattered affected plants among a community of plants, scattered leaves or shoots on a single plant, or scattered spots on a single leaf.
Determine the Time Development of the Damage
If damage does not spread or there is a clear line of demarcation between damaged and nondamaged tissues, this is indicative of nonliving factors. Spread of the damage from plant to plant or to other plant parts over time indicates damage by a living organism.
Look for Specific Symptoms and Signs
Look for signs such as fungal fruiting bodies, mycelial threads, bacterial slime, presence of insects, mites, or holes with frass. Look for (or ask questions about) symptoms of nonliving factors that may be caused by extremes of temperature, light, or water. Ask about mechanical factors or chemical factors as indicated by uniform patterns. Check references for probable diseases of the identified plant. You may have to send the sample to an appropriate laboratory to continue to identify possibilities.
Once you have determined that a real problem exists and is caused by a living organism, you need to decide what type of organism may be causing the damage. There are many fungal and viral diseases of plants and a few caused by bacteria and nematodes. Some insect problems can mimic diseases; however, this next discussion will focus only on pathogens.
“Preliminary Diagnosis of Plant Diseases by Visual Symptoms” by C. Leach is a useful article; it is briefly described here. The discussion focuses on biological causes and also discusses many physical factors such as extremes in environment.
Begin by establishing which plant part or growth stage is showing symptoms. Are symptoms showing on roots, tubers, bulbs, corms, seedling, foliage, stem, branches, trunks, flowers, fruit, or on the entire plant? Often, you must next decide whether the symptoms are on the outside of the plant or whether you need to cut into it to see the symptoms.
External root symptoms include galls, discoloration, or death of roots or parts of roots. Crown gall caused by the bacterium Agrobacterium tumefaciensis common on many plants. Some fungal diseases, such as club root of cabbage, also cause galls. Root-knot nematodes, Meloidogyne spp., can cause large or small irregular galls. Small discolored, dead areas may be caused by a large variety of fungi and root-lesion nematodes, Pratylenchus spp. General death of the entire root system or just feeder roots is indicative of many fungi. Remember, injury to the root system often includes yellowing, stunting, or wilting of aboveground parts. Many fungi, such as Verticillium and Fusarium, will cause an internal vascular discoloration as will some bacterial wilts.
Symptoms on Storage Organs (Tubers, Bulbs, Corms, etc.)
Discolored or dead areas that penetrate deep into the storage organs are caused by many fungi and some bacteria. Dry rots are often caused by fungi, which may produce mycelia or spores. Soft rots are usually associated with bacteria such as Erwinia and can be accompanied by strong, repulsive smells. Many times bacterial soft rots will closely follow rots caused by fungi, making diagnosis difficult. Scurfy dead tissue on the surface may be caused by a variety of myxomycetes, such as powdery scab of potato. The filamentous bacterium Actinomyces scabies causes common scab of potato. Galling of storage organs can be caused by both fungi and nematodes. Internal problems, such as ring rot of potato, can be caused by bacteria fungi or by several viruses.
If seedlings fail to emerge, or fall over and die, this usually is referred to as damping-off. Fungi such as Rhizoctonia, Pythium, and Fusarium are common and affect seedlings just at or below the soil line. Dead areas on cotyledons, the first leaves or stems, are usually caused by fungi but occasionally by bacteria. Spots caused by fungi may contain hyphae or fruiting bodies while spots caused by bacteria may appear water soaked around the margins. General yellowing, or mottled or patterned yellowing, can be caused by several viruses. Ringspots, blotches, and streaks of yellow or green may also occur with viral infections. Nematodes such as Ditylenchus can cause swelling or galling of stems or leaves. Rusty red, brown, or black spots or stripes are indicative of rust and smut fungi. White moldy growth can be caused by powdery mildew or downy mildew fungi.
Discoloration (yellowing or shades of green), which is localized or in distinct patterns, usually indicates a virus. Other leaf symptoms may occur with viral infections. A general or uniform yellowing may indicate a root rot of some kind, so remember to examine the entire plant. Dead areas on leaves can be caused by fungi or bacteria. Necrotic areas caused by fungi may contain hyphae or fruiting bodies, particularly after incubation in a warm, moist environment. Necrotic areas caused by bacteria may be distinguished by water-soaked margins or bacterial streaming. Small rusty red, brown, or black spots or stripes may be caused by rust and smut fungi. Distinctive spores can usually be found in these spots. Leaf distortion (elongated, dwarfed, etc.) can be caused by several viruses. Leaf galls usually are caused by fungi, such as peach leaf curl and azalea leaf gall. Viruses and nematodes rarely cause galls on the leaf. Moldy white appearance of leaves indicates powdery or downy mildew. Wilting indicates lack of water, which may be due to one of the vascular wilt fungi, root rots, or bacterial wilts. Other parts may need to be examined for an accurate diagnosis.
Stem, Branch, and Trunk Disorders
The leaf disease remarks above pertain to most stem disorders of annuals or herbaceous perennials. Complete or partial death of woody stems or branches, usually referred to as cankers, can be caused by a large variety of fungi and a few bacteria. Cutting into the wood with a knife may reveal a sharp border between healthy and infected tissue. Look for spore-bearing structures of fungi or induce sporulation in a moist chamber. Some bacterial cankers will excrete a sticky ooze in the spring. Wood rots and decays are caused mainly by fungi, which may produce large conks or bracketlike fruiting structures. Galling can be produced by the crown gall bacterium Agrobacterium tumefaciens. A few galls or swellings are caused by fungi, such as white pine blister rust and Exobasidium sp. Witches’ brooms (excessive branching) can be caused by several fungi and by a group of parasitic plants called mistletoes.
Abnormal color changes and/or distortions can be caused by several viruses. Partial or complete death of flower parts can be caused by fungi and bacteria. Fungi usually produce characteristic spores; bacterial infections can appear water soaked. Smut of grasses convert individual flowers or whole inflorescences into masses of black spores. Ergot, stamen blight of blackberry, and anther mold of red clover are other fungal examples of converted flower parts. Some nematodes also can convert flower parts for their own use.
Fungi cause a wide variety of decays, rots, and superficial spotting or russetting. Important symptoms include specific color of rotted tissue, firmness of the tissue, and signs such as spores or spore-bearing structures. Viruses can cause discolorations and malformations. Bacteria may cause discrete spots on fruit in certain field situations or soft rots in storage.
Principles of Plant Disease Control
After you diagnose a plant disease, you are only half finished. The equally challenging task of designing the proper control recommendation is next. Understanding the specific disease or the life cycle of the pathogen involved is necessary to make an adequate control recommendation.
The Disease Triangle
Three major factors contribute to the development of a plant disease: a susceptible host, a virulent pathogen, and a favorable environment. A plant disease results when these three factors occur simultaneously (Figure 1). If one or more of these factors do not occur, then the disease does not occur. The genetic makeup of the host plant determines its susceptibility to disease. This susceptibility or resistance may be determined by various physical and biochemical factors. Plant stature, growth habit, cuticule thickness, and stomatal shape are a few physical factors that influence disease development. The plant’s developmental stage also may influence disease development. Pathogens differ in their ability to survive, spread, and reproduce. Environmental extremes of temperature, light, or moisture can accentuate many diseases. Cool, moist conditions are ideal for many fungal pathogens.
The Disease Cycle
Understanding the disease cycle is important when considering control options. Learning the chain of events that contribute to a disease helps point out the weakest links. Control measures can then be used to break the cycle. Most pathogens must survive an adverse period, usually winter, when they do not actively incite plant diseases. This overwintering inoculum reinfects or continues infecting the plant host in the spring. Some diseases are characterized by a single cycle during the year. Other diseases continually produce new inoculum, repeating the cycle many times during the course of a single growing season.
The five basic principles of plant disease control are: exclusion, avoidance, eradication, protection, and resistance. These principles work at federal, state, county, and personal levels.
Exclusion This includes quarantines, inspections, and certification. Plant material is examined to prevent entry of a disease that does not already occur in a particular country, state, or geographic area. The most notable use of this method is in California. As you cross the state border, you must stop at an agricultural inspection station because they screen for fresh fruit, vegetables, and plants coming from areas known to have certain diseases (or insects). There are many quarantines on importing plant materials into the country as well as into the Pacific Northwest. For example, several grape viruses, Dutch elm disease, chestnut blight, eastern filbert blight, azalea flower spot, rust-susceptible varieties of barberry, mahonia (Oregon grape) or mahoberberis, and several peach viruses all are quarantined to prevent import into Oregon.
Quarantines are effective but may not always protect an industry. Quarantines on the movement of Corylus sp. infected with eastern filbert blight into the Pacific Northwest from the east coast were effective for more than a century. In 1972, an infection center was discovered in Washington and has since moved into major production areas of Oregon.
Avoidance If the disease does occur in your area, there may be techniques to avoid disease development. Choice of planting site, time of planting, storage conditions, or avoiding wounds are a few of these techniques. Phytophthora root rots can be avoided by not planting in heavy, poorly drained soils. Planting later in the year when soils are dryer and warmer will avoid some damping-off diseases common to many vegetables. Wounding can cause entry points for pathogens or weaken a plant to the point that it cannot defend itself. Avoiding wounds also helps to control the crown gall bacterium, which needs an injury to begin the infection process. Planting certified virus-free stock is a good way to avoid virus-related diseases.
Good horticultural practices such as proper fertility, pruning, watering, and proper training go a long way to help control plant diseases.
Eradication When a plant is infected or an area is infested with a plant pathogen, eradication can eliminate or reduce the disease threat. Rotation, sanitation, heat treatment, eliminating the alternate host, and certain chemicals can be used to reduce or eliminate diseases. Crop rotation is a common method in both commercial agriculture and home gardens. It is necessary to know the pathogen and its host range. Rotation reduces soil populations of fungi or nematodes only if nonhost plants are used.
Removing plant debris (sanitation) is important where pathogens may overwinter. Raking leaves, removing rotted fruit, picking up old vines, and pruning out dead wood or canes all are part of sanitation. Once collected, dispose of debris by burning, burying, or hot composting. If you decide to compost, it must be done correctly or completely or you will not benefit from your efforts. Field burning is another method of sanitation, which destroys grass stubble where plant pathogens may overwinter.
Rusts are a group of fungi that can complete their life cycle on two or more different hosts. Eliminating an alternate host may help reduce pressure from these diseases. Heat treatment is usually used to eliminate viruses from propagation material.
Certain chemicals can be used to eliminate infections or infestations. Soil can be fumigated to reduce populations of certain fungi and nematodes. Lime sulfur is used during the dormant season to denature and kill fungal fruiting bodies or spores. Some fungicides have “kickback” activity; meaning that infections of some fungi can be stopped if the chemical is applied within a few days after the infection has started.
Protection Protection is treating a healthy plant before it becomes diseased. There are both biological and chemical means of protection. One of the most successful biological protections has been the use of a bacterium to protect against a bacterial disease known as crown gall. The roots of a seedling or nursery plant are merely dipped into a suspension of a commercial preparation of the bacterium prior to planting.
Chemical protection is one of the most widely used means of control. Some fungicides (such as copper and sulfur products) are allowed for use under several “organic” growing guidelines. Many fungicides are on the market but few can be obtained easily by the homeowner. It is necessary to know the disease cycle and host susceptibility to get good control using fungicides. Proper timing, coverage, and selection of fungicides are also needed.
Resistance Resistance is a term sometimes mistakenly used interchangeably or in conjunction with “immunity,” “tolerance,” and “susceptibility.” These terms describe the plant’s inherent genetic makeup and thus its reaction to plant pathogens. Resistance and its opposite, susceptibility, are levels or degrees of a plant’s reaction. Some cultivars of a plant can be more or less resistant (or susceptible) than another cultivar. Resistant cultivars can still become diseased but not as much as (or more than) another. If a plant does not ever become diseased, then the term “immune” can be used. Tolerance describes a plant (usually a food crop) that may become diseased but produces yields similar to a healthy plant.
Lists of resistant plants can be found in many texts and seed catalogues. Planning ahead is essential and planting resistant cultivars is the easiest means of disease control.
Knowing what diseases a plant is susceptible or resistant to can help in the diagnostic process. One can eliminate possibilities by knowing which diseases are likely to occur.
Experience and practice are the best teachers of plant disease diagnosis. Examination of the plant’s physical environment and management history are essential. Observing patterns and specific symptoms and signs are important in arriving at a correct diagnosis. Once diagnosed, the proper control measures can be formulated. Knowledge of the host, pathogen life cycle and environmental factors also aid selection of the most effective control measures. A combination of exclusion, avoidance, resistance, eradication, and protection will control most plant diseases.
For Further Reading and Reference
*Benyus, J.M. 1983. Christmas Tree Pest Manual. USDA Forest Service, North Central Forest Experiment Station, 1992 Folwell Avenue, St. Paul, MN 55108.
*Bobbitt, V., Antonelli, A., Foss, C, Davidson, R., Byther, R., and Maleike, R. 1996. Pacific Northwest Landscape IPM Manual. Puyallup, WA: Washington State University.
*Chase, A.R. 1997. Foliage Plant Diseases: Diagnosis and Control. St. Paul, MN: APS Press
*Chase, A.R., Daughtrey, M., and Simone, G.W. 1995. Diseases of Annuals and Perennials. Geneva, IL: Ball Publishing.
*Converse, J. 1982. Scott’s Guide to the Identification of Turfgrass Diseases and Insects. Marysville, OH: The O. M. Scott and Sons Company.
Costello, L.R., Perry, E.J., Matheny, N.P., Henry, J.M., and Geisel, P.M. 2003. Abiotic Disorders of Landscape Plants. A Diagnostic Guide. Univ. of California Agriculture and Natural Resources Publication 3420.
*Daughtrey, M., and Chase, A.R. 1992. Ball Field Guide to Diseases of Greenhouse Ornamentals. Geneva, IL: Ball Publishing.
de Bokx, J.A. (ed.). 1972. Viruses of Potatoes and Seed-potato Production. Wageningen Centre for Agricultural Publishing and Documentation, The Netherlands. An advanced treatment of potato viruses and certification systems, including virus descriptions, vectors, diagnostic tools, and resistant varieties.
Dickson, J.G. 1947. Diseases of Field Crops. New York: McGraw-Hill Book Co., Inc. Detailed descriptions of most field crop diseases. Strictly a scientific text; very useful to agents working with field crops.
*The Disease Compendia Series. American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. A continuing series of specialized booklets on individual crops and crop groups: alfalfa, allium, apple, barley, beans, beets, blueberry, chrysanthemum, citrus, conifers, corn, cotton, cranberry, cucurbit, elm, grape, lettuce, ornamental foliage plants, ornamental palms, pea, peanut, pepper, potato, potted flowering plants, raspberry and blackberry, rhododendron and azalea, rice, rose, sorghum, soybean, stone fruit, strawberry, sweet potato, temperate zone nut crops, tobacco, tomato, tropical fruits, turfgrass, umbelliferous crops and wheat. They have clearly written descriptions and colored illustrations. Second and third editions of some books have been published.
Dreistadt, S.H. 2001. Integrated Pest Management for Floriculture and Nurseries. Univ. of California Agriculture and Natural Resources Publication 3402.
*Ecke, P., Matkin, D.A., and Hartley, D.E. 1990. The Poinsettia Manual, 3rd ed. Encinitas, CA: Ecke Poinsettias.
Hadidi, A., Barba, M., Candresse, T., and Jelkmann, W. 2011. Virus and Virus-like Diseases of Pome and Stone Fruits. St. Paul, MN: APS Press.
*Henley, R.W. (ed.). 1983. A Pictorial Atlas of Foliage Plant Problems. Florida Foliage Association, P.O. Box Y, Apopka, FL 32703.
Horst, R.K. 2008. Westcott’s Plant Disease Handbook, 7th ed. Springer Dordrecht, Berlin, Heidelberg, NY. This book has some excellent plant disease descriptive information and gives accurate and detailed control measures, particularly on garden disease problems. Written in a semipopular style but presupposes a knowledge of the field. To get the most out of this book, the reader needs a little training in plant pathology.
*Howard, R.J., Garland, J.A., and Seaman, W.L. 1994. Diseases and Pests of Vegetable Crops in Canada. Canadian Phytopathological Society and Entomological Society of Canada. Entomological Society of Canada, 393 Winston Avenue, Ottawa, Ontario, Canada K2A 1Y8.
*Jones, A.L., and Sutton,T.B. 1996. Diseases of Tree Fruits in the East. North Central Regional Extension Publication 45. For copies, write: Bulletin Office TFD, Michigan State University, 10-B Agriculture Hall, East Lansing, MI 48824-1039.
*Jones, R.K., and Benson, D.M. 2001. Diseases of Woody Ornamentals and Trees in Nurseries. St. Paul, MN: APS Press.
*MacNab, A.A., Sherf, A.F., and Springer, J.K. 1983. Identifying Diseases of Vegetables. University Park, PA: The Pennsylvania State University Extension Service.
*Nichols, L.P. 1973. Tree Diseases Description and Control. The Pennsylvania State University College of Agriculture, Extension Service, Special Circular 175. University Park, PA 16802.
Ogawa, J.M., and English, H. 1991. Diseases of Temperate Zone Tree Fruit and Nut Crops. University of California Publication 3345.
Pirone, P.P. 1978. Diseases and Pests of Ornamental Plants, 5th ed. New York: John Wiley & Sons, Inc. A readable book of considerable value to agents and recommended as a general reference for disease descriptions.
*The Plant Health Management Series. American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. This new series of guidebooks integrates information from all disciplines required to produce a healthy crop. The first two publications in the series are on wheat and potatoes.
*Riffle, J.W., and Peterson, G.W., tech. coords. 1986. Diseases of Trees in the Great Plains. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, General Technical Report RM-129. Fort Collins, CO 80526.
*Scharpf, R.A., tech. coord. 1993. Diseases of Pacific Coast Conifers (revised). USDA Forest Service Agriculture Handbook 521. Washington, DC: U.S. Government Printing Office, Superintendent of Documents.
*Schwartz, H.P., Westra, P., and Cranshaw, W. 1990. Colorado Onion Integrated Pest Management. Colorado State University Cooperative Extension Service Bulletin 547A. Fort Collins, CO 80523.
Shaw, D.C., Oester, P.T., and Filip, G.M. 2009. Managing Insects and Diseases in Oregon Conifers. Oregon State University Extension Service. EM 8980.
Sherf, A.F., and MacNab, A.A. 1986. Vegetable Diseases and Their Control, 2nd ed. New York: John Wiley and Sons, Inc.
Shurtleff, M.C. 1966. How to Control Plant Diseases in Home and Garden, 2nd ed. Ames, IA: Iowa State University Press. Identifies the diseases that may affect house plants, lawn grasses, trees, shrubs, vines, flowers, vegetables, and fruits. Also gives control measures.
*Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 2005. Diseases of Trees and Shrubs. 2nd ed. Ithaca, NY: Cornell University Press. Excellent book with color illustrations and very detailed descriptions of each disease.
*Skelly, J.M. et al. (eds.). 1987. Diagnosing Injury to Eastern Forest Trees. University Park, PA: The Pennsylvania State University College of Agriculture.
*Virus Diseases of Small Fruits. 1988. USDA Agriculture Handbook 631. Superintendent of Documents, U.S. Government Printing Office. Washington, DC. This is the standard reference on the subject and contains numerous colored illustrations.
*Whipker, B.E., Cloyd, R., and Warfield, C. 2012. Troubleshooting Petunia Disorders. 2nd ed. pp 68.
Ziller, W.G. 1974. The Tree Rusts of Western Canada. Canadian Forestry Service Publication 1329.
The following can be obtained from: WSU Extension Bulletin Office, Washington State University, P.O. Box 645912, Pullman, WA 99164-5912
Free leaflets that list Extension bulletins by subject include:
Plant Disease C0880
Gardening in Eastern Washington C0825
Western Washington Fruits and Vegetables C0837
Other WSU Extension publications available include:
*Antonelli, A.L. et al. 1999 (rev.). How to Identify Rhododendron and Azalea Problems. Washington State University Extension Bulletin Stock No. EB1229.
*Gould, C.J., and Byther, R.S. 1980. Diseases of Narcissus. Washington State University Extension Bulletin Stock No. EB0709.
*Gould, C.J., and Byther, R.S. 1980. Diseases of Tulips. Washington State University Extension Bulletin Stock No. EB0711.
Gould, C.J., Goss, R.L., and Byther, R.S. 1999 (rev.). Diseases of Turfgrass. Washington State University Extension Bulletin Stock No. EB0713.
Eastwell, K.C. et al. 2005 (rev.). Field Guide to Sweet Cherry Diseases of Washington. Washington State University Extension Bulletin Stock No. EB1323. Available only online, at https://pubs.wsu.edu/
The following can be obtained from: Publications, Divisions of Agriculture and Natural Resources, University of California, 6701 San Pablo Avenue, Oakland, CA 94608. Tel. (toll free) 1-800-994-8849. Website http://anrcatalog.ucdavis.edu/InOrder/Shop/Shop.asp
*Dreistadt, S.H., Clark, J.K., and Flint, M.L. 2004 (rev.). Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 2nd ed. Publication 3359.
*Flaherty, D.L. et al. (eds.). 1992 (rev.). Grape Pest Management, 2nd ed. Publication 3343.
*Has good, useful color pictures