- Author: Steven A. Tjosvold
The previous post described the importance of inspecting incoming plants and keeping a clean nursery to avoid pest introductions. Many important root pathogens and other pathogens, weeds, and insects can be introduced in contaminated soil and growing media, and so the focus of this post will be on ways to prevent introductions from these potential sources.
Soil- inhabiting plant pathogens can be found in growing media and associated root and crop debris. Anything that has contacted the ground, such as equipment, tools, irrigation hoses, and workers' hands and shoes could be contaminated. Pots should not sit directly on the ground. Phytophthora infecting the roots of just one potted plant can produce thousands of propagules that can move in water draining from the pot and infect roots of nearby plants.
Likewise, during a vigorous rain storm, these propagules can be splashed from the contaminated pot or ground onto nearby plants. Benches or similar structures that support plants above the ground can eliminate or minimize this. In greenhouse structures, concrete floors or other impervious surfaces are ideal for walkways between benches. Floor surfaces should be kept clean of plant debris, soil, or growing media. After a crop cycle, benches should be cleaned of plant debris, washed and dried. Drying can kill sensitive plant pathogens.
Benches can be sprayed with diluted chlorine bleach (0.5 % sodium hypochlorite solution) or other suitable disinfectant. Potting media and plant debris will inhibit the activity of most disinfectants.
There are many clever ways to raise pots and containers off the ground.
The bottom of clean shoes can be sanitized with disinfectants such as quaternary ammonium compounds .
Store and handle growing media so it does not come in contact with the ground or water runoff. Cover the media when not in use.
- Author: Steven A. Tjosvold
A pest-free nursery should be the resolve of every grower in the New Year. Start the New Year with a clean nursery and keep pests from being introduced. It is more important than ever now because there is increasing risk that serious invasive pests and diseases can be moved through the nursery trade. Phytophthora ramorum (the cause of Sudden Oak Death) and light brown apple moth (LBAM) are current notable examples that could cause ecological or economic damage and trigger regulatory oversight.
Keeping a nursery clean and preventing introductions of new pests and diseases is often difficult in the complex and fast-paced nursery industry. But here's a short list of things that you could do:
Know the source of propagative material you plant, and insure that the propagator is doing everything possible to provide healthy seed or plants to you. Inspect seed, transplants, or other plants when they are delivered to your nursery or greenhouse. Make sure no diseases or pests are evident. Use your hand lens (See link below).
Pull transplants or other plants out of their containers and look for healthy root tips. Insects such as aphids, thrips or mealybugs hide in young folded leaves or tightly angled stems. There are field test kits to detect Phytophthora and common viruses such as tomato spotted wilt, impatiens necrotic spot and cucumber mosaic. If a pathogen or insect infestation is detected, controlling the problem before you plant in the field is much easier than after the problem is established in the field. “Controlling the problem” sometimes means destroying the plants before they are planted or introduced into a nursery or greenhouse.
Start clean and stay clean in the New Year, and have a Happy New Year!
- Author: Steven A. Tjosvold
To diagnose plant problems, one must take the approach of a detective. Begin with identifying the plant or plants where the problem is occurring. Characterize the problem by describing the symptoms. Are there leaf, stem or root galls, wilted or yellowed leaves, discolored or rotted roots, distorted leaves, chewed or stippled leaves? When did symptoms occur? Look for evidence or signs of the pathogen, insect or other pest. Look for insect sooty mold or honeydew or other byproducts of an insect infestation. Look for patterns of the symptoms or plant damage. Gather appropriate information on plant culture, pesticide applications, and weather or the greenhouse environment. Is there something extraordinary that occurred that could be the cause of the problem? Maybe a sample needs to be taken to a diagnostic laboratory for culturing or pest identification.
Once the information has been gathered, it's time to review the possible problems by using references. Do the symptoms, and environmental conditions fit those listed and described? These references need to give enough information to aid identification and enough information to help manage the problem. But what references to use? For California ornamental production, I have relied on a few general references that have aided in the identification and management of plant diseases, insects and vertebrates. These are UC publications, and I have provided links where they can be obtained.
In this reference, crop tables give you a symptom-based guide for diagnosing problems and the recommended controls for problems affecting 120 major flower and foliage species. It's loaded with outstanding color photos and diagrams, references, suppliers, and a comprehensive index. You'll learn how to establish an IPM program for your nursery, techniques for managing pests in flower and nursery crops, how good cultural practices can nip problems in the bud, disease control techniques for root and crown decays, vascular wilt diseases, and pathogens infecting flowers and foliage, how to identify and manage aphids, leafminers, thrips, whiteflies, mites, nematodes and weeds.
https://anrcatalog.ucanr.edu/Details.aspx?itemNo=3402
Pests of Landscape Trees and Shrubs
Although this reference is directed to the professional working with trees and shrubs in the landscape, I have found it to be very useful for identifying problems on woody plants in our California nurseries. The pests and information listed in this reference often supplements those that are covered in “IPM for Floriculture and Nursery”. It contains two tables that can be used for problem solving. One table uses the “describe symptom first” approach and the other table starts with “identifying the host first” approach. This manual covers hundreds of pests, including insects, mites, nematodes, plant diseases, and weeds that can damage California woody ornamentals.
https://anrcatalog.ucanr.edu/Details.aspx?itemNo=3359
Pests of the Garden and Small Farm
Because many of our California nurseries often produce a wide range of crops, including edible crops, it is often necessary to have a good pest reference for vegetable, herb, and fruit tree pests. Again, in this manual, there are good crop-specific lists that include common pests and diseases found in California. It covers the IPM approach, and the methods recommended rely primarily on organically acceptable alternatives.
https://anrcatalog.ucanr.edu/Details.aspx?itemNo=3332
Wildlife Pest Control Around Gardens and Homes
Where do you turn for information if you suspect a vertebrate pest in your nursery or greenhouse? These are sometimes the most fascinating problems. (I have seen evidence of mice eating only the nectaries from carnation flowers in greenhouses; but only one particular variety, was chosen from about 50 available. It was clearly the most fragrant of all and smelled just like cloves!). These problems are often easy to identify (I have seen wild pigs rampaging through a field of hydrangeas), but what is their biology and how do you manage them? It's something plant people sometimes need to know. This is when this reference really becomes useful.
https://anrcatalog.ucanr.edu/Details.aspx?itemNo=21385
UC IPM
UC Statewide Integrated Pest Management Program
This is an online version of much of the information given in the references described above. However, this online version is not going to beat the quality and quantity of images, figures, and tables, and depth of information found in the references. What the references lack, however, UC IPM online has. It gives current pesticide recommendations for the listed pests and diseases.
Merry Christmas and Happy Holidays!
- Author: Steven A. Tjosvold
The first factor in the disease triangle to evaluate is the host plant. Know what a healthy plant looks like-- both its aboveground portions and roots-- so that abnormalities can be recognized. Identify the plant species or cultivars that the unknown disease is occurring on. Look for patterns where abnormalities are occurring. Are there certain species or many species? Are there patterns that could suggest abnormalities caused by abiotic factors such as excesses in temperature, light, fertilizer, or pesticide application?
The second factor is the pathogen. Identifying biotic diseases is sometimes facilitated if the pathogen is visible. Particularly with some fungi, distinctive mycelium and spores can be visible on diseased tissue. Sometimes the pathogen can be seen with the naked eye but sometimes a hand lens can aid in seeing them. Gray mold (Botrytis), powdery mildews, and rusts are good examples of diseases that have distinctive spores and mycelium that are usually quite visible. Often, however, diseased tissue must be cultured in the laboratory to get the pathogen to produce mycelium, spores, or spore-producing structures that could aid identification. A light microscope can help the plant pathologist see and identify the specific pathogen in the laboratory. With viruses or other similar microscopic pathogens an electron microscope is needed to see and identify the pathogen. Sometimes evidence of a secondary fungus, bacteria or insect is visible and not directly related to the primary cause of disease.
The third factor is the environment. The diagnostician can collect information on the environmental conditions associated with the disease. Does irrigation frequency, dew, greenhouse condensation support disease occurrence? Do temperatures support the pathogen? Have temperatures been excessively high or low for plant growth? Have there been excesses applied of fertilizer, soil amendments, or pesticides that have been associated with the occurrence of disease?
References that aid in the diagnostic process are often organized so that they use and describe these three factors. To use these references, you need to first identify the host plant. After host identification, you can often go to a section where the host plant's common diseases are listed. The disease symptoms are described along with a description of the pathogen's mycelium, spores, or spore producing structures. There usually is a description of the specific disease's life cycle and any environmental conditions that support infection and development of a disease. Together, the information helps to focus the diagnostic process and formulate a likely diagnosis of an unknown plant disease.
Next: Suggested references that aid in identifying diseases and pests of ornamental plants.
For a more comprehensive look at the diagnostic process see attachment below:
UCNFA News as PDF57650 diagnosis
- Author: Steven A. Tjosvold
The disease triangle is a fundamental principle illustrating the factors involved in the occurrence and severity of plant disease. Disease caused by a living agent requires the interaction of a susceptible host, a virulent pathogen, all in the context of a favorable environment. Plant disease is prevented when any one of these three components are eliminated. The three factors can be manipulated to reduce the risk of disease development (See previous blog).
Some plant pathologists have expanded the usefulness of the disease triangle concept by adding one or more factors such as “human activity” or “vectors” to represent special-case applications. Another useful factor is “time” and it can be represented by one of the vertices of a three-dimensional pyramid (here with the top of the pyramid pointing out at you).
The dimension of time can convey that disease occurrence and intensity are affected by the duration that the three primary factors are aligned. Disease may not occur instantly once the three parameters are favorably aligned but will occur after some time.
For example, the initiation of gray mold disease on rose petals (caused by Botrytis cinerea) can occur in just 4 to 6 hours. With free moisture on rose petals and cool temperatures, Botrytis spores germinate, penetrate the petal's surface, and its hyphae infect and develop within the plants cells.
Figure to the left: Botrytis spores formed in clusters (upper left), germinate (upper center) and penetrate the petal's surface , and its hyphae infect and develop within the plants cells (lower two figures).
The physical manifestation of gray mold is first seen as pinpoint-like lesions surrounded by a reddish border. In very favorable conditions, the hyphae develop quickly through the petal and abundant spores are formed on the surface in a distinctive gray mat in about 24 hours.
Figure to the left: Once infection is initiated, symptoms begin to manifest themselves. Gray mold on rose petals is first seen as pinpoint lesions with reddish borders (left), but later the lesions coalesce and form a gray mat composed of thousands of spores (right and close up below).
Gray mold diseases exhibit symptoms relatively quickly after initiation of Botrytis infection. Other diseases such as those caused by Phytophthora cinnamomi may take much longer. A young avocado tree, may take up to a year to exhibit symptoms after its roots are infected. An older avocado tree may take several years to exhibit symptoms.
This time lag or latency of disease expression leads to many diseased nursery stock plants to be inadvertently sold and moved in the nursery trade. An infamous example of this was in 2004 when hundreds of infected, but symptomless, camellia plants were inadvertently shipped with Sudden Oak Death (caused by Phytophthora ramorum) from a southern California nursery to many parts of the United States and Canada.
Management factors such as fungicide applications are influenced by the time factor. Most fungicides are preventative, they should be applied before infection occurs. But some other fungicides have the ability to slow or stop infection and/or sporulation. The urgency of application will depend on the disease and the fungicide tools available to control it.
Next: Using the disease triangle in plant disease diagnosis
