Identifying and combating plant diseases
By Donna Prock

Last month I talked generally about how to prevent insect and disease problems in the garden. Now I am going to discuss the most common diseases in our area and some basics in how to control them.

In order for pathogenic plant disease to occur, three conditions must be met: 1) the host plant must be susceptible, 2) an active, living pathogen must be present and 3) the environment must favor disease development. If one of these factors is absent, disease does not occur. The more the circles overlap, the more serious is the disease problem.

Identifying Diseases

When identifying problems with your plants, the first step is to rule out environmental factor—nutrient deficiencies, temperature extremes, soil pH, over- and under-watering. If you have ruled these out, you can begin to consult with experts and search in books and websites about plant diseases.

There are many ways to learn about plants and the specific diseases that afflict them. The Oregon State Extension Service provides two excellent resources for identification. Their Master Gardener program (http://www.washingtoncountymastergardeners.org/) has volunteers who answer questions over the phone and takes walk-in queries. Call 503-821-1150, option 2; or bring in a sample of the damage when you visit their office at 18640 NW Walker Road, Suite 1400, in the south end of the Capital Center complex.

Their 600-page Pacific Northwest (PNW) Plant Disease Control Handbook can be ordered for $45 at: http://extension.oregonstate.edu/catalog/pdf/plant/plant.pdf.

Building on the information I provided last month, the five basic principles of plant disease control are: exclusion, avoidance, eradication, protection and resistance.

Exclude disease by purchasing only healthy plants, thus eliminating the movement of diseased plants into a new area.

Avoid problems by choosing the right plant for the right site. Handle them carefully to avoid creating wounds that create entry points for disease. Look for cultivars that have resistance to known diseases.

Eradicate any diseased plant material through crop rotation, sanitation, elimination of alternate hosts, and chemical or heat treatments.

Protect a healthy plant before it becomes diseased by methods such as dipping the roots of a young plant in a protective medium before planting.

In controlling a plant disease, it is helpful to understand the life cycle of the disease and to apply the proper treatment at the proper stage of the disease. Pictured is the life cycle of the fungus that causes apple scab, which scars apples with brown indented splotches. Since the fungus over-winters in infected leaves on the ground, this diagram shows why good sanitation is essential in eradicating, or at least reducing, apple scab. While apple scab can be treated to some degree with fungicides, very few fungicides are available to home gardeners and are difficult to obtain in small quantities. If you do decide to try a fungicide, the instructions on the label are the legal requirements to use the chemical.

Plant pathogens are microscopic living organisms that cause infectious plant disease. The four main kinds of microorganisms that cause plant diseases are fungi, bacteria, viruses, and parasitic nematodes.

Parasitic fungi get their nutrients by feeding on host plants. Fungi are usually visible as dots on discolored parts of a plant. Microbial fungicides, mixtures of beneficial fungi that destroy parasitic fungi, can be purchased for treating fungal infections. These are very sensitive to soil temperature and only operate when warm enough. Black spot is a common fungal disease that attacks roses and other garden plants.

A garlic fungicide can be made by blending 5 garlic cloves with one pint of water and then straining. Sulfur is an option for soils severely infested with fungal diseases. All fungicides should be considered only as a last resort. The best way to prevent fungi and all plant diseases is to nurture soil by adding organic matter and by purchasing disease-resistant plant varieties when available.

Disease-causing bacteria cannot be seen with the naked eye, but their effects on a plant are usually clearly visible. These bacteria are very hard to control, but the best way to prevent them from spreading is by properly disposing of plant debris (burning, depositing in trash can, and hot composting) and always washing garden tools so as not to spread bacteria.

A common bacterial plant disease is bacterial leaf spot. Infected plants will develop dark-colored, water-soaked spots that surrounded by yellowing halos. Continuous rain and moisture will cause the spots to coalesce. Severely infected leaves will drop prematurely. Bacterial leaf spot is a common nuisance of stone fruit trees and vegetables, as well as other indoor and outdoor foliage plants. There is no cure for bacterial leaf spot. However, you can reduce the potential for bacterial leaf spot by keeping the area free of decomposing debris and watering your plants at soil level.Viruses can rarely be seen, even with a microscope.

Viruses reproduce while inside plant cells. They can be spread through garden tools and on hands, as well as on aphids, insects, and nematodes.

Cucumber Mosaic Virus is a common virus affecting a number of vegetables and ornamental plants, including pepper, tomato, cucumber, melons, squash, spinach, celery, beets and petunia. The disease appears with yellow and mottled older leaves, which gradually start to twist and curl downwards, taking on a "shoestring" look. Plant growth and fruit production is also stunted. Aphids commonly transmit cucumber mosaic virus. There are no controls after a plant is infected. The best preventive measure is to inspect all new plants to make sure they are healthy. Remove and destroy diseased plants immediately.

Parasitic nematodes attach to plant roots and lay eggs on them. These parasites puncture plant cells and suck out their contents. Nematodes spread slowly on their own but can be easily spread by water and on garden tools. Nematode infestations can be prevented by adding nitrogen to soil. (NOTE: some nematodes are beneficial and attack insect parasites. This is a good reason not to use chemical poisons in your soil)

The Root-knot Nematodes change the root structures to form small knots on the infected root, hence earning their nickname. These nematodes enter the roots of plants as juveniles and settle down in the root cells to make home. They swell up as they continue development. The hormones that they give off create the swollen plant areas called galls or root knots. Males leave the root when they reach adulthood while the females stay to fatten up. The female lays her eggs in the soil and thus the cycle begins again. The image displays a classic example of an infected carrot root. As you can see, its root system is weak, stunted, and deformed. In cases as such, the damage is permanent, rendering plants like potato tubers and carrot taproots inedible.

Nematode control is essentially prevention, because once a plant is parasitized it is impossible to kill the nematode without also destroying the host. The most sustainable approach to nematode control will integrate several tools and strategies, including cover crops, crop rotation, soil solarization (a technique that exposes soil to heat by covering with plastic sheeting), least-toxic pesticides, and plant varieties resistant to nematode damage. These methods work best in the context of a healthy soil environment with sufficient organic matter to support diverse populations of microorganisms. A balanced soil ecosystem will support a wide variety of biological control organisms that will help keep nematode pest populations in check.

A rich assortment of organic/non-chemical disease and pest control methods can be found at: http://www.howtogoorganic.com/index.php?page=pest-disease-management. The Resource Guide for Organic Insect and Disease Management is also an excellent reference: http://web.pppmb.cals.cornell.edu/resourceguide

Next month we'll talk about insects and other pests.

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