Nip it in the bud

It’s best to catch plant diseases early

Special to the Democrat-Gazette/JANET B. CARSON
For HomeStyle story about plant diseases, shelf fungus on a tree is a sign the trunk is rotting.
Special to the Democrat-Gazette/JANET B. CARSON For HomeStyle story about plant diseases, shelf fungus on a tree is a sign the trunk is rotting.

An amazing summer for Arkansas, with lower temperatures and periodic rain, has been a boon for our gardens. But it has also benefited a few diseases.

Scouting your garden on a regular basis can help you spot a problem in time to stop or (at least) learn what to worry about or not worry about.

A plant disease is usually defined as abnormal growth of a plant - the result of some disturbance in the normal life process.

There are more than 80,000 different diseases of plants, and they have been causing problems since the beginning of time. Think of the great potato famine of the 19th century.

Some diseases threaten the life of the plant, and others simply cause a slowdown in production or growth.

Diseases can be the result of living (biotic) or nonliving (abiotic) causes. Biotic diseases are caused by living organisms such as fungi, bacteria, viruses and parasites. Abiotic diseases are caused by environmental conditions such as late frosts, drought, hail and chemical burn from pesticides or fertilizer.

Unfortunately, the damage caused by these various living and nonliving agents can look very similar. Even with close observation, a positive diagnosis can be difficult.

SEE THE SYMPTOMS

When trying to decide what is causing problems, we need to first look at the symptoms. Diseases can show up on leaves, stems and roots, but they also can damage flowers and fruit.

Leaves: For leaf symptoms, there can be spots of various colors, powdery substances covering the upper or lower surface of the leaves and distorted foliage with mottled leaf tissue.

Stems: Stem damage can be sunken lesions that are soft or discolored. The damage can occur above ground, at the soil level or slightly below ground.

Larger trees that have internal decay may often sprout mushroom-like growth on the trunks which signal damage within. Hypoxylen canker is a damaging disease of oak trees. By the time the outer bark sloughs off, exposing the dark black or light gray mycelium, the tree is pretty far gone, and saving it is a lost cause; but this disease only attacks weak trees - which unfortunately we have a lot of after two hot, dry summers.

Roots: The harder diseases to identify are those that affect the roots, since we don’t want to uproot a plant to see how its roots are growing. We can do that on houseplants or small annuals and perennials, but with trees and shrubs, we often find out about the root problem as we are digging it up to dispose of a dead plant.

Root rots typically are worse in poorly drained sites. If you are examining roots on a small plant, the roots should be full and white. If they are black and slimy or give off a musty odor, they need to be cut off and cleaned before replanting in fresh soil - if the plant isn’t already too far gone to save.

ABIOTIC FACTORS

As we are examining the plant symptoms and damage, we also need to consider the weather and cultural conditions. Weak plants are often more susceptible to disease and insect problems. Some diseases occur more commonly in cool, wet weather, while others are more common in warm, damp weather.

Moisture that stays on plants overnight can often take its toll with diseases. Keep in mind the disease triangle that must occur for diseases to be present. Diseases require:

a susceptible plant

a disease organism

the right environment for that disease to grow.

When we have all three, disease can happen.

When we have a lot of one plant in the same garden and a disease hits, that disease can then multiply much more quickly. This is why agents of the University of Arkansas’s Division of Agriculture County Extension Service recommend having diversity in the garden.

Most diseases are host-specific. For example, black spot on roses won’t grow on hollies, and fire blight of apples and pears won’t grow on peaches.

Some plants are commonly prone to specific diseases.

Hybrid tea roses are plagued by black spot, and if you don’t spray on a regular basis, the disease will annihilate your plants. New Earth-Kind varieties and some of the antique rose varieties are not susceptible to the disease, and these are becoming increasingly popular since many gardeners want to avoid spraying pesticides.

Red top photenia used to be one of the most popular hedges in the South, but it is now highly susceptible to entomosporium leaf spot. Plants have been dying out for years, so we are seeing fewer and fewer photenias planted.

Crape myrtles and lilacs often suffer from powdery mildew. Moderate, humid conditions often make it worse. In the hotter, drier summers, it often is not as bad as in mild, humid years. New varieties of both plants have been bred with disease resistance.

Tomatoes suffer from many diseases, many of which are soil borne. Several wilt diseases including verticillium and fusarium wilt are common in Arkansas, but new varieties of tomatoes (with the initials VF following their variety name) are resistant to the disease. But if you plant in the same spot every year, the disease organisms build up and problems will arise even if you use resistant varieties.

MIX IT UP

Crop rotation - not planting the same thing in the same spot for three years - can discourage diseases. Even in flower beds, if diseases attack flowers (such as this year theimpatiens with downy mildew or periwinkle with stem canker) avoid planting those same annuals in the same spot for a few years. Increase your chances of success.

If you have annual plants in the flower or vegetable garden that get plagued with diseases, toss them and plant something else. If you have deciduous plants that were infested with leaf diseases or have dead twigs, clean them up in the fall, getting rid of the damaged plant materials. Don’t let them lie on the ground or you will start the process over again the following spring.

If you have flowers or fruits that were diseased and they are lingering on the plants, remove them and dispose of them. They can mummify and stay attached to the plants all winter, and these mummies contain the disease spores to reinfect the plants.

So monitor your gardens. Even if you are giving your plants the care they need, problems will arise. Some of the plant diseases can mimic fertilizer burn or insect injury. Proper identification is crucial. There are products that can control most fungus and bacteria organisms, but all work best if the problem is caught quickly.

In the case of plants that get the same diseases year after year, such as roses and fruit trees, preventive sprays are your best mode of control, or select disease-resistant plants that you won’t have to spray. There are no controls for virus diseases, so sanitation is crucial.

ASK THE EXPERTS

If you are not sure what the problem is, take a good sample to your local county extension office. It is best to take in samples early in the week so the samples make it to the disease diagnostic lab in good shape.

Take a representative sample - various stages of the problem. Don’t take a long dead plant. Give the agents leaves, stems and even roots (if needed) to look at.

Samples should be kept cool until you can get them to the county office. Put the samples in a plastic bag, but do not add any moisture or the plants could rot during shipment. If you have roots with soil clinging, wrap them in a separate plastic bag, again adding no moisture. A good sample will make correct diagnosis easier.

To find your county agent, look in the phone book or on the website uaex.edu.

HomeStyle, Pages 35 on 08/17/2013

Upcoming Events