Most garden diseases are caused by invisible agents — fungi, bacteria, and viruses. We cannot see them without a microscope, but we can see the diseases they cause, such as leaf spots, blights, and root or stem decays.
If you have not yet seen evidence of the more common diseases in your garden, state and federal experiment station publications, books, and articles in garden magazines will help you to become familiar with them.
Types Of Diseases
Some fungus diseases, such as botrytis blight of tulips and blackspot of roses, are so generally prevalent that most of you are well acquainted with them.
Others, such as root rots, are more difficult to recognize because the symptoms are visible only by lifting the plants and washing the soil from the roots.
Virus diseases are even more difficult to diagnose because their symptoms resemble those caused by nutrient deficiencies, insects, and other agents.
I wish I could offer a simple set of directions for growing healthy plants. But, unfortunately, I cannot because growing disease-free plants requires a working acquaintance of the causes of plant diseases, some knowledge of the principles on which disease-control practices are based, and, perhaps above all, constant vigilance.
But occasionally, one finds a gardener who, year after year, has few or no diseases in his garden. This, despite any thorough knowledge on his part of the cause and control of plant diseases.
If we delve into the reasons for his success, we usually find one or all of the following factors at work:
Factors To Discourage Plant Diseases
- First, the soil in his garden was well prepared before the gardening season started. That is, it was deeply dug; considerable organic matter was present at the start or was added to it, and the soil had good air and water drainage.
- Second, the garden was so located that it received lots of sunshine, was well away from the roots of large trees, and the plants were not crowded. Third, weeds in and around the garden were almost nonexistent. Finally, he buys only top-quality plants and seeds.
All four factors tend to discourage diseases or at least prevent their gaining a strong foothold in the garden.
Soil Preparation
In my experience, more garden failures result from improper soil preparation than any other single factor.
A soil spayed when still quite wet rarely produces good plants during the growing season.
One that is heavy and poorly drained cannot grow healthy plants. Incorporating ground limestone (in all but arid regions and naturally limey soils), organic matter of various kinds and perhaps installing agricultural drainage tiles are suggested for such soils.
All plants except mushrooms need lots of sunshine. When grown in fairly heavy shade, they suffer from a lack of light and are more susceptible to fungus and bacterial parasites.
Large trees’ long, dense roots compete with garden plants for moisture and food.
A Reservoir For Plant Viruses
Besides competing for moisture and nutrients, weeds are breeding places for many destructive insects and serve as reservoirs for plant viruses.
Chickweed, for example, can harbor the virus known as spotted wilt, which insects readily transmit to tomatoes and dahlias growing nearby. In addition, several weeds are known to carry the “yellows” virus, which is so destructive to China asters.
Some diseases are introduced into a garden on newly purchased plants even though such plants appear healthy. Unfortunately, there is no easy way of detecting incipient infections on these, and isolating them in some out-of-the-way spot in the garden for a few years is not usually feasible.
Thus the best procedure is to purchase new plants from reliable nurseries or sell houses and to examine each plant carefully before setting it in its new home.
Disease Resistant Strains
Another way to keep diseases in check is to purchase, when available, so-called disease-resistant kinds.
For example, good gladiolus varieties are resistant to the fungus disease known as Fusarium yellows and certain chrysanthemums are resistant to the highly destructive verticillium wilt.
Some kinds of dahlias, though not completely resistant, are highly tolerant to the virus disease known as “stunt.”
Cotoneasters, snapdragons, and many vegetable seeds produce plants that are highly resistant to certain fungi.
Therefore, the gardener would do well to study nursery and seed catalogs carefully and select those that are said to resist diseases. In addition, lists of disease-resistant plants are available from state and federal experiment stations.
Lastly, overcrowding plants is a poor practice because it favors the development of diseases like mildew. Phlox and roses, for example, are always more subject to mildew when they are overcrowded or where the air circulation is poor.
Unfortunately, a few diseases will appear in some gardens despite all the precautions mentioned above. In such cases, therefore, the gardener must resort to sprays or dust, although, occasionally, diseases gain a foothold despite their use.
Failure To Control Diseases?
What, then, is the reason for the failure to control diseases? The most common is that the sprays or dust are not applied in time.
Fungus and bacterial diseases of the leaves start at least several weeks before they become spotted, yellowed, or blighted.
Hence it is necessary to have the spray materials on the plants weeks before the appearance of the symptoms. In other words, one must anticipate infections and beat the fungus spores or bacteria ‘‘to the punch.”
Time To Start Spraying Roses
A good gardener knows that the time to spray roses for blackspot is not midsummer when the leaves become spotted and yellowed. Instead, the time to start is just as soon as the leaves emerge in the spring.
By coating the newly developing leaves’ upper and lower surfaces, there is little chance that the blackspot fungus spores can germinate on and penetrate the leaves.
Of course, he must spray the roses periodically to cover new leaves as they emerge or to cover new areas on the older leaves.
The more rain in late spring, the more frequent the applications. In rainy weather, it may be necessary to spray roses every four or five days; in dry spells, it may be necessary to spray up to every two weeks.
Frequent Spraying
I do not recommend that all plants in the garden receive as many sprays as roses. The gardener will soon find, either from experience or from periodicals, which plants require frequent sprays and which can be grown with few to no sprays.
Generally speaking, roses, chrysanthemums, phlox, and mountain laurels are so subject to fungus diseases that they require persistent sprayings annually. Others, such as tulips, hollyhocks, iris, and delphinium, do not require periodic spraying for disease control.
Some of these, however, may require an occasional application of insecticide to keep insects and related pests under control.
For plants that do not require a regular spray program, early detection of the first stages of disease and removal and burning of infected parts are usually sufficient to keep most diseases in check.
Fungicides Available
There are many excellent fungicides on the market against fungus diseases of leaves and stems.
Perhaps the most widely used in recent years is Fermate, a black, sootlike powder that is extremely effective against fungus diseases like blackspots of roses and scabs of apples.
It is not so effective against mildew diseases, however. For these, sulfur compounds do a far better job.
Also available are copper compounds and certain organic mercurials which control bacterial and some fungus diseases. Combinations of fungicides and insecticides are also available to be used as directed.
Besides applying the fungicides in time, the gardener should do a thorough job, using a good duster or sprayer.
44659 by P. P. Pirone