Malnutrition in human beings is considered a disease. But, for some reason, we do not think in the same terms as plants.
Most gardeners feel that if a plant lacks some element, such as nitrogen, all that is needed is to supply the missing dement.
Although partially true, this is only partially accurate. For example, if the lack of some food elements adversely affects growth in the seedling stage, we can often reverse this condition by proper feeding with complete plant food.
At the same time, if allowed to progress too far, a point is reached where the damage is irreversible.
At times, even the experienced plant disease expert is puzzled and has difficulty deciding whether a certain condition is caused by disease or lack of food.
Those caused by viruses are particularly bothersome since they tend to change the inner color of the leaf in the same areas where color changes occur because of a lack of certain food elements.
How To Distinguish Between Virus Disease And Food Deficiency
The best way to distinguish between virus disease and food deficiency is to know the feeding program.
Virus diseases attack the healthiest, most vigorous plants. The virus agent (a complex near-living protein substance) needs young, rapidly-growing tissue to develop.
If the plants have been doing well before abnormal symptoms arise, and if the soil has been fed liberally with good mixed plant food and the pH is within a range where all elements should be available (pH range of 6.0 to 7.5), the chances of either major or trace element deficiencies are not great. In such cases, suspect virus infection.
Check For Discoloration
Another partial check is the nature of the discoloration. Most virus diseases cause changes in the green coloring matter of the leaf. Hence the color change, whether mottling, ring spots, or an all-over-yellowing, is either green and white or yellow and white.
The purple, brown, or reddish colors, which appear in connection with most deficiency symptoms, are lacking.
It is easy to tell the difference between yellowing caused by a lack of manganese, magnesium, or iron and that caused by a virus.
In the case of chemical deficiencies, the veins of the leaves tend to remain green, while the areas between the leaves bleach out.
Virus infection is either mottled, in rings, or mosaic pattern over the entire leaf.
Checking For Aster Yellows
The trouble with this method of diagnosing comes when we are dealing with the early stages of nitrogen or iron deficiencies or, in some cases, excessive nitrogen. Here we might confuse the symptoms with those of aster yellows.
The experienced gardener knows that the yellow virus travels down the leaf (beginning in the puncture made by an infected leaf hopper) through a vein. It can be traced as a streak down the stalk into the root.
When it moves upwards, it is diffused through the entire plant and may cause complete yellowing of all parts.
Watch for the streaks caused by an early yellow infection in plants susceptible to this disease. Destroy infected specimens as soon as found, so they cannot act as “Typhoid Marys” to infect the rest of the plants.
From the above, we find we can distinguish between a virus disease marked by all-over yellowing by
- Supplying quickly available nitrogen
- Feeding with chelated iron
- Checking for typical symptoms of aster yellows
If the plants become green again, they are all right. An easy way to do this is to spray the plants with a complete fertilizer in soluble form (foliar feeding).
This should correct deficiencies temporarily and serve as a diagnostic tool. Since foliar feeding is only temporary, follow with a sail application of a good mineral or chemical fertilizer containing all 13 essential elements.
A few “crinkle” diseases are caused by viruses, such as shoestring or string top of tomatoes and spindle tuber disease of potatoes, which are nutritional troubles. The foliar feeding test should show whether lack of food is the trouble in about five days.
Disease Or Mineral Deficiency
Diseases caused by fungi are not often confused with mineral deficiencies. However, the inexperienced gardener might be fooled by some of these, such as a black spot of roses.
However, the external nature of these diseases should be apparent if the leaf is carefully studied. Rusts and mildews are also external. Examining the leaf under low magnification should make the difference easy to detect.
Every serious-minded gardener needs a small magnifying glass or a jeweler’s loupe. It should be powerful enough to show details but not so powerful that the field of vision is restricted. Anywhere from 10 to 20 power is adequate.
Incidentally, if you are trying to interest children in gardening, try showing them leaves, aphids, flower parts, and other garden details through a glass. They find this a fascinating pastime and soon become interested in the entire field of plant care.
Bacterial diseases can be more confusing. Stem, crown, and root galls —specific diseases caused by bacteria —are the only ills that do not resemble nutritional deficiencies.
Many of these diseases stunt the plant, suggesting poor nutrition. For example, bacterial canker of tomato seedlings checks growth in much the same way as food deficiencies.
Soft rot of calla (which also attacks geraniums, hyacinths, iris, sansevieria, and several vegetables) causes brown patches in the leaves with marginal yellowing.
Usually, bacterial diseases are rather rapid in their effect, and the gardener soon learns that disease rather than nutrition is at fault.
Since practically none of the diseases caused by bacteria can be cured in garden plants, infected specimens should be destroyed as soon as discovered.
No virus disease can be considered curable, although symptoms may vanish without apparent cause. Most such diseases display no symptoms in cool weather but show up rapidly with rising temperatures.
The mildews can usually be destroyed among fungus diseases if only on the surface, but this is not too certain.
Treatment For Plant Diseases
The best course of treatment for plant diseases is prevention, not cure. Once the tissues are invaded, in most instances, it is too late.
Even if only a leaf is lost, further inoculation is possible from the dead leaf. However, the constant loss of foliage will eventually result in the death of the plant.
Finally, the amateur should understand that the methods recommended for checking are not scientific. Nor are they infallible.
The only accurate tests require such complex procedures as tissue analyses, bacterial cultures, transfers of infective agents by grafting, binocular microscopes, and other costly materials and undertakings beyond the capacities of the home gardener.
My rough methods may work 90% percent of the time, but answers cannot be taken for anything final.
44659 by R. Milton Carleton