How Can We Combat Plant Diseases?
At one time to stop epidemic spread of plant diseases was virtually
impossible; farmers had to abandon fields and crops. Such catastrophes
caused by microbes have changed the course of history. For example, the
Irish famines of the 1840’s resulted from the potato blight and caused
mass emigrations from Ireland.
In this country today plant diseases result in losses estimated at $3
billion a year. So far, the most economical means of reducing the
ravages of plant diseases has been to breed resistant plant varieties.
Although such a variety may cost $100,000 to develop, its cost is
usually repaid within a year or two.
But the victory is only temporary. Although plants are bred to resist
the pathogen (fungus) of the moment, Nature is constantly changing the
microbial population by mutation and hybridization. Within a few years
virulent strains of fungi which can attack the “resistant” variety
increase to such an extent that the new variety must be replaced.
For crops that provide high per-acre income such as some vegetables and
vine and tree fruits, chemical control of fungous diseases is
economically possible; in fact, it is a real necessity. But such
treatment is too costly for most field crops, unless some cheap seed
treatment or fertilizer additive can be found.
A general breakthrough in control of plant diseases is yet to come.
Because of thousands of pathogenic species, with hundreds of strains, it
does not seem possible that the following questions could be answered
about each one. What is the life cycle of the microbe? What conditions
of temperature and humidity encourage it to spread? What plant species
does it attack? How does it enter? What chemical changes within the
cells of the plant determine whether they resist or succumb to the
invader? How long can germs remain potent? How far can they travel by
wind or water? What combination of resistant varieties, cultural
methods, and chemical treatment will control the disease?
With tracers it is possible for the first time to measure chemical
uptake in single spores and to follow chemicals through the plant.
Perhaps the most enlightening information from such studies is that some
fungicides are 10,000 times less effective per unit of “body weight”
than are other chemicals used to destroy weeds and insects. Obviously
the breakthrough in chemical control of plant diseases is yet to come.
Why Do Chemicals Destroy Some Plants?
Weeds cost this country an estimated $5 billion annually, which is more
than the loss to either plant diseases or insects. Selective chemical
weed killers such as “2, 4-D” have become so widely used that more than
$135 million worth was sold in the United States in 1959. In proper
concentration these compounds will destroy many unwanted plants without
harming lawn grasses or crop plants.
[Illustration: Fig. 3—Tagged weed-killing chemicals (A) are taken in and
transported alike in grassy (B) and broad-leaved (C) plants, but only in
the latter are killed.]
As in many other instances, beneficial use of the chemicals has far
outreached an understanding of how they work. The still scanty knowledge
of the process has come almost entirely from tracer studies.
All plants readily absorb selective weed killers (“herbicides”), which
are not destroyed within the plants. Resistant plants show no effect of
the chemicals, but sensitive plants suffer damage in actively growing
roots and shoots. Sugar formation during photosynthesis is disrupted in
these plants, and phosphorus movement is retarded. In order to predict
what new classes of chemicals might be of value as herbicides, we must
await the results of research using radioactive tracers.