CHAPTER VI
PROTECTING PLANTS FROM THINGS THAT PREY ON THEM
Plants are preyed on by insects and fungi; and they are subject to
various kinds of disease that, for the most part, are not yet
understood. They are often injured also by mice and rabbits (p. 144), by
moles, dogs, cats, and chickens; and fruit is eaten by birds. Moles may
be troublesome on sandy land; they heave the ground by their burrowing
and may often be killed by stamping when the burrow is being raised;
there are mole traps that are more or less successful. Dogs and cats
work injury mostly by walking across newly made gardens or lying in
them. These animals, as well as chickens, should be kept within their
proper place (p. 160); or if they roam at will, the garden must be
inclosed in a tight wire fence or the beds protected by brush laid
closely over them.
The insects and diseases that attack garden plants are legion; and yet,
for the most part, they are not very difficult to combat if one is
timely and thorough in his operations. These difficulties may be divided
into three great categories: the injuries wrought by insects; the
injuries of parasitic fungi; the various types of so-called
constitutional diseases, some of which are caused by germs or bacteria,
and many of which have not yet been worked out by investigators.
[Illustration: Fig. 211. Shot-hole disease of plum.]
[Illustration: Fig. 212. Hollyhock rust.]
The diseases caused by parasitic fungi are usually distinguished by
distinct marks, spots or blisters on the leaves or stems, and the
gradual weakening or death of the part; and, in many cases, the leaves
drop bodily. For the most part, these spots on the leaves or stems
sooner or later exhibit a mildew-like or rusty appearance, due to the
development of the spores or fruiting bodies. Fig. 211 illustrates the
ravages of one of the parasitic fungi, the shot-hole fungus of the plum.
Each spot probably represents a distinct attack of the fungus, and in
this particular disease these injured parts of tissue are liable to fall
out, leaving holes in the leaf. Plum leaves that are attacked early in
the season by this disease usually drop prematurely; but sometimes the
leaves persist, being riddled by holes at the close of the season. Fig.
212 is the rust of the hollyhock. In this case the pustules of the
fungus are very definite on the under side of the leaf. The blisters of
leaf-curl are shown in Fig. 213. The ragged work of apple scab fungus is
shown in Fig. 214.
[Illustration: Fig. 213. Leaf-curl of peach, due to a fungus.]
The constitutional and bacterial diseases usually affect the whole
plant, or at least large portions of it; and the seat of attack is
commonly not so much in the individual leaves as in the stems, the
sources of food supply being thereby cut off from the foliage. The
symptoms of this class of diseases are general weakening of plant when
the disease affects the plant as a whole or when it attacks large
branches; or sometimes the leaves shrivel and die about the edges or in
large irregular discolored spots, but without the distinct pustular
marks of the parasitic fungi. There is a general tendency for the
foliage on plants affected with such diseases to shrivel and to hang on
the stem for a time. One of the best illustrations of this type of
disease is the pear-blight. Sometimes the plant gives rise to abnormal
growths, as in the "willow shoots" of peaches affected with yellows
(Fig. 215).
[Illustration: Fig. 214. Leaves and fruits injured by fungi, chiefly
apple-scab.]
Another class of diseases are the root-galls. They are of various kinds.
The root-gall of raspberries, crown-gall of peaches, apples, and other
trees, is the most popularly recognized of this class of troubles (Fig.
216). It has long been known as a disease of nursery stock. Many states
have laws against the sale of trees showing this disease. Its cause was
unknown, until in 1907 Smith and Townsend, of the Bureau of Plant
Industry, United States Department of Agriculture, undertook an
investigation. They proved that it is a bacterial disease (caused by
Bacterium tumefaciens); but just how the bacteria gain entrance to the
root is not known. The same bacterium may cause galls on the stems of
other plants, as, for example, on certain of the daisies. The
"hairy-root" of apples, and certain galls that often appear on the
limbs of large apple-trees, are also known to be caused by this same
bacterium. The disease seems to be most serious and destructive on the
raspberry, particularly the Cuthbert variety. The best thing to be done
when the raspberry patch becomes infested is to root out the plants and
destroy them, planting a new patch with clean stock on land that has not
grown berries for some time. Notwithstanding the laws that have been
made against the distribution of root-gall from nurseries, the evidence
seems to show that it is not a serious disease of apples or peaches, at
least not in the northeastern United States. It is not determined how
far it may injure such trees.
[Illustration: Fig. 215. The slender tufted growth indicating peach
yellows. The cause of this disease is undetermined.]
[Illustration: Fig. 216. Gall on a raspberry root.]
[Illustration: Fig. 217. Canker-worm.]
Of obvious insect injuries, there are two general types,--those wrought
by insects that bite or chew their food, as the ordinary beetles and
worms, and those wrought by insects that puncture the surface of the
plant and derive their food by sucking the juices, as scale-insects and
plant-lice. The canker-worm (Fig. 217) is a notable example of the
former class; and many of these insects may be dispatched by the
application of poison to the parts that they eat. It is apparent,
however, that insects which suck the juice of the plant are not poisoned
by any liquid that may be applied to the surface. They may be killed by
various materials that act upon them externally, as the soap washes,
miscible oils, kerosene emulsions, lime-and-sulfur sprays, and the like.
There has been much activity in recent years in the identification and
study of insects, fungi, and microorganisms that injure plants; and
great numbers of bulletins and monographs have been published; and yet
the gardener who has tried assiduously to follow these investigations is
likely to go to his garden any morning and find troubles that he cannot
identify and which perhaps even an investigator himself might not
understand. It is important, therefore, that the gardener inform himself
not only on particular kinds of insects and diseases, but that he
develop a resourcefulness of his own. He should be able to do something,
even if he does not know a complete remedy or specific. Some of the
procedure, preventive and remedial, that needs always to be considered,
is as follows:--
Keep the place clean, and free from infection. Next to keeping the
plants vigorous and strong, this is the first and best means of averting
trouble from insects and fungi. Rubbish and all places in which the
insects can hibernate and the fungi can propagate should be done away
with. All fallen leaves from plants that have been attacked by fungi
should be raked up and burned, and in the fall all diseased wood should
be cut out and destroyed. It is important that diseased plants are not
thrown on the manure heap, to be distributed through the garden the
following season.
Practice a rotation or alternation of crops (p. 114). Some of the
diseases remain in the soil and attack the plant year after year.
Whenever any crop shows signs of root disease, or soil disease, it is
particularly important that another crop be grown on the place.
[Illustration: Fig. 218. A garden hand syringe.]
[Illustration: Fig. 219. A knapsack pump.]
See that the disease or insect is not bred on weeds or other plants that
are botanically related to the crop you grow. If the wild mallow, or
plant known to children as "cheeses" (Malva rotundifolia), is
destroyed, there will be much less difficulty with hollyhock rust. Do
not let the cabbage club-root disease breed on wild turnips and other
mustards, or black-knot on plum sprouts and wild cherries, or
tent-caterpillars on wild cherries and other trees.
|