brassicæ_, and it belongs to the _Myxomycetes_, or ‘slime-fungi,’
which, as a rule, live upon decaying vegetable material. The protoplasm
of the fungus ramifies among and within the tissues of the roots of
attacked plants, and eventually produces an amazing number of spores so
small that more than thirty millions would be required to cover a
superficial inch. A microscope of great power is necessary to reveal
them to human vision.
[Illustration: Fungus of Finger-and-Toe Disease] FUNGUS OF
FINGER-AND-TOE DISEASE
_Plasmodiophora brassicæ_
The spores are capable of resting in a state of vitality for a long
time, and can easily withstand the frosts of winter. The illustration
shows at A the fungus in its protoplasmic condition, and at B its
ultimate sporiferous or ‘seed’-producing stage, after the protoplasm
has changed to a mass of minute spores (enlarged five hundred and
twenty diameters). When a spore in due course germinates, its
protoplasmic contents escape through a small aperture in its wall and
begin moving about of their own accord in a slow writhing manner. The
movement is so much like that of the microscopic animal organism found
in ponds, and called _Amœba_, that this tiny mass of moving protoplasm
is called _Myxamœba_, to denote that it is an amœba-like form produced
by one of the _Myxomycetes_. Each myxamœba is drawn out at one spot
into a fine delicate tail or cilium, as at C, D, E, and is capable of a
creeping motion in moisture. When quite free from the spores,
transparent expansions or limbs extend from the bodies of the myxamœbæ,
as at F, G, and when these organisms, after existing in the soil for a
longer or shorter time, reach the roots of cruciferous plants, which
they apparently enter through the root-hairs, they again assume the
protoplasmic condition shown at A, and live within the cells, at the
expense of the nurse-plant. Other cruciferous plants are less seriously
damaged by the pest than are Turnips and Cabbages; but it is evident
that if diseased Charlock is near Turnips, the latter are very likely
to fall a prey to the disease. We advise the sowing of the best seeds,
the eradication of cruciferous weeds, and the destruction by fire of
all decaying Finger-and-toe material, for it is in this material that
the spores of the disease rest ready for continuing the disease in the
following season. It is also desirable that cruciferous plants should
not be continuously grown in the same quarter—in other words, it would
be prudent after an attack of Anbury not to repeat a cruciferous crop
on the same ground, but to follow on with a crop of some other class.
Numerous experiments have shown that slaked lime can be relied on to
destroy the spores of Finger-and-toe in infested land. An application
of from fourteen to twenty-eight pounds per pole may suffice in the
case of light soils, but fifty-six pounds per pole will not be too much
on heavy land, and the dressing should be given either six or eighteen
months before a Cabbage or Turnip crop is sown; the longer period is
the more certain in its effect. Preference should be given to stone or
rock lime over chalk lime. The former is much more powerful and
efficient. It may be necessary to repeat the dressing twelve months
after the first application. As regards the occurrence of Anbury in
seed-beds, frequent transplantation is a very effectual mode of
stopping its progress, for the little galls can be pinched off by the
workman, and burned as he proceeds; and the plant, being invigorated by
change of soil, will soon grow away from the affection. In
transplanting Cabbages it is a good plan to discard and burn such
plants as are obviously affected with Anbury. It is worthy of remark
that in market-gardens this disease is by no means so prevalent as to
interfere with the routine of cultivation, although the Cabbages,
Broccoli, and Cauliflowers grown in these grounds are, under other
circumstances, especially liable to attack. By ‘other circumstances’ we
mean that market-gardens are generally kept under high cultivation, the
land being perpetually turned and heavily manured; and these measures
appear to be a preventive of Anbury, while they result in heavy crops.
But on land less energetically tilled Anbury may prevail to such an
extent as to interfere seriously with the order of cropping. Another
very important mode of keeping down the pest consists in burning
instead of burying the stumps and all other refuse of the crop that
cannot be turned to account.
Confusion may be prevented if we point out that Club-root, Anbury, or
Finger-and-toe—whichever name may be used—is quite distinct from an
apparently similar malformation of the root which is sometimes induced
by certain characteristics of soil, seed, or manure, and is in fact a
case of reversion to the original wild type. Instead of a shapely,
solid Turnip, the bulb is divided into a number of coarse, worthless
tap-roots, caused by either poverty of the soil, careless cultivation,
or a degenerated stock of seed. Those who save their own seed
continuously for years are almost certain to become well acquainted
with this malady. They will find a change of seed necessary, and at the
same time an alteration in the routine of culture. A healthy, vigorous
plant, derived from a pure seed-stock, does not easily make
Finger-and-toe, but a sound root that stands for food and money.
‘Grub.’—The wart-like growths formed upon the roots of Turnip and
Cabbage by the little hard beetle known as the Turnip-gall Weevil,
_Ceutorhynchus pleurostigma_, are also quite distinct from
Finger-and-toe. By cutting across a malformed root of Turnip or Cabbage
it is usually not difficult to determine the cause of the mischief. If
it is Finger-and-toe the root will be found filled with decaying
matter; in the case of Weevil attack the small legless maggots,
commonly called ‘Grub,’ will be brought into view; and if it is merely
an instance of reversion the cut root will appear to be healthy.
Potato Disease.—The fungus which causes the Potato Disease, or ‘Blight’
as it is sometimes called, was formerly known as _Peronospora
infestans_; now it is recognised by scientific authorities as
_Phytophthora infestans_. The mark of its pestilent touch on the
foliage, and its destructive effect on the tubers, are unfortunately
too familiar in gardens and on farms. In dry seasons its energies are
restricted, but the scourge is never absent, and during wet summers the
parasite may do its deadly work on such a vast scale as to cause a
Potato famine. Moisture is a necessity of its existence, and in rotting
haulm, decayed tubers, and damp soil the spores remain in a resting
condition until they are afforded an opportunity of multiplying with
the marvellous rapidity that invests the disease with its terrible
power. A series of six illustrations, five of which are highly
magnified, will enable the reader to follow the development of
_Phytophthora infestans_.[1]
[Illustration: No. 1.] No. 1.
The illustration No. 1 shows a Potato leaf on a reduced scale
disfigured by the attack of the fungus. The _Phytophthora_ is sending
mycelial threads (called hyphæ) in all directions through the substance
of the leaf, feeding on the protoplasm of the cells and destroying the
chlorophyll, or leaf-green, in those cells.
[Illustration: No. 2. and 3.] No. 2. & No. 3.
No. 2 shows the fungal threads at work. In a diseased Potato plant
these threads, or mycelial hyphæ, make their way through the substance
of the leaves, and down the haulm into the tubers, from which they
consume the food stored there.
No. 3 exhibits the various stages of germination of one of the conidia
of _Phytophthora infestans_: (_a_) the ripe conidium in water; (_b_)
protoplasmic contents breaking up into blocks, which separate and
escape (_c_ and _d_) as minute kidney-shaped zoospores (_e_) each with
two cilia; (_f_ and _g_) the zoospore coming to rest and losing its
cilia; (_h_, _i_, _j_, and _k_) successive stages of germination of the
zoospore.
[Illustration: No. 4.] No. 4.
No. 4 represents a longitudinal section of Potato-stalk with
germinating zoospore, the germ-tube of which has pierced the cell-wall,
and is growing inside the cell, as shown at +.
[Illustration: No. 5. and 6.] No. 5. & No. 6
No. 5 affords a view of another piece of tissue of the stem of a Potato
plant, and shows the hyphæ of _Phytophthora infestans_ running in the
cell-walls; (_a_) nucleus of a cell; the other contents shown are
crystals and chlorophyll corpuscles.
No. 6 is a section of a Potato tuber: A, the cell-walls; B, the starch
grains; C, the mycelial hyphæ.
Spraying Potato plants twice or thrice with Bordeaux mixture has proved
effective in warding off the attack of _Phytophthora infestans_, and
the practice is now freely adopted, especially in humid districts. The
first application should be given towards the end of June or early in
July, immediately the haulm is sufficiently developed. The Bordeaux
mixture is made in the proportion of four pounds of pure copper
sulphate and two pounds of quicklime to forty gallons of water. The
foregoing quantities will give what is known as the _one per cent._
mixture. For the _two per cent._ mixture the quantities of copper
sulphate and quicklime must be doubled, but the amount of water should
remain at forty gallons. In its effect on the fungus, however, little
difference is to be found between the two solutions. The copper
sulphate is stirred into a few gallons of hot water placed in a wooden
tub or earthenware vessel. When quite dissolved, add twenty or thirty
gallons of cold water. The lime, which must be freshly burnt quicklime,
is then slaked in another vessel and thoroughly stirred with two or
three gallons of water until it is of the consistency of thin cream. As
soon as the liquid is quite cold, filter it through coarse sacking into
the copper sulphate solution and add water to make a total of forty
gallons. To be effective, Bordeaux mixture must be applied in the form
of a fine spray, and not with a coarse-holed syringe.
The Burgundy mixture, the use of which is preferred by some, acts in a
very similar manner to the Bordeaux mixture, and is made in the same
way as the latter, except that washing soda (five pounds) is
substituted for quicklime.
Those who leave Potatoes to rot in the ground because the crop is not
worth digging, or who bury diseased haulm and tubers in a shallow
trench, under the impression that it is a safe way of getting rid of
worthless vegetation, are simply storing _Phytophthora_ for another
attack in the event of Potatoes being planted in the same land again.
If buried at all, it must be at a considerable depth, but the effectual
method is to destroy all Potato refuse by fire.
Wart Disease (Black Scab) of Potatoes (_Synchytrium endobioticum_,
Percival).—This extremely infectious and destructive disease of the
Potato has been given a variety of names in different parts of the
country, but it is now generally known as the Wart or Cauliflower
Disease, the latter term being attributable to the Cauliflower-like
appearance of the outgrowth of the fungus. This outgrowth first shows
in the eyes of the young Potato in the form of small wrinkled warts.
These multiply and combine, thus creating a dark spongy scab which
eventually decomposes. Where the disease is very rife it attacks haulm
as well as tubers, and a yellowish-green mass may sometimes be found
just above or just below the surface of the soil. As a rule, however,
no outward indication of its existence is to be seen in the crop during
the early stages of growth, but towards the end of the season the haulm
of badly diseased plants often retains a fresh green appearance when
the foliage of others, which are healthy or only slightly attacked, is
dying off.
Infection is perhaps most commonly spread by the planting of diseased
tubers. Another frequent means of dissemination is caused by consigning
infected haulm to the waste heap instead of to the fire. The spores may
also be introduced in manure from animals fed on diseased Potatoes in a
raw state, and they may even be carried from one plot to another on
garden implements or the boots of those who walk across infected
ground. Immediately any sign of the disease is observed it should be
dealt with promptly and in no uncertain manner. Every particle of the
infected material must be carefully collected and burned. Dig out the
soil around all diseased plants and burn this also. On infected land it
is important that some crop other than Potatoes be taken in the season
following the outbreak, and, if possible, such land should not be used
for Potatoes for at least five or six years. But where garden space is
limited, a contaminated plot may have to be requisitioned for Potatoes
within two or three years. In such cases it is an excellent plan to
dust the sets freely with sulphur at the time of planting and to repeat
the application before earthing up.
Although for some years the unremitting labour of experts has been
devoted to the investigation of Wart Disease, and innumerable
experiments have been undertaken, no effectual remedy has yet been
discovered. It has been found, however, that certain Potatoes are
resistant to the disease, and by order of the Ministry of Agriculture
and Fisheries none but ‘immune’ varieties may be planted in districts
scheduled as infected areas. A notification of the existence of Wart
Disease must be made to the Ministry immediately it is observed.
Leaf Spot of Celery.—This disease, which is caused by a minute fungus
(_Septoria apii_, Chester), is capable of inflicting serious damage to
the Celery crop unless prompt measures are taken to exterminate it. The
first sign of its appearance is to be found in the leaves in the form
of small brown patches. These are, however, quite distinct from the
spots deficient of leaf-green due to the attack of the Celery Fly
larvæ, and on close examination may be recognised by the presence of a
number of very small black points. From the leaves the fungus quickly
spreads over the leaf-stalks and finally to the heart of the plant,
ending in its total collapse. So rapid is the multiplication of the
spores, especially in moist weather, that a few diseased plants are
capable of infecting a large plot within two or three weeks.
Immediately discoloration of a leaf is noticed the affected portion of
the plant should be picked off. If the stage of the disease is so far
advanced that the outer leaf-stalks have become decayed, the entire
plant should be removed and destroyed. It is of the utmost importance
that every particle of diseased material be consigned to the fire and
not to the waste heap. Spraying three or four times with Bordeaux
mixture at intervals of two or three weeks may be helpful in the case
of a light attack, but the safest course always is to remove and
destroy any plant on which the fungus is found. One of the most
frequent means of introducing Leaf Spot of Celery is through the use of
infected seed, and therefore only seed which has been treated for the
destruction of the fungus should be sown.
Lettuce Mildew.—This fungus is named _Bremia lactucæ_, formerly known
as _Peronospora ganglioniformis_, and is sometimes of the most
destructive character. It covers Lettuce leaves with a fine white
bloom, which decomposes the leaves, and makes them adhere together in
one putrescent mass. It should be looked for in its earliest stages,
and be hand-picked and burned. Old Lettuce stumps should likewise be
pulled and burned, otherwise they may harbour the disease.
Onion Mildew is caused by the fungus _Peronospora Schleideni_, which is
occasionally disastrous in its effects, more especially in cold, wet
seasons. It occurs at uncertain intervals of time with extraordinary
virulence, and then utterly destroys the crops. Autumn sowing is
considered a good preventive by many growers, as the disease is
frequently fatal to spring seedlings. In its early stages the mildew
may be successfully dealt with by freely dusting the plants with
flowers of sulphur when wet with dew, or by the application of sulphide
of potassium in the proportion of one ounce to a gallon of water.
Otherwise all diseased material should be removed and burned.
Pea Disease.—Although garden Peas often suffer badly from the attacks
of _Peronospora viciæ_, which is the cause of Pea Mould, yet the most
deadly foe to Peas, especially late Peas, is a fungus of a totally
different character. To such an extent does the Pea Blight sometimes
devastate the later Peas, particularly in dry summers, that the whole
crop is in some gardens completely annihilated. The name of the fungus
of the Pea Blight or Mildew is _Erysiphe Martii_. Its attack is often
made suddenly; the leaves then lose their natural green colour, and
become yellowish and densely coated with a fine white bloom; this bloom
becomes at length dusted over with innumerable minute black bodies,
which look, under a lens, like tiny spiders’-eggs in the web. These
little black bodies are filled with extremely small transparent
vessels, and each vessel contains from four to eight spores or seeds.
Our illustration shows this _Erysiphe_ enlarged one hundred diameters,
with two of the vessels containing the spores removed from the globular
spots and further enlarged. The only safe way of dealing with infested
Pea plants is to burn them. Many other species of fungi belonging to
the same genus attack fruit trees, vegetables, and garden flowers. It
is, however, unnecessary to illustrate them, as they more or less
resemble the fungus of Pea Blight. They all arise from an _Oïdium_
condition, similar to the _Oïdium_ or Mildew of the Vine, and it is in
this condition alone, as in the case of the Vine, that they can be
reached by any fungicide.
[Illustration: Fungus of Pea] FUNGUS OF PEA MILDEW
_Erysiphe Martii_
Tomato Diseases.—The Tomato, like its near relative, the Potato, is
subject to a number of destructive diseases which spread rapidly if
allowed to become established. The most serious of these epidemics are
found among crops cultivated under glass, where the forcing treatment
which they often receive, and the soil and atmospheric conditions,
render the plants abnormally susceptible to the attacks of fungi and
insect pests. Perhaps the most virulent forms of disease with which the
Tomato-grower is troubled arise from the attacks of parasitic fungi and
bacteria, among which the following are most frequently met with:—
SLEEPY DISEASE, or TOMATO WILT.—In its outward symptoms and effects
this disease somewhat resembles an attack of Root-knot Eelworm, but the
swellings are absent from the root. The plants for a time appear quite
vigorous and healthy, but when full-grown they suddenly wilt and die
within a few days. The malady is caused by the fungus _Fusarium
lycopersici_, which first invades the roots and ultimately eats its way
through the substance of the collar or stem near the surface of the
soil, in consequence of which the supply of water taken up by the roots
is cut off from the leaves above ground and the plant collapses. There
is no remedy for the Sleepy Disease of Tomato, and plants which bear
evidence of infection should be carefully dug up and burned.
TOMATO ‘STRIPE.’—This disease of the Tomato is comparatively common,
and although the attacks are sometimes slight its ravages may be
disastrous when conditions are favourable for its development. The
presence of Tomato Stripe is usually first noticed about the time fruit
is forming. The stems of the diseased plants then exhibit dark spots
and elongated sunken stripes of a brown tint, and yellow patches, which
turn brown later, appear on the leaves. Brown pits or depressions
develop on the fruits and spoil their appearance. The disease has been
traced to the action of a bacterium which closely resembles, or is
identical with, that causing Stripe among Sweet Peas. This organism
probably resides in the soil, and the signs of its attack are often
visible in young plants. In severe cases the soil of the house should
be removed and replaced with fresh loam. But when only slight traces of
the disease are apparent, partial sterilisation of the soil by means of
carbolic acid, as recommended for Root-knot Eelworm on page 425, may be
adopted. One of the surest means of guarding against losses by Stripe
disease, is to promote robust healthy growth, and to avoid extreme
forcing conditions, particularly by the excessive use of nitrogenous
manures. Where, however, forcing manures may have been employed in too
large a quantity, an application of potash (in the form of kainit or
sulphate of potash) and phosphatic fertilisers should be given to
counteract the effect of the nitrogen. Immediately any trace of the
disease is found, remove the affected part of the plant, if it is
possible to do so without serious injury, but otherwise the entire
plant should be uprooted and destroyed by fire. It should be remembered
that the organism can be carried on the fingers and on tools, and
therefore knives with which affected plants have been trimmed should be
sterilised with lysol or some other antiseptic solution before being
used on healthy plants.
TOMATO-LEAF RUST.—The leaves of the plant attacked by this disease
rapidly become covered with a dull brownish velvety mould, or fungus,
known as _Cladosporium fulvum_. From the mouldy spots and patches
thousands of spores are readily carried by a slight current of air to
the surrounding healthy crop, and unless prompt measures are taken to
check the pest the whole house is rapidly involved. Excessive
atmospheric moisture encourages the mould, and it is spread extensively
if diseased plants are sprayed with water in the presence of healthy
ones. Judicious management in air-giving, which is one of the
fundamental principles of successful Tomato culture, will do much to
prevent the attack of _Cladosporium fulvum_. Under regular examination
the presence of the disease will be revealed before considerable damage
can be inflicted, and when only a few leaves are affected, carefully
remove and consign them to the fire. Spraying with the Bordeaux mixture
at half the usual strength is recommended when the disease is first
noticed. When the plants are bearing flowers or fruit, fungicides
containing copper must not be used, but a solution of liver of sulphur,
one ounce dissolved in six gallons of water, employed instead.
ROOT-KNOT EELWORM.—A dangerous insect pest which frequently attacks the
Tomato, in common with the Cucumber and Melon, is the Root-knot Eelworm
(_Heterodera radicicola_). The root on which the swollen pea-like knots
develop do not carry on their ordinary functions, and the leaves droop,
the stem becomes limp, and the whole plant soon collapses and dies if
the trouble is severe. The treatment suggested on page 425 should be
adopted.
Sometimes the outdoor Tomato crop is attacked by _Phytophthora
infestans_, the fungus responsible for the Potato Disease: Bordeaux
mixture should be used to check it.
Directions for preparing the Bordeaux mixture are given on page 440.
Another useful preparation which checks many fungus diseases may be
made by dissolving one ounce of potassium sulphide (liver of sulphur)
in three or four gallons of water, to which should be added an ounce or
two of soft soap. The last named greatly assists in the complete and
uniform wetting of all parts of the foliage.
THE FUNGUS PESTS OF CERTAIN FLOWERS
Cineraria and Senecio Disease.—_Senecio pulcher_, soon after its
introduction into England, was attacked, and in some gardens completely
destroyed, by a fungus named _Puccinia glomerata_, or rather the
_Uredo_ stage of this fungus with simple, not compound, spores. The
fungus is well known, being closely allied to that which causes the
rust or mildew of corn crops. It is very common on the wild species of
Groundsel in England, being especially frequent and virulent on the
Ragwort Groundsel, _Senecio Jacobea_, from August to October. The
leaves of infected plants are covered with rust-coloured dusty
pustules, the _Uredo_ condition of the fungus, and known in this stage
as _Uredo senecionis_, sometimes termed _Trichobasis senecionis_. The
fungus has a _Puccinia_ stage of growth very similar to that of the
Hollyhock fungus, _Puccinia malvacearum_.