DISEASES WHICH AFFECT MANY KINDS OF PLANTS
DAMPING OFF AND FOOT ROT (Pythium and Phytophthora species)
Damping-off is a common trouble which attacks very young, causing them to collapse at level, wither and die. It can also occur among young plants at a later stage when they are pricked-off into boxes or small pots. It is then usually known as foot rot. This trouble is caused by a fungus at the base of the stems or in the roots, and is encouraged by thick sowing and too wet soil conditions, the latter probably due to unsuitable being used, or over-watering and poor ventilation.
It helps to water the compost with Cheshunt compound or to dust with one of the ‘seed saver’ preparations, but it is essential to use good compost and provide suitable growing conditions both for seeds and young plants in their early stages.
THE GREY MOULD FUNGUS (Botrytis cinerea)
Grey mould causes a great deal of trouble in greenhouses because it likes humid conditions, can live on dead and decaying matter (facultative saprophyte) and can survive almost anywhere. It is thus able to get a hold on crop plants, especially if a dead or dying shoot — such as a snag left when trimming tomatoes,, etc. — gives it an opportunity. It flourishes on such yellowing stubs and from there grows on into the main stem or sends spores out to alight on fruits, leaves and so on. Given the right conditions it can do enormous damage to of such plants as salvias, (geraniums), and in greenhouses and among the flowers of zinnias, carnations, etc. Pot plants such as cyclamen, calceolarias and pelargoniums often suffer and tomatoes can be attacked not only in the stem but in the ripening fruits, which quickly rot.
Even on outdoor plants botrytis can quickly enter a wound or dead bud in soft stems of roses and other shrubs, killing the branch. In wet summers, and particularly if they are close-planted, petunias, clarkias, godetias, zinnias and similar bedding plants may be killed. It is the stems of these plants which are attacked.
Tomato, strawberry and raspberry fruits are destroyed as they ripen, and the same fate can overtake apples and pears in store. Grapes and other imported fruits are likely to suffer seriously if the conditions in transit are not correct.
Moist air seems to be the factor of utmost importance to this disease. If the humidity is high the grey mould fungus will flourish through an extraordinarily wide range of temperatures.
Keep down the humidity as far as possible by prudent ventilation and watering, in the latter case paying attention not only to the quantity given but also to the method of application.
The substances known asTCNB (tetra-chloronitrobenzene) and PCNB (penta-chloronitrobenzene) seem to be very effective against Botrytis cinerea. They are now used very extensively as smokes and dusts in greenhouses, as well as in powder form raked into the soil outdoors against other species of botrytis which are closely related to the grey mould fungus. (See tulip fire and tulip grey bulb rot.)
The powdery mildews are a group of fungi which include many genera, but all are closely related and all resemble one another in the way they grow on the host plants they attack. They are the type of disease most often seen in ordinary gardens.
Their habit of growth is fairly conspicuous. They produce a white powdery coating on the leaves, shoots and some-times fruits of their hosts. This white covering consists of a network of threads of the mildew fungus sending down its suckers to feed in the epidermal (outer) cells, and producing on many of the threads long chains of spores. These are cut off in regular succession and released in enormous numbers to spread disease. Such mildews are commonly seen on roses,, Michaelmas daisies, clematis, cyclamen, hawthorn, , cucumbers, swedes, strawberries, gooseberries, grapes, apples, young oaks and many other plants.
Before the arrival of winter the white coating develops small dot-like bodies (perithecia), which survive the winter and produce another kind of spore to begin the disease again in spring.
Moist conditions often encourage these mildews to attack plants, especially in greenhouses and frames, but dryness at the roots also encourages them to attack. This appears to lower their resistance to mildew, and should be guarded against by watering and mulching. The best spray to use against this kind of mildew is Karathane.
These mildews are very different from the ones just described. The downy mildews grow deeply into the plant but they also produce a velvety, mould-like growth on the surface to release their spores. They include many serious troubles of flowers and vegetables.
Among vegetables they attack onion,, pea, parsnip and , and among flowers there are similar mildews which attack antirrhinum, veronica, meconopsis, stock and other plants.
It is the young plants that are seriously affected by these mildews, and a timely spray with zineb (dithane) is often required.
At no time allow the plants to suffer dryness at the roots.
CROWN GALL AND LEAFY GALL
Crown gall is the result of infection by Crown gall takes the form of a sphere varying in size from a pea to a cricket ball. It is hard with a smooth or rough surface and it usually occurs on the roots. Sometimes, however, it may show on aerial shoots of blackberry and other plants.
Leafy gall resembles a-like mass of shortened shoots, growing usually at the base of the stem of the plant. It is usually rather soft. On sweet peas the shoots are very flattened, stunted and not so numerous.
Although crown gall may be seen on many— , dahlias, etc. — it mostly attacks the roots of trees and shrubs — roses, apple and other fruit trees and even some conifers.
Any plants showing signs of leafy gall should be burnt and no cuttings should be taken from them except possibly in the case of pelargoniums, where the cutting material comes from upper shoots well away from the infected base of the stem.
Rust diseases are easily recognized as they commonly show on leaves and stems as yellow- brown- or orange-coloured pustules. These have a rusty appearance, hence the common name.
They are highly specialized and produce spore stages of different kinds in a definite order. The rusts are interesting, because although some produce all their spore stages on the same plant hetereocious rusts) many produce them on different plants (heteroecious rusts). In the latter the parasitic rust fungus is said to use alternate hosts, a factor of great importance when considering control measures.
It is of extreme importance to know the alternate host and the method by which the fungus lives and persists on its different host plants.
Some common examples of rust diseases are mint rust which produces all its spore stages on mint plants during the season, and currant rust, which has -some spore stages on the black currant leaves in summer but also lives and produces other spores on the Weymouth pine and similar five-needled pines.
Rust diseases are checked by a good spraying with Bordeaux mixture or colloidal copper.
In autoecious rusts, where the parasite lives on the one kind of plant, control is fairly simple by destroying diseased plants and spraying.
With heteroecious rusts it may also be necessary to seek out the alternate host and either destroy it or treat it by spraying.
THE HONEY FUNGUS (Armillaria mellea)
The honey fungus is an underground parasite which attacks the roots of trees and shrubs. It rarely shows above ground, except when it produces its honey-coloured clusters of toadstools on old stumps or at the base of trees which it has killed.
When it has killed a tree or shrub it sends out long, black, cord-like strands (rhizomorphs) which may be many feet in length. These grow through the soil to attack other trees.
Some people claim that it can only attack roots which have been injured or which are weakened by some other cause; many others believe it to be a more serious parasite. It is widespread, causes many losses and no tree or shrub is known to be immune. Where soil conditions are known to be adverse — excessively wet, for instance — the honey fungus seems to flourish, rather than in good, well-drained soils.
It is risky to clear woodland of old scrub and use the land immediately for young fruit trees. Hone)’ fungus on old tree stumps and pieces of root may try to attack the young trees.
Where trees are killed by this parasite, remove the stumps and all possible roots for burning. If this cannot be done, make a trench in a circle round each dead stump and throw the soil inward. Then fork over the soil inside the trench and saturate it with 2 per cent formalin solution in the usual manner. If the stumps are left treat them with creosote, pouring this into holes bored near the outer edge. This will discourage further fungal infection.
SILVER LEAF (Stereum purpureum)
This disease is well known but not well understood. The main symptom is the silvering of the leaves, but this is not entirely reliable because other agencies such as cold and aphid attack can some-times cause foliage to become silvered. In trees or branches affected by the true silver leaf fungus a brown irregular stain can usually be seen in the wood when the infected branch is cut across.
The real proof is the appearance of the typical purple-coloured fungus out-growths, but these appear only on the dead wood.
The plum varieties Victoria and Czar are said to be very susceptible to this disease, but it also attacks peach, morello cherry, laburnum and Portugal laurel.
It is less commonly seen on currants, gooseberries and roses.
Infection can occur only through a wound, and the way to prevent this disease from gaining entry is to treat all wounds with a good protective paint and to burn all dead wood before 15th July each year as required by the Silver Leaf Order of 1923. The intention is to prevent the fungus flourishing in dead wood.