Diseases Of Leaves And Shoots (contd.).
( Originally Published 1915 )
Black-Leg or Wilt Disease of Asters. Tomato-leaf rust or mould Leaf-blotch of Cucumber. Mildews of Roses, Goose-berries. etc.
The Black-Leg or wilt disease of asters is extremely prevalent wherever asters are grown. It is caused by a species of Phytophthora, which differs in several respects from Phytophthora infestans, the cause of the Late-Blight disease of potatoes. The disease may manifest itself at any stage in the life of the aster, but the initial attack is always upon the seedlings. In severe cases the seedlings collapse, as in the " Damping-off " disease caused by Pythium, but more frequently they harbour the fungus without showing any outward sign of disease at this stage.
Such seedlings may wilt and collapse when transplanted, but many succeed in surviving even to the flowering stage without showing external signs of injury. In the latter cases the plants succumb quite suddenly, almost without warning. The leaves wilt, hang limp and flaccid, and in a few days the whole plant shrivels and dies. Even though affected plants show little direct sign of the disease until the wilt sets in, they are often much dwarfed in size and produce fewer flowering branches than healthy plants.
The fact that this disease may be present in apparently healthy asters, which only wilt after they come into flower, renders it all the more objectionable and difficult to combat. When wilted plants are pulled up, the parts of the stern a few inches above the level of the ground are found to be blackened and often decaying; these symptoms have given the disease its popular name.
The parasitic fungus which causes the aster disease, unlike Phytophthora infestans of the potato, never attacks the leaves directly but enters the plant from the soil through the root of the seedling. Once within the young plant it may, either grow slowly not seriously interfering with the work of the vital parts of the seedling, or it may extend rapidly through these and cause immediate collapse. Microscopic examination of the blackened portion of the stem shows the distribution of the fungus in the tissues. The cells of the rind as well as, the spaces between them are occupied by filaments of the fungus. The former are not so rapidly killed by the fungus as are those of the potato plant by Phytophthora infestans.
The ultimate collapse of the plant, however, is brought about by the extension of the fungus to the vascular cylinder of the stem, and the consequent reduction of the supply of water to the leaves.
In moist weather the fungus gives rise to conidia on the diseased stem. Unlike those of Phytophthora infestans they are only produced under water, and do not become detached from the filaments bearing them. A few hours after their first appearance they burst at the apex liberating about fourteen motile spores which, after swimming about for a short time, come to rest and are able to infect other seedlings.
In studying this disease the writer has up to the present failed to find any resting spores of the Phytphthora in the tissues of diseased asters, and attempts to obtain such spores by artificial cultivation have been equally unsuccessful. Experience however has shown quite conclusively that the disease originates each season from the presence of the fungus in the soil, especially of the seed bed and the ease with which it may be cultivated on dead organic substances suggests that it may be able to persist in a vegetative condition in the rich soil used for seed-beds.
On the other hand, further research may prove the existence of resting spores.
It is often stated that this disease is caused by a species of Fusarinm, which is frequently found on the decaying tissues of diseased asters. The writer, however, has proved that a species of Phytophthora is the primary cause of the Black-Leg disease and that Fusarium only appears later as a saprophyte living upon tissues previously killed by the other fungus. The Phytophthora is always present in the diseased asters even in the earliest stages; it has been isolated and grown separately from all other organ-isms, and the disease has been produced artificially from such growths. Other organisms found on diseased asters, including Fusarium, are unable to infect and produce wilting in healthy plants; such saprophytes only succeed after the tissues have been killed by the Phytophthora.
The insidious nature of this disease renders it extremely difficult to deal with, for it is usually almost impossible to detect diseased plants until the wilting actually sets in. Whilst sufficient scientific trials of remedial measures have not yet been made to warrant promise of complete success in all cases still some precautions may be indicated. The soil of the seed-bed should be partially sterilised by steam or hot water and asters should not be planted in around which produced diseased plants the previous sea-son. All diseased material should be removed and burned and the infested soil thoroughly drenched with Formalin (1pint per 10 galls. water) and covered with sacking for a few days. Many growers raise aster seedlings on hot-beds of stabile manure, but these conditions should be avoided since they are much more favourable to the fungus than when alkaline artificial manures are used.
As a first example of the common diseases of leaves, the Tomato leaf rust or mould caused by Cladosporium fulvum may next he considered.. This is more strictly a disease of the leaves than is the Late-Blight of potato, but even in this case the whole tomato plant suffers because of the attack on the leaves. The disease, which has been known in this country for over a quarter of a century, first appears on the leaves in the form of small, yellowish spots. These gradually increase in size and often run together, and the under surface of the leaf in the diseased areas becomes covered with a rusty, velvety growth. This is the reproductive part of the fungus producing the disease. The vegetative filaments ramifying in the tissues of the leaf give rise to branches which pass out through the stomata and stand erect from the surface of the leaf. Food material is absorbed from the host plant by the vegetative filaments and is passed on to the reproductive branches. These produce large numbers of small two-celled spores, which being readily detached are disseminated by air currents. Under suitable conditions they germinate immediately on the surface of tomato leaves; each of the cells of the spore may send out a filament, which growing through one of the stomata, into the interior of the leaf produces a new infection. The careless watering .of slightly diseased plants may carry spores to healthy leaves and thus spread the disease which is highly infectious.
The disease only occurs in this country on tomatoes grown under glass, and though the spores described above are the only kind known, it is certain that they are able to survive the winter in the greenhouse and give rise to. infection the following season. If the houses are kept sufficiently well ventilated the disease seldom assumes serious importance; neglect of this precaution may, on the other hand, prove disastrous. It is possible to prevent any bad outbreak by regularly spraying with a Bordeaux mixture of half the usual strength, until the young fruit is beginning to set, when, owing to the poisonous character of Bordeaux mixture, Liver of Sulphur (1 oz. per 4 galls. of water) should be substituted. In order to prevent the disease recurring, all diseased leaves should be picked off immediately and dropped into a vessel containing a solution of copper sulphate; at the end of the season remains of plants should be burned and the greenhouse disinfected in the manner to be described in connection with the Cucumber-leaf disease.
Several other diseases of the tomato plant, which are readily distinguished from the Leaf-rust described above, may be mentioned. The Sleepy disease caused by Fusariurn. lycopersici, is a wilt disease somewhat similar in symptoms to the Black-Leg o f asters although caused by a very different fungus. The Black-Stripe disease shows itself on the fruit and also sometimes on the stem as a dark, velvety growth of fungus. This disease, caused by Macrosporium solani, should not be confused with the Bacteriosis of the tomato in which blackening of the parts attacked is also produced. In the latter case, Bacillus solanacearum is the cause. The Septoria disease of the leaves produced by Septoria lycopersici is the only other malady likely to be confused with the leaf-rust, but in this case the spots are always small and concentric, and the spores arc produced in minute black bodies scattered over the patches.
The Leaf-Blotch of the Cucumber caused by Cercospora Gadonis is another destructive disease of leaves. It was first described in this country by Dr. M. C. Cooke in 1916, and since then it has become so wide-spread that many horticulturists have been compelled to cease growing cucumbers. Once the fungus appears in greenhouses it is extremely difficult to 'eradicate. The leaves arc most often attacked, but the fungus frequently spreads to the fruit.
An outbreak of the disease is usually first indicated by the appearance of pale, scattered spots on the leaves.
These spots gradually increase in size, become brown, and the leaves are so rapidly killed that death of the plant may soon result.
Microscopic examination shows that, in the region of the spots, the tissues of the leaf are occupied by filaments of the fungus, that the chlorophyll bodies are pale in colour and many of the cells of the leaf are shrivelling and dying. From the fungus within the leaf stiff branched filaments grow out and stand more or less, erect from the surface. These aerial threads are dark in colour, and bear numbers of large conidia which fall free as they mature. Each somewhat spindle-shaped conidium is divided into about seven or eight cells, and may germinate in a warm, moist atmosphere by sending out filamentous germ-tubes from any of the cells. The germ-tubes may then produce new infections by growing through the stomata into the healthy tissues of the leaf. This spreading by means of conidia that germinate immediately, takes place very rapidly under favourable conditions. In addition to producing large clonidia on the leaves the fungus is said to grow as a saprophyte on decaying leaves and damp soil producing myriads of smaller spores which also rapidly spread the disease. If the conditions are unfavourable to the germination of the spores, and especially at the end of the season, the large conidia persist alive as resting spores. Filaments of the fungus are also able to pass into a resting condition in the soil, only to begin active growth with the }production of spores, when the conditions are once more favourable. In this way the disease survives in greenhouses from, one season to the next, and once a house is infected the disease is almost certain to recur year after year unless precautions are taken to thoroughly disinfect the soil and all parts of the house.
The practice of disinfecting greenhouses with burning sulphur is largely employed in some districts. Whilst this is an excellent preventative of insect pests and of certain mildews, it is useless against the Leaf-Blotch of cucumbers. The writer recently established this with certainty. A large house which had been badly infected with the disease was cleared out and thoroughly disinfected with burning sulphur. A few fragments of diseased leaves and fruits were then collected from the soil of this greenhouse and brought into the laboratory.
Spores of the Cercospora, taken from this material, germinated in water in a few hours and cultures of the fungus were readily obtained on suitable media. From this experiment it is clear that the spores and resting filaments of Cercospora melonis remain uninjured in houses disinfected by burning sulphur alone. Probably more certain results would be obtained by spraying the house thoroughly, and also drenching the soil with Formalin (1 pint per 20 galls. of water). The Board of Agriculture recommends the use of Jeyes fluid (1 oz. per gall. of water) for this purpose, but the writer has no experience of this as a disinfectant against parasitic fungi. Needless to say all diseased material should be destroyed by burning. It is possible to control this disease to, some extent by spraying with Liver of Sulphur (potassium sulphide) two ounces in three gallons of water, to which two ounces of soft soap is added to facilitate the sticking of the solution to the leaves, which should be thoroughly wetted by the spray.
The disease is only prevalent where cucumbers are forced under glass, but if the ventilation is carefully regulated much can be done to reduce the possibility of an epidemic.
A most effective way of combating it is by growing disease-resisting varieties of the cucumber, of which there are a number on the market. The most reliable of these have rather coarse, hard foliage, but unfortunately the fruit is not so highly valued as that of some of the more susceptible varieties. At the same time it ought to be quite possible to produce, by crossing, a variety which combines the qualities of disease-resistance and those most acceptable in the fruit.
Many diseases of leaves belong to the class spoken of as mildews and arc caused by fungi belonging to the family Erysiphaceae. The Rose mildew, caused by Shaerotheca pannosa, is one of the most familiar of these diseases and is typical of the class. The diseased leaves become covered with a white, powdery growth of the fungus, which causes them to curl up and die. The fungus grows mainly on the surface of the leaves, swollen branches from the filaments acting as sucker-like organs of attachment, while other branches penetrate the outer walls of the epidermal cells and swell out within the surface cells in the form of bladder-like haustoria. These absorb food material from the cells occupied as well as from the adjoining cells. The substances thus absorbed from the living leaf are passed on to the vegetative filaments of the fungus outside which is thus enabled to grow and multiply. Erect threads arise from the creeping filaments .on the surface and bear single chains of colourless, thin-walled, oval spores or conidia. Myriads of these are produced on the surface of mildewed leaves and give to the latter the characteristic powdery appearance. Being extremely light they are easily spread by the wind to healthy leaves and produce new infections throughout the summer. Later in the season, the production of conidia gradually gives place to another means of spreading. The fungus on the shrivelling leaves and also on the twigs assumes a brown colour and gives rise to minute dark bodies about the size of a pin head. This is the winter or resting stage of the fungus. Each of the minute dark bodies or perithecia is furnished with a thick wall made up of a number of closely interwoven fungal threads and within this resistant coat a club-shaped spore-case (ascus) containing eight oval spores, gradually develops. When mature the perithecium splits across and the ascus is squeezed out. The latter then opens at the apex and the oval spores are forcibly ejected. These are able to infect leaves with the mildew and serve to start the disease afresh each spring. As has been stated above, many destructive mildews are caused by fungi having a life history closely corresponding to that of the Rose mildew. The mildews or apple, chrysanthemums, peas, hops, strawberries, and gooseberries are all common in this country. The American Gooseberry mildew is the most destructive of these, and growers of gooseberries have to observe certain restrictions prescribed 1)y the Government in regard to this disease. Full particulars of the symptoms and treatment. in ay be obtained from the Board of Agriculture.
Preventive measures against most of these mildews are similar. The main facts to consider are the means of spreading during the season and the method of carrying over from autumn to spring. Sulphur, or one of its compounds, is the most effective fungicide for use in combatting mildews. Plants may be dusted with flowers of sulphur or sprayed with liver of sulphur. Sprays, containing lime and sulphur, are now extensively used with success, especially in America, against the mildews of hops and gooseberries.t The treatment with sulphur, or its compounds, however, is only effective against the summer stage of the fungus and other measures must be taken to destroy the resting fungus. The only satisfactory method of dealing with this is by the removal and immediate destruction of all branches and leaves bearing perithecia. Otherwise no amount of spraying will prevent the disease recurring year after year.