Diseases Of Leaves (contd.)
( Originally Published 1915 )
Rusts. The Wheat Rust. Life Story of the fungus, relation to the Barberry. The Mint Rust and Hollyhock Rust as examples of other Rust diseases.
The Rusts constitute a very important group of plant diseases, which includes some of the most widely distributed and highly destructive of parasitic fungi. The grain-producing plants and grasses of our fields, the plants of our gardens and greenhouses, and even forest and fruit trees are attacked by members of the rust family of fungi. Of all plant diseases they are in. many respects the most remarkable, as, they are also the most difficult to combat. Whilst the nature and the life history of the fungi causing them have only been known for a relatively short period, the blighting effects caused by outbreaks of these maladies are referred to by writers of the remotest antiquity. Throughout the centuries wheat rusts have been responsible for great damage wherever wheat has been grown, and at the present time the loss caused to the wheat crop alone throughout the world amounts to many millions of pounds annually.
Since the life story of the fungus causing the Black Rust of wheat (Puccinia graminis) is typical of many rusts, it will be described in some detail. The disease usually appears in the summer on the leaves and stalks of the growing wheat in the form of yellowish streaks, at first shining through the epidermis. The orange patches on the leaves consist of the developing uredospores of the fungus causing the disease. As this progresses, the epidermis is ruptured and the bright orange uredospores are liberated as a fine powder. These spores bring about the rapid spread of the malady from one plant to another throughout the summer months, so that in a short space of time whole fields of wheat may he rusted. As the disease pustules increase in number and size the leaves lose colour and become paler day by day and a badly diseased field may thus give the appearance of premature ripening.
With the advance of the season the streaks on the plants gradually change colour from orange almost to black, and although for a time some uredospores continue to arise in the pustules, mingled with them are now dark, brown spores of a different appearance. Finally, in the autumn, none but the dark spores are produced in the pustules. These spores are the teleutospores which serve for carrying the fungus over to the next season. Before considering in detail the form and behaviour of the uredospores and teleutospores, reference must he made to an opinion which prevailed at least a hundred and fifty years ago with regard to the wheat rust.
It was strongly held by farmers that the presence of bushes of the common barberry near to wheat fields, bore some relation to outbreaks of rust, but no very definite reasons were given for this belief. The farmers of Massachusetts were so convinced of the connection between the barberry plant and the wheat rust, that a law was passed in 1755 compelling the destruction of all the barberry bushes. About a century ago Sir Joseph Banks suggested that a certain bright yellow fungus common on the bar-berry might be the same as that causing the rust of wheat. This fungus on the barberry, however, when examined microscopically was so unlike the fungus on wheat that for a long time the relationship was not understood.
If a leaf of wheat is cut across through one of the yellow streaks and examined under the microscope, it is found that the pustule is formed by the rupture of the epidermis of the leaf. Large numbers of the orange uredospores arise under the skin and burst through to the outside. The torn edges of the epidermis thus act as a boundary to the pustule. The filaments of the fungus ramify in the tissues of the leaf often sending special branches called haustoria into the living cells from which nutriment is thus absorbed. The reproductive branches of the fungus grow outwards and accumulate in rows under the skin of the leaf. The round, 'orange uredosores are produced on the tips of such branches,, and are liberated when the epidermis bursts under the pressure. Each uredospore is a single, oval cell about a thousandth of an. Inch in length, and readily falls free from the stalk bearing it. The wall of the spore is studded with minute warts and has four thin round pores near the middle part. The uredospores are able to germinate, immediately they are liberated, in a film of water or in damp air. "Trough the pores mentioned above two or three fine filaments grow out; one of those usually out-strips the rest and may become a long, branched, wavy filament. If in water alone, or indeed apart from a living leaf of the wheat plant:, this filament is only able to survive until the small store of reserve food material in the spore is exhausted. If, however, the spore germinates on the leaf of a wheat plant the germ tube grows to one of the stomata, and passing through the pore enters the tissues of the leaf. Here the fungus absorbing food material grows and produces a new pustule with uredospores in about a fortnight. In this way very large numbers of uredospores are produced during the summer months and hence the disease spreads rapidly.
The teleutospores, like the uredospores, arise from branches of the filaments of the fungus in the leaf and they appear towards the end of summer in the same pustules as the uredospores. With the advance of the season, however, the pustules give rise to teleutospores only. Microscopically, these are longer than the uredospores, are more spindle-shaped, but are also borne on stalks. Each spore consists of two cells and is furnished with a thick, resistant wall. Unlike the uredospores the telcutospores do not germinate immediately when placed under moist conditions, but they require to rest for a period of months. If, however, teleutospores which are produced in the autumn are allowed to remain exposed to the weather through the winter, they will, under suitable conditions,, germinate in a few hours in March or April. In this case the process of germination of the spore differs from any of those dealt with in previous chapters. Each of the two cells of the spore sends out a delicate tube, which, after growing to two or three times the length 'of the spore, becomes divided into four cells or segments. Then a delicate peg-like branch is put out from each of these segments, and a minute, oval spore is formed at the tip of each peg. The question now arises as to what becomes of these teleutospores and the small oval spores or sporidia they produce. For many years this was not understood, although trials again and again proved that the teleutospores or their sporidia were unable to cause new infection on the wheat plant.
The clue to the problem was discovered by De Bary in the old belief of farmers that outbreaks of rust were in some way connected with the presence of barberry. He first made a careful study of the yellow fungus which occurs on barberry, and then proved the connection of this with the rust on wheat. If one of the diseased areas on the barberry is closely examined it is found to arise on a swollen part of the leaf, and on the under side a number of small, yellow, cup-like bodies are produced. From these clustercups yellow spores, known as accidiospores, are liberated as a fine dust. A section through the diseased area shows that the cluster-cups arise as round masses of fungal filaments beneath the skin of the leaf. Within the ball of fungus, aecidiospores are produced in chains, and when this growing ball bursts the skin of the leaf, it opens at the apex and the edges turning back give the structure the appearance of a minute cup or bowl. The aecidiospores arise from threads at the base of this, and as they ripen and are set free new spores arc produced. Now De Bary discovered that when sporidia from the teleutospores on wheat are sown on barberry leaves in the spring, infection takes place and the cluster-cups just described are produced in two or three weeks. Not only did he prove this, but he also showed that when the aecidiospores from the cluster-cups are sown on the leaves of wheat, they germinate, and like the uredospores send germ tubes through the stomata and thus infect the leaf. Pustules containing uredospores and later teleutospores are produced from this infection. The connection between the fungus on the barberry and that on wheat was thus established, and it was shown that, although the appearances of the fungus on the two host plants differ so materially, each represents a stage in the life history of one fungus.
Since De Bary's remarkable discovery very many rust fungi, showing this type of life history, have been studied. The rust which frequently appears on the leaves of the pear, for example, is the aecidiospore stage of Gymnosporangium which produces swellings on the branches of juniper, where the teleutospores arise. The Blister-Rust of the Weymouth or white pine, a disease which is very prevalent on the continent of Europe, is the aecidiospore stage of a fungus which forms its uredospores and teleutospores on the leaves of currants. Similarly, the Gooseberry rust is the cluster-cup stage of a fungus which forms the uredospores and teleutospores on the leaves of sedges. Many other similar cases occur as diseases of cultivated plants and trees.
When the facts became known for the wheat rust it was thought that in order to eliminate the disease from a given district all that was necessary was to destroy the barberry plants in the district. It was soon found, however, that in some countries, for example, Australia, where the rusts of wheat are most destructive, the barberry is almost unknown. In such cases it seems likely that the rust fungus is able to maintain itself without passing through the barberry. It probably does this by means of the uredospores, which have been shown to be capable of resisting a mild winter.
Apart from the destruction of the barberry other means of combatting the wheat rust had to. be devised. Up to the present this has proved a very difficult problem. The most satisfactory progress has been attained along the lines of breeding disease-resistant wheats. It has long been known that certain wheats are much more resistant to rust diseases than others. The chief difficulty lay in the fact that the most resistant forms, e.g., certain semi-wild wheats, were almost valueless as crops. In recent years, however, much has been done, especially by Professor Biffen, of Cambridge, to produce by crossing, wheats which combine qualities of rust-resistance with good cropping and milling capacities. The subject is nevertheless still beset with difficulties, both in regard to the permanence of the rust-resistance, and to the fact that a wheat, which is resistant to one form of rust may be equally susceptible to another. Further, wheats which are resistant, say in England, are not necessarily so in India or Australia. It has been essential therefore for each country to establish its own rust-resistant varieties of wheat.
Many of our common garden plants are liable to attacks by different. rust fungi. The Mint rust caused by Puccinia mentae is one of the most prevalent of these in some districts. In this example the three forms of spore appear in succession, and there is no intervention of a second host plant in the life cycle of the rust. In the spring diseased plants send up shoots which are often swollen and distorted, and bear the cluster-cups of the fungus. The aecidiospores are liberated as a bright yellow dust and infect the leaves of healthy shoots so spreading the disease. The pustules produced by this infection are brown in colour and are scattered, as minute dark spots, over the leaves. Uredospores arise from these pustules throughout the summer, and towards the end of the season teleutospores are produced from similar disease spots. The teleutospores remain in a resting condition in the soil for some months, but germinate in the early spring giving rise to sporidia, which infect the young buds on the underground stem. Such infected buds are not killed outright by the fungus, but grow out to produce the distorted shoots bearing the aecidiaspores, described above. It has also been shown that once the underground stem is infected, the fungus lives there perennially and grows into the young buds as they are formed. This renders the disease all the more difficult to eradicate.
Indeed, the best plan is to destroy all infected material and only use for planting, rhizomes known to be free from the fungus. Care should also be taken to prevent infection by teleutospores from soil which has grown diseased plants.
As an example of a rust caused by a fungus with a simpler life history, the Hollyhock rust produced by Puccinia malvacearum may be considered. This disease occurs on a large number of the members of the Mallow family; it is widely distributed throughout the world and is abundant in this country, both ,on wild mallows and on the cultivated Hollyhock. The fungus, which is a native of Chile, was introduced into Europe about 1875, and for some years its ravages were so severe that it was scarcely possible to grow the Hollyhock free from rust. Even at present the disease causes considerable trouble in some districts.
The disease pustules chiefly occur on the leaves, but the stem and even the flower buds and fruits are often attacked. As in the rusts already described, the developing cushion of spores bursts through the epidermis. The filaments of the fungus ramify in the tissues of the host sending haustoria into the living cells, which are slowly depleted of nutritive substances. The pustules arise on both surfaces of the leaf, are small and circular in outline and produce teleutospores only. These germinate in situ under suitable conditions, immediately they mature, giving rise to sporidia as in other rusts. The sporidia, falling on to the surface of any part of a hollyhock plant germinate and penetrate the epidermis producing an infection. From this region a new pustule of teleutospores is produced in about fourteen days the Hollyhock rust thus omits both the aecidiospore and uredospore stages from its life story.
The fungus probably passes the winter by teleutospores which fail to germinate owing to unfavourable conditions. In addition it is likely that it also is carried over in the few radical leaves which, generally survive the winter in this country. The writer has frequently observed incipient disease spots on such leaves in the winter, and it is probable that these develop much more slowly in the cold weather than those produced in the warmer months. By destroying diseased leaves as soon as the spots appear, it is usually possible to restrict the damage caused by this rust.
From the examples described in this chapter it will be evident that, although the fungi causing the rust diseases belong to one group, yet they present certain differences from one another. A large number, like the rust of wheat, produce accidiospores on one host, and the uredospores and teleutospores on an entirely different species of plant. Others, however, like the rust of mint pass the whole life cycle on one host and give rise to aecidiospores, uredospores and teleutospores in succession on the same species. Still others like the Hollyhock rust, in addition to living
wholly on one host plant, altogether omit some of the spore forms from the life story. Whilst showing these variations, however, the rust fungi are all highly specialised parasites ; they cannot grow apart from living plants, and with rare exceptions, a specific rust fungus is restricted to a single species of plant.