|Standard Cyclopedia of Horticulture|
Rusts. The plant rusts are fungi constituting the large and well-defined order Uredinales, which contains approximately 3,000 species distributed in about forty genera. They are all obligatory parasites, mostly on angiosperms, few on gymnosperms, and a small number on ferns.
Relations between host and parasite.
The mycelium of the rust fungi (see Fungi) lives symbiotically within the tissues of the host. It is generally confined to the intercellular spaces, but short branches or haustoria penetrate the cells and absorb nutriment from them. Usually not more than one or two haustoria are found within a single cell and only a small proportion of the host cells are thus invaded, although the mycelium itself is abundant in the intercellular spaces.
The extent to which the mycelium spreads within the tissues of the host and its duration of life vary greatly with different species of rusts. In most of the species which inhabit annual parts of plants, as leaves and stems, the mycelium resulting from individual infections is localized within a more or less restricted area, as in case of hollyhock rust, carnation rust, and wheat rust, and generally persists as long as the infected parts remain alive. That plants thus infected nevertheless become completely covered with rust spots is attributable to the large number of separate infections which occur and not to the spreading of the mycelium through the whole plant. When biennial or perennial parts are locally infected, the mycelium often persists for two years, as in the rust of the red cedar, Gymnosporangium globosum and G. juniperi-virginianae, or becomes perennial, as in the rust of juniper, Gymnosporangium clavariaeforme, and the blister rust on stems of pines, Peridermium cerebrum. Many rusts have mycelia which extend throughout the tissues of the host. This type of mycelial distribution is characteristic of the orange rust of the species of brambles, Gymnoconia interstitialis, and of a number of rusts infecting herbaceous perennial plants, as the rust of Canada thistle, Puccinia suaveolens and the pea rust (Uromyces pisi) on the cypress spurge (Euphorbia Cyparissias). In such cases the mycelium of the fungus remains dormant in the rhizomes or in the roots and in spring grows out into the developing shoots which are usually characterized by spindling growth with long internodes and small and somewhat deformed leaves. Shoots which are normally trailing or prostrate and branches which are naturally horizontal tend to become erect. This effect is sometimes observed in annual plants also, as in the case of Euphorbia maculata infected by Uromyces euphorbiae. Blackberry canes growing out from roots infected by Gymnoconia, besides having the characteristic growth described above, are free from thorns.
In spite of the intimate association of rust fungi with their hosts, very little apparent injury results to the plants during the vegetative growth of the parasites. Deformations of various kinds are common in plants attacked by these fungi. Aside from those described above there is in many cases a marked stimulation to abnormal growth which results in the formation of galls, as the well-known cedar apples, or in enlarged and distorted flowers, fruits, and branches, or in the production of witches' brooms. The greater number of rusts, however, do not cause any abnormal deformation of their hosts, and the presence of the fungus is usually not evident until the time of spore-production, although it may be assumed, and is sometimes apparent, that the plants have been weakened or retarded in growth, particularly when the mycelium is widely dispersed in the tissue. At the time of spore-production, the injury to the host becomes unmistakable. The spores are produced in pustules beneath the epidermis. This is finally ruptured and the spores break forth forming the characteristic orange, brown, or black spots and patches to which these fungi owe their name. The injury is brought about both by the withdrawal of nutriment from the host cells and by the extensive destruction of the epidermis of the host and the resulting loss of water from the tissues beneath. The magnitude of the injury differs with the extent to which the plant is infected. Plants which are severely infected often lose their leaves, which wither and die prematurely. In annual or biennial plants this injury hastens the death of the plants, as in the hollyhock and the cereals, or, as in greenhouse carnations, impairs their vigor to such a degree that the plants are of little commercial value. In perennial plants like asparagus, the yearly injury of the assimilating parts results in the slow weakening of the roots through malnutrition and finally in the death of the plants. Apple trees are frequently defoliated as a result of infection by the cedar rust (Gymnosporangium). In plants grown for crops, whether flowers, fruits or other parts are sought, these injuries diminish the yield, but in ornamentals the mere presence of the rust pustules over the surface greatly detracts from the appearance of the plants.
Specialization among rust fungi.
Rust fungi do not attack various plants indiscriminately. In general, each morphologically distinguishable species is confined to one or to a relatively small group of closely related host plants. Nevertheless, the degree of adaptation to particular hosts varies much with different species of fungi. Some species are truly plurivorous. A striking example of this habit is furnished by the hollyhock rust (Puccinia malvacearum) which inhabits about forty species belonging to many different genera of the subfamily Malveae, and passes readily from one host to another. Many rusts which are apparently plurivorous have been found on closer investigation to be divisible into a number of so-called biological or physiological races, each of which is restricted to a comparatively small group of host plants. This type is illustrated by the common cereal rust (Puccinia graminis). This occurs on all the common cultivated cereals and on about 180 species of wild grasses. The forms on the different hosts are not morphologically distinguishable but culture experiments with this rust on the cereals and the common grasses have shown that it falls into a number of biologic races each of which is more or less restricted to a small group of host plants, and cannot readily be transferred to plants outside of that group. Thus the form on oat infects also orchard grass and a few other grasses, but not wheat, barley, or rye; the form on rye infects also barley and some other grasses, but not wheat; and the form on wheat infects less readily barley, oat, rye, and some other grasses. This type of specialization is very common and occurs in many species which have a wide range of host plants. The separation into physiological races is not always sharp and clear-cut and often a transfer of a race from one host to another can be accomplished by so-called bridging species of host plants, i. e., species which act as a common host to two races of rust. It appears also that the degree of specialization of different forms is not the same in different geographical regions. It is readily seen that the matter of specialization of rusts is one of considerable economic significance. In the case of truly plurivorous species of rusts, many wild plants may be the source of infection for cultivated plants. Thus the hollyhock is easily infected from Malva rotundifolia and other wild mallows; but, in the case of species which, like the cereal rust, have become differentiated into a number of physiological races, there is comparatively little danger of infection from wild plants since the rust on each host has become more or less strictly adapted to its particular host. Thus, for example, different members of the pink family are inhabited by the carnation rust (Uromyces caryophyllinus) but each genus has its own physiological race which does not infect the members of other genera of the family. The specialization of rusts to particular hosts has also another economic bearing. When a given rust is restricted to one or more species of a genus but does not infect the others, these are said to be immune. Just as there are immune species, there may be immune varieties within a species, as the phrases "disease-resistant cowpeas," or "rust-resistant carnations," indicate. This fact, that the cultivated varieties of a given species show varying degrees of resistance, furnishes the basis for the breeding of immune varieties, which is one of the most promising means of overcoming the danger from attacks of rust.
Of the various groups of fungi, the rusts are the most difficult to combat. For most forms, especially those infecting cereals and other agricultural crops, no satisfactory methods of control have been developed since the usual methods of disease-prevention are either unprofitable or not applicable here. Even with horticultural crops, direct remedial measures have proved successful only in few cases, as with the apple rust, which can be successfully controlled by spraying with bordeaux mixture. Various mixtures, such as copper sulfate (one pound to fifteen gallons of water) and potassium sulfide (one ounce to one gallon), have been recommended and tried for carnation rust, but the growers are far from being in accord as to the effectiveness of these remedies. Environmental conditions have much to do with the prevalence of rust. Thus the severity of outbreaks of asparagus rust and probably of other rusts also is dependent on the amount of dew. In greenhouses it has also been found that the environment has much to do with the presence of rust on carnations and chrysanthemums, and that the maintenance of the best cultural conditions is one of the surest means of controlling rust on these plants. This method of control is, however, applicable in the field only in a restricted degree.
While the methods of direct control of rusts have not on the whole proved very successful, the indirect method by the breeding of resistant varieties seems to offer the most promising solution of the problem. Although the so-called rustproof varieties of plants have not generally proved to be entirely resistant, yet different degrees of resistance have long been noticed by growers, and varieties particularly susceptible to rust have been gradually eliminated. The history of the carnation rust in the United States probably furnishes an illustration of this process. Twenty years ago horticultural literature was replete with discussions of the carnation rust which caused much agitation among growers. At that time much was written of "rustproof" varieties, and mention is frequently made of varieties thrown out on account of rust. At the present time the rust once regarded as the "most dreaded of the carnation diseases" attracts but little attention, and within
Botanically the rusts are of great interest because their life-cycle consists typically of two distinct generations. These are technically known as the gametophytic generation and the sporophytic generation, but for simplicity they may be designated respectively as the sccidial stage and the telial stage, terms derived from the names of the principal spore-producing structures which characterize the two phases. Each of these generations may produce one or more spore forms. The life-cycle of a rust is best understood by means of an example. One of the most familiar is the wheat rust.
If barberry bushes in the neighborhood of wheatfields are examined in spring (May and June), there will usually be found on some of the leaves yellowish spots. Within the discolored area, on the under side of the leaf, there are a number of small cup-like openings with fringed margins. (Fig. 3513.) These are termed aecidia and from them yellow aecidiospores, which can be seen scattered like dust around the cups, are discharged. About the same time or a little earlier, flasklike pycnidia break through the epidermis on the upper side of the leaf. These discharge minute sporelike bodies whose function is not known. They do not infect either the wheat or the barberry. The aecidiospores are incapable of reinfccting the barberry. They can infect only the wheat and a few other grasses susceptible to this particular biologic race. On the wheat the germ-tubes of the aecidiospores penetrate the stomata of the leaf and stem and produce local colonies of mycelium which gives rise to uredospores (Fig. 3514). These are one-celled spores which are produced in many generations and which serve to propagate and spread the fungus during the summer. To them the rusty appearance of infected grain-fields is due. They can reinfect only wheat and the other graminaceous hosts of the fungus but not the barberry. Toward autumn the same mycelium which produces uredospores during the summer gives rise to teleutospores. These occur as black streaks and patches on the stems and leaves, upon which they remain during the winter. In spring the teleutospores germinate in place and produce short germ-tubes termed promycelia from which four minute sporidia are abjointed. The sporidia are borne away by the wind and when they alight on the barberry the secidia are again produced. The sporidia do not reinfect grain. Puccinia graminis represents the complete life-cycle of a rust of the most complex type. Rusts of this character, which alternate regularly between two hosts, are said to be heteroecious. Those which produce aecidiospores on the same host on which the other spore forms are borne, are said to be autoecious. Not all rusts have the entire complement of spore forms, one or more of which may be dropped from the cycle. Thus, neglecting the pycnospores which, so far as known, have no significance in the life of a rust fungus, the red cedar rust (Gymnosporangium) has aecidiospores on the apple and teleutospores on the red cedar; the hollyhock rust (P. malvacearum) has only teleutospores which germinate immediately or which may survive the winter and reinfect plants in spring; the blackberry rust (Gymnoconia) has only aecidiospores which germinate like teleutospores. Many rusts, like some of the grain rusts, are able to maintain themselves by means of uredospores which are capable of enduring the winter. This method of persistence through unfavorable seasons must be considered as a special and accidental adaptation to particular conditions, for it is not likely that any form of rust has become permanently reduced to the uredospore stage since this is merely a propagative spore-form of title morphological significance. Continuous uredospore production is likely to occur where plants are kept uninterruptedly in a growing condition like carnations in greenhouses, or fig trees and species of Vitis in tropical and subtropical regions. Under such conditions teleutospores are produced with comparative rarity.
Some common rusts.
The following are some of the common rusts on horticultural plants:
Uromyces caryophyllinus, the common carnation rust, occurs on several members of the pink family but is known in America chiefly in its uredo stage on the carnation upon which teleutospores also are sometimes found. Different physiological races occur on various members of the pink family, some of the European forms on tunica and on saponaria have aecidia on Euphorbia Gerardiana, but in America the aecidial stage of the race on dianthus is not known. The presence of the fungus is indicated by the appearance of lead-colored pustules which appear first on the lower leaves and stems of the plants. The pustules soon rupture and discharge sooty powder made up of uredospores. These are blown or carried by spray to other plants. For germination of the spores a thin film of moisture such as that produced by dew is necessary. Avoidance of conditions which favor the germination of spores will tend to decrease the rust, but only such remedial measures as have been mentioned above can be suggested.
Uromyces appendiculatus, on bean, cowpea and related genera. Sometimes causes damage but not common. Aecidia and uredospores on the leaves, teleutospores mostly on the stems, on which they remain during the winter. The infected material should be destroyed.
Uromyces pisi produces its uredospores and teleutospores on the pea. The sporidia produced by the teleutospores infect the dormant buds of the subterranean shoots of Euphorbia Cyparissias in which the mycelium becomes perennial. From the infected rhisomes deformed shoots arise, on the leaves of which aecidia are borne.
Puccinia asparaffi has all its spore-forms on Asparagus officinalis. This is the most serious parasite of the garden asparagus. It was first noticed as a menace in the eastern United States in 1896. It has since spread over the entire country. As a result of its ravages the system of asparagus-growing has been greatly modified in some regions while in other sections the commercial cultivation of asparagus has been practically abandoned. The palmetto varieties appear to be somewhat resistant to the disease. Sprays of sulfur-soda soap, and of bordeaux mixture, and dusting with sulfur in dry regions have proved more or less successful. Ornamental species grown in the United States are not affected by this rust.
Puccinia graminis, the black rust of cereals and grasses, while of great importance agriculturally is of interest to horticulturists only for the reason that the xcidial stage sometimes disfigures the leaves of Berberis vulgaris planted for ornamental purposes.
Puccinia malvacearum, the hollyhock rust, has only teleutospores which are produced in successive generations. The teleutospores produced during the summer germinate immediately and rapidly spread the rust. In regions where the winter is not too severe, those produced late in the season sometimes survive the winter, but in the northern states the fungus lives through the winter in the mycelial stage in the stems and petioles of Malva rotundifolia, on which developing sori can be found during the entire season. Destruction of wild hosts and of diseased portions of plants is a partly successful method of control.
Puccinia chrysanthemi, the chrysanthemum rust, is known in the United States chiefly in the uredospore stage on chrysanthemums of which only some varieties appear to be susceptible. The rust rarely produces serious damage, and is readily controlled by the elimination of susceptible varieties.
Gymnosporangium juniperi-virginianae, cedar rust, apple rust. The telial stage produces the galls known as cedar apples on the red cedar. After rains in spring the teleutospores ooze out from the galls in the form of horn-like gelatinous masses, an inch or more in length. They germinate in place and the sporidia infect the leaves and fruits of the apple on which the aecidial stage is produced. No appreciable damage is caused to the cedar, but apple trees are sometimes defoliated by this rust and the damage to orchards is often extensive where cedar trees are abundant. Removal of cedar trees prevents the occurrence of this rust on the apple. The fungus can be controlled also by spraying with bordeaux mixture.
Gymnosporangium globosum, another species much like the foregoing, is the common cause of apple rust Id the East.
Cronartium ribicola has uredospores and teleutospores on various species of currants, but is chiefly of importance on account of the destructiveness of its aecidial stage to the white pine and other fiveleaved nines. Introduced from Europe probably about 1900. Local in northeastern United States at present.
Coleoaporium solidaginis occurs on aster, solidago, and other Compositae. It is chiefly of interest because it also attacks the cultivated aster (Callistephus hortensis) causing considerable damage. The aecidial stage occurs on pines. The intervention of this stage is, however, not essential for the maintenance of the fungus since the uredo sori persist through the winter on the rosette leaves of solidago and other Compositae.
Melampsora tremulae is the common orange rust of poplars in the United States. Several races exist which have aecidia on larix, pine, and other plants. Numerous other species or subspecies of Melampaora occur on willows. The injury caused by these is not great.
Gymnoconia interstitialis, of the blackberry, has but one spore form, the aecidiospores which germinate like teleutospores. The rust is exceedingly common on the blackberry and raspberry covering the whole under surface of the leaves with blisters which burst and display the brilliant orange spore-powder. The mycelium is perennial and permeates the entire host. No satisfactory remedy has been suggested.
Uredo fici, an unattached uredo-form which is common on fig trees, causing a rusty brown appearance and premature fulling of the leaves. Where figs are grown for fruit, considerable damage results to the crop from the loss of leaves. Common also in the tropics.
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