3040 



RUSTS 



RUSTS 



the last few years has been scarcely mentioned in 

 florists' journals. That a part of this result at least is 

 due to the gradual elimination of susceptible varieties 

 can scarcely be doubted, but the fact should 'not be 

 overlooked that the seriousness of the early rust inva- 

 sion may have been exaggerated, for even then conserva- 

 tive men felt impelled to warn against the agitated 

 state of mind which caused growers to see rust in every 

 discolored leaf. (McBride, Am. Flo. 8 p. 930, 1893; 

 Herr, ibid. p. 980.) 



Botanical features . 



Botanically the rusts are of great interest because 

 their life-cycle consists typically of two distinct genera- 

 tions. These are technically known as the gametophy- 

 tic generation and the sporophytic generation, but for 



3513. The cluster -cup 

 stage of the wheat rust 

 fungus growing on barberry 

 leaf. 



3514. Colonies of 

 rust on the stem of 

 wheat. 



simplicity they may be designated respectively as the 

 aecidial 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 wheat- 

 fields 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 

 secidia and from them yellow secidiospores, which can 

 be seen scattered like dust around the cups, are dis- 

 charged. About the same time or a little earlier, flask- 

 like pycnidia break through the epidermis on the 

 upper side of the leaf. These discharge minute spore- 

 like bodies whose function is not known. They do not 

 infect either the wheat or the barberry. The secidio- 

 spores are incapable of reinfecting the barberry. They 

 can infect only the wheat and a few other grasses sus- 

 ceptible to this particular biologic race. On the wheat 

 the germ-tubes of the secidiospores 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 uredo- 

 spores 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 pro- 

 duce 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 sporidiz 

 do not reinfect grain. 



Puccinia graminis represents the complete life-cycle 

 of a rust of the most complex type. Rusts of this char- 

 acter, which alternate regularly between two hosts, are 

 said to be heteroscious. Those which produce secidio- 

 spores 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 secidiospores on the 

 apple and teleutospores on the red cedar; the holly- 

 hock rust (P. malvacearwri) has only teleutospores which 

 germinate immediately or which may survive the winter 

 and reinfect plants in spring; the blackberry rust 

 (Gymnoconia) has only secidiospores 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 sea- 

 sons 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 propaga- 

 tive spore-form of little 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 horti- 

 cultural 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 secidia on Euphorbia Gerardiana, 

 but in America the socidial 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. ^Ecidia and uredo- 

 spores 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 

 rhizomes deformed shoots arise, on the leaves of which secidia are 

 borne. 



Puccinia asparagi 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 aspara- 

 gus has been practically abandoned. The palmetto varieties appear 

 to be somewhat resistant to the disease. Sprays of sulfur-soda S9ap, 

 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 fficidial stage sometimes disfigures the leaves 

 of Berberis vulgaris planted for ornamental purposes. 



Puccinia mahacearum, 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 Malta rotundifolia, on which 

 developing sori can be found during the entire season. Destruction 



