RUSTS 



RUSTS 



3039 



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 produc- 

 tion 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, par- 

 ticularly 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 (Gymnosporan- 

 gium). 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 indiscrimi- 

 nately. In general, each morphologically distinguisha- 

 ble 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 fur- 

 nished 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 experi- 

 ments with this rust on the cereals and the common 

 grasses have shown that it falls into a number of bio- 

 logic races each of which is more or less restricted to a 

 small group of host plants, and cannot readily be trans- 

 ferred 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 physio- 

 logical 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 speci- 

 alization 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 mal- 

 lows ; but, in the case of species which, Uke the cereal rust, 

 have become differentiated into a number of physio- 

 logical 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 particu- 

 lar host. Thus, for example, different members of the 

 pink family are inhabited by the carnation rust (Uromy- 

 ces caryophyllinus) but each genus has its own physio- 

 logical 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. 



Remedial measures. 



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 satis- 

 factory methods of control have been developed since 

 the usual methods of disease-prevention are either 

 unprofitable or not applicable here. Even with horti- 

 cultural 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 sul- 

 fate (one pound to fifteen gallons of water) and potas- 

 sium sulfide (one ounce to one gallon), have been recom- 

 mended 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 chrysan- 

 themums, and that the maintenance of the best cul- 

 tural conditions is one of the surest means of controlling 

 rust on these plants. This method of control is, how- 

 ever, 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 horti- 

 cultural 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 carna- 

 tion diseases" attracts but little attention, and within 



