PLANTS 



223 



frequent, but the type is less severe, the disease 

 wears itself out. The potato disease of 1845 has 

 continued annually in greater or less severity 

 since that time ; out from that year, and the 

 two or three years immediately succeeding, it 

 has ceased to be so formidable. Other instances 

 might be mentioned in illustration of this resem- 

 blance of remarkable plant diseases to epidemics 

 among animals the vine disease (Oidium), the 

 hollyhock disease (Puccinia malvacearum), the 

 celery disease (P. Apii), all of which made 

 their appearance suddenly with such virulence and 

 widespread rapidity as to threaten extinction of 

 the species attacked, but are now sources of neither 

 trouble nor alarm. 



The direct action of parasitic fungi in causing 

 disease in plants is through the mycelium or spawn 

 injuring the host plants either by depriving them 

 of nourishment, by impairing their power of assimi- 

 lation, or by abnormally accelerating or retarding 

 growth. The extent of injury inflicted is extremely 

 variable, in some cases exterminating in degree, 

 and in others, though widespread and general, 

 having little ill effect upon the health of the 

 subjects. Adverse external circumstances such 

 as unsuitable temperature, excess of dryness or 

 moisture in the air and in the soil, deficiency of 

 light, the presence of deleterious elements in the 

 soil or of noxious gases in the atmosphere by 

 debilitating the plants render them more liable to 

 attacks of fungi, and aggravate their severity. 



The effects of disease in plants are extremely 

 various. One of the most common manifestations 

 of the presence of parasitic fungi in the tissues is 

 hypertrophy in the parts affected. This may be 

 either local or general ; the roots, the stems, the 

 leaves are all liable to this peculiar disorder. 

 Many of the conifers are particularly subject to 

 hypertrophies in disease. The so-called Cedar- 

 apples of the United States, which occur in great 

 abundance on the branches of Junineriis virginutna, 

 are caused by the spawn of Gymnosporanqium 

 macropus. They are reniform tumours, and, as 

 has l>een pointed out by Professor Farlow, originate 

 by the mycelium entering a leaf and growing down- 

 wards into the bark of the smaller branches. Its 

 presence acts as a source of irritation to the cells. 

 The stem and branches of Juniper us communis are 

 subject to hypertrophies caused by the spawn of 

 G. claviariffforme. This is frequently to be 

 observed in Britain, and the enlargements are 

 of a very persistent character, and in effect im- 

 pede the supply of sap to the branches beyond 

 them. Similar tumours occur on the branches of 

 the silver fir, which are caused by the spawn of 

 Peridermium elatinum. But the most remarkable 

 example of hypertrophy in connection with the 

 diseases of any of the conifers is that which occurs 

 in the larch disease, which is caused by the spawn 

 of Peziza calyrina. The presence of the spawn 

 threads in the cortex, cambium, and woody tissues 

 causes their death ; but hypertrophy of the tissues of 

 the surrounding parts is set up. The death and 

 lixsuring of the bark of the affected parts follows in 

 due course, and the branch attacked eventually 

 dies, ami sooner or later also the tree succumbs 

 to the disease. There are many other examples of 

 this form of plant disease, nor are they peculiar to 

 ligneous plants, but occur in humble herbaceous 

 subjects, such as the violet, garden and other 

 species of anemone, ranunculus, and even in 

 grasses ; but space will not permit of more 

 extended notice of particular cases. 



' Finger-ami -toe, ' Anbury,' and ' Club-root ' are 

 tin; common names given to a disease which attacks 

 tlie turnip, cabbage, cauliflower, and other members 

 of the important natural order of Cmcifenv. It 

 assumes the form of tumours on the roots gener- 



ally, but they also appear on the bulb of the 

 turnip. They increase in number and in size as 

 the plants grow, but eventually the plants cease 

 to develop and die ; the tumours becoming foetid 

 masses, and leaving the spores of the fungus 

 (Plasmodiophora brassicce) in the soil. 



Diseases of plants of traumatic origin are those 

 which result as a consequence of wounds which 

 may have been received by a tree or shrub. Nature 

 may have succeeded in covering over the wound, 

 and superficially all may appear well ; but it often 

 occurs that some fungus, perhaps harmless to the 

 subject while its bark is intact, finds a lodgment 

 in tne wound, and sets up parasitic disease. 



Canker, though most familiar to us upon the 

 apple-tree, is not uncommon upon such trees as the 

 oak, ash, elm, beech, &c. It is caused by the same 

 fungus (Nectria distissima) in every case. Gum- 

 mosis, which is similar to canker, is caused by 

 Gleospora gummifera, which occurs in several 

 forms. This destructive disease, it has been sug- 

 gested, was caused by bacteria, but Beyrinck has 

 conclusively settled the point by inoculative ex- 

 periments. It is quite innocuous when inoculated 

 into other trees, such as the apple, pear, oak, and 

 maple. 



Some parasitic fungi cause disease and death by 

 fastening on the woody tissues of trees and shrubs 

 when these are exposed by reason of wounds and 

 bruises of the bark. The spawn penetrates gradu- 

 ally to the core of even the largest trees, and effects 

 their decay and death. Unlike the microscopic 

 species which induce disease by penetrating the 

 herbaceous parts of plants, these wound parasites 

 are mostly large conspicuous fungi. Examples are 

 to be found on the ash and the elm in Polyporus 

 squamorus, and in two species of Agaricus, A. 

 ulmariits and A. adiposus, the latter attacking 

 wounded ash and beech trees, and the former 

 wounded elm-trees, while A. ostreatus attacks 

 injured laburnums. 



Rot or gangrene in the stems of fir-trees is caused 

 by A. melletis, the mycelium of which finds 

 its way into the woody tissues by the roots and 

 gradually ascends the trunk, inducing decay either 

 in the form of moist or dry rot. 



Atrophy occurs in the pine, in wheat, and in 

 other plants in consequence of attacks of various 

 minute parasitic fungi. Uredo pinitorquum attacks 

 the pine trilje, and by arresting the flow of sap at 

 the. points attacked starves the branches beyond, 

 causing sterility and eventually death. In like 

 manner Puccinia graminis attacks the straw of 

 wheat, and renders the ear abortive. But sterility 

 is also caused by parasitic fungi attacking the 

 organs of reproduction directly. Certain species 

 attack the male organs only, as Ustilago violacea, 

 some, such as Thecaphora kyalina, affect only the 

 female organs, while Ustilago cm-bo effects the 

 destruction of all the floral organs. 



A singular feature in the economy of many of 

 the minute parasitic fungi is that in one stage of 

 their existence they are capable of living only on 

 one kind of host plant, and at another stage on a 

 species quite distinct; so distinct indeed, as in the 

 case of tne Puccinia above named, that in one form 

 it attacks the barberry and in another stage wheat. 

 It has long been a matter of common observation 

 among farmers in Britain, on the continent of 

 Europe, and in parts of the United States, where 

 the common barberry has been introduced, that 

 the proximity of that shrub to wheat-fields had 

 some mysterious connection with mildew in wheat. 

 In the state of Massachusetts a law was passed in 

 1760 enjoining the extirpation of the shrub, in con- 

 sequence of the belief that it caused or at least in- 

 tensified the wheat disease. The problem was 

 solved by the late Professor De Bary in 1864, who. 



