DISCOVERY 



179 



Immunity and Plant 

 Diseases 



By Geo. G. Gough, A.R.G.Sc, B.Sc. 



Immunity from, or resistance to, a disease in animals 

 is an old tale, and a vast amount of literature in many 

 countries testifies to the work that has been done on 

 the subject. Toxins and antitoxins are now ahnost 

 household words. The immunity of plants to disease 

 has only recently come into the realms of scientific 

 research, but with animals it is now possible to inject 

 a serum containing an antitoxin which will give im- 

 munity to a specific disease — recent notable examples 

 being diphtheria and tetanus — such serums being 

 prepared in large quantities and on a commercial 

 scale. 



Thousands of pounds are lost yearly by the action 

 of plant and animal parasites on plants, and at least 

 10 per cent, of each crop is lost by such parasitic action. 

 It is evident, therefore, that a considerable increase of 

 food could be obtained if such diseases could be held in 

 check. Generally speaking, animal pests are more 

 easily controlled, whether naturally by means of their 

 I'll .Lsites, or by spraying the plants with some poisonous 



V.M^h. 



On the other hand, diseases caused by fungi are not 

 SD readily checked, as in very many cases the first 

 I signs of their presence are the wilting and death of 

 the plant. 



It would therefore be of the greatest service if it were 

 }j')35ible to confer immunity from disease on plants. 

 Our knowledge of the cause of immunity in plants is 

 , so slight that the question of the preparation of anti- 

 1 toxins is not a practical proposition at the present 

 time, and even if it were, it would be seldom possible 

 I to apply them to the plants. Wliat little knowledge 

 I we have suggests that immunity may be due to various 

 1 causes, such as the death of the germ tube of developing 

 spore of the causal fungus on entering the cells, the 

 presence of certain salts or acids, or even the amount 

 j of bloom produced on a leaf. In the latter case resist- 

 jance is only due to the fact that water, in which spores 

 jean develop, rolls off. 



I Fortunately this natural resistance to certain diseases 

 I in many varieties of plants can be made use of by 

 [hybridists who have been able to produce immune 

 varieties. 



I It is regrettable that such natural resistance is often 

 associated with poor cropping or feeding qualities, and 

 the efforts of breeders are put forward to associate 

 immunity with other good qualities. ■ In this connec- 

 tion mention must be made of Professor Biffen's efforts 



to associate immunity from rust ' in wheat with a good 

 cropping " strong " v;u-iety, as these have met with 

 gratifying success. At least two of his varieties are 

 now being grown commercially. 



In Australia, Farrar has produced wheat hybrids 

 which are rust-resistant, and one of these is being 

 grown by farmers ; while in America, Norton has pro- 

 duced a rust-resisting asparagus, which is grown on a 

 commercial scale. 



Pure selection has been used to a considerable extent 

 in the United States. A plant is noted in a field as 

 being resistant to some epidemic disease which has 

 killed all its neighbours. The progeny of this is then 

 subjected to attack by the same disease, and by a 

 similar weeding-out process a strain is produced which 

 is mainly, if not entirely, resistant to the disease in 

 question. Thus strains of cabbage and tomatoes re- 

 sistant to wilt disease have been raised in this fashion. 



It is important to realise that the resistance thus 

 conferred is like that amongst animals, only to a specific 

 disease, so that a plant or animal may be rendered 

 immune to one disease only to fall a victim to another. 



The most important instance of immunity is that 

 of certain potatoes from a virulent disease known as 

 Wart Disease. This disease has been known for some 

 years as a soil pest in certain districts, and is gradually 

 spreading over Great Britain. It is caused by a lowly 

 fungus which may lie in the soil for many years, and as 

 no cure is known for it, it continues to attack potatoes, 

 if planted year after year, causing ugly outgrowths on 

 tubers and stalk. In very few years warts only are 

 found, and no potatoes are formed. The disease is so 

 destructive and widespread that it has been made 

 notifiable by the Ministry of Agriculture, and the 

 movement of seed potatoes from infected districts is 

 regulated. In 1908 an inquiry was made by the 

 author, who noticed that certain varieties were grown 

 by cottagers and others in infected land without being 

 attacked by the fungus, and this led to trials of dilferent 

 varieties in infected soO. 



Official trials are now conducted every season by 

 the Ministry, and as a result the list of varieties which 

 have proved immune from this disease is now over 100. 

 Although the reason for it is still unknown, the immunity 

 is absolute, and no variety which has been officially 

 classed as immune has taken the disease. 



A striking experiment in this connection is to plant 

 side by side in infected soU tubers of Great Scot and 

 Arran Chief (or King George and British Queen), so 

 that plants grow up intertwined. The tubers are 

 similar, and would not be distinguished by the ordinary 

 person. Yet on lifting the crop at the end of the 



' Rust, a fungus disease showing first as rusty streaks on 

 leaves of cereals, and causing a great reduction of crop. Many 

 other plants have their " rusts." 



