338 



THE AGRICULTURAL NEWS. 



October 26, 1912. 



attacked from a. It has been found in some cases that 

 the fungus parasitic on a can attack a third species x 

 and that when it has grown on x for one or more gen- 

 erations, spores from x can infect b. Thus « serrcs as 

 a bridging species to carry the fangus from a to b, and 

 the introduction of x into a cultivation where a is 

 attacked and h is immune would naturally result in 

 the breaking down of h's immunity. Finally, it has 

 also been shown that the immunity of the species h to 

 the fungus strain on a may be partly broken down, if 

 the parts of b liable to attack are damaged by adverse 

 conditions, wounds, or the depredations of insects. 



" Very little, if anything, is known of the existence 

 of biological species in the tropics; yet the matter is 

 clearly worthy of attention, particularly in relation to the 

 production or introduction of immune varieties of host 

 plants and in considering legislation restricting the 

 introduction of plants from one country into another. 

 These applications are so evident that they do not call 

 for further elaboration here. In conclusion it may be 

 added that the related genera Colletotrichum and 

 Gloeosporium, to mention only two, might well repay 

 investigation from this point of view. 



The next point raised by Professor Salmon is that 

 of the degree of importance, from the economic point 

 of view, of the saprophytic stage of a fungus causing 

 a plant disease. It has been found that the mycelium 

 producing conidial fructifications of a fungus may live 

 as a parasite, while that producing the ascigerous fruit 

 lives as a saprophyte on dead and often fallen portions 

 of the same plant. In countries with a very marked 

 change of climate in summer and winter this power 

 may be of considerable economic value, since the 

 saprophytic stage may serve to carry the fungus through 

 the winter and give rise to new outbreaks of disease in 

 the succeeding spring. In mild winters the parasitic 

 stage may persist, 'but under extreme conditions the 

 saprophytic form may alone be able to survive. An 

 investigation of this problem is of more importance in 

 temperate countries than in the more uniform climatic 

 conditions of the tropics, yet even there it should 

 hardly be entirely neglected. It is possible, for example, 

 that definite knowledge of the part played in spreading 

 infection by the ascospores of Rosellinia bunodes, the 

 black root disease fungus, would be of value. The 

 perithecia in this instance always develop on a sapro- 

 phytic mycelium, some time afcer the tree is dead. Their 

 growth is slow and the spores have a thick outer coat — 

 all facts which point to this stage as intended to carry 

 the fungus thi'ough unfavourable conditions. It would 

 appear, however, that most fungi perpetuate themselves 



in the tropics largely by means of conidia, since the 

 ascigerous stage is often either entirely absent or only 

 rarely formed. 



The question of the co«|^tion6 under which some 

 saprophytic species of fungi^^piue parasites is one of 

 very great importance in thei^^ics, and one on which 

 some information, of g, rather^reliminary nature, has 

 been obtained. Quite a large number of the more 

 serious diseases of crops are caused by fungi that are far 

 more usually saprophytic than parasitic in habit. As 

 an example may be taken the ubiquitous Thyridaria 

 tarda found as a saprophyte on an immense number of 

 ditferent plants, and as a wound parasite on cacao, 

 Hevea and tea, among other hosts. Its parasitism is 

 largely dependent on conditions unfavourable to the 

 growth of the host, as well as on other factors. Again, 

 the root disease of Para rubber in the East is due to 

 a fungus (Fomes semitostus) usually saprophytic on 

 forest stumps. Its parasitism depends on the presence 

 of large quantities of decaying wood which afford it 

 food for vigorous vegetative development before it 

 begins its attack, and on the presence of an 

 ample supply of moisture. The same is probably 

 true to some extent of Rosellinia bunodes referred 

 to above. The solution of the problem in connexion 

 with many species of the family of bracket fungi 

 ( Polyporaceae ), to which Fomes semitostus belongs, is 

 a matter of some importance in the tropics, as many of 

 them appear to act occasionally as wound parasites or 

 as root parasites on trees planted in newly cleared 

 forest land. Similar investigations would be valuable 

 in the case of many of the toadstools (Agaricaceae) 

 and of the genera Colletotrichum and Gloeosporium, of 

 which many forms are found on ripe or fallen fruits. 



Of the last of Professor Salmon's questions, namely 

 what are the conditions under which a parasitic fungus 

 attacks a new host species, nothing appears to be known 

 in the tropics, since the records as a rule do not go back 

 far enough to show that when a parasitic fungus is 

 found on an apparently new host plant, it has never 

 actually occurred on that host before in the same locali- 

 ity, or in some other. This again is a problem worthy 

 of attention. 



Other problems of some economic importance also 

 occur in connexion with the life-histories of fungi, 

 besides those mentioned bj' Professor Salmon. One isi 

 to what extent a strain of a parasitic fungus may lose 

 its virulence when growing for some time on the same 

 ho<t plant in a limited area, exhibiting fairly uniform 



