346 tiii: farmers' handbook. 



characteristics that the cultivator requires. With the recognition of the 

 work of Mendel, and the method of segregation pointed out by him, the old 

 vague conception that crossing " broke the type " has given way to methods- 

 of greater range and precision in plant-breeding. Breeding for immunity 

 from disease along Mendelian lines presents special difficulties, but among 

 plants wheat presents one of the most favourable subjects, and some pro- 

 gress has been made as indicated above by Prof. Biffen. The problem pre- 

 sented by many plants attacked by fungi — and perhaps also even by wheat 

 in regard to Rust — is that when the life-history of the fungus has been 

 worked out it does not indicate the primary cause of disease, which, in 

 the majority of cases, is a physiological one. Our knowledge of the physio- 

 logy of plants, though extending, has given us so far no clue to the answer 

 to this problem. There is no doubt that immune plants ran be found 

 in any given place if sufficient attention is given to the subject, but com- 

 mon experience has proved that such plants, when cultivated at a distance 

 from their place of origin, lose their immune properties. 



In the attempt to find and produce immune forms, the idea is sometimes 

 held that a plant can be so modified that its fungus parasite is unable to 

 attack it, but it must be remembered that a fungus is as capable of modify- 

 ing its mode of attack as its host plant is of modifying its method of 

 repelling it, and, in fact, in adaptability the fungi are usually superior to 

 green plants. 



The recent work of Stakman and others on the American Busts has shown 

 that the species Puccinia graminis is a complex one, and consists of a 

 number of races or biologic forms, and that while some wheats are resistant 

 to several forms they are not resistant to all. Quite recently Mr. W. L. 

 Waterhouse has undertaken work on the biologic forms of the species 

 existing in Australia, and there is thus being made available to the wheat- 

 breeder a mass of information which may eventually lead to our coping with 

 the Bust problem in our wheat varieties by a scientific breeding against 

 clearly recognised races of the Bust in particular districts. While super- 

 ficially many of these races appear quite identical, when analysed they show 

 differences which are of the. greatest importance in breeding for rust- 

 resistance. 



At present the farmer's gi*eatest hope of combating Bust lies in growing 

 the varieties which the Department of Agriculture recommends for growth 

 in his district. Bust avoidance is considered as one of the vital factors in 

 drawing up the variety lists for the different districts. 



Wheat Mildew (Erysiphe graminis). 



This fungus disease is prevalent in damp seasons. It is known in some 

 localities as " flag fungus." 



It grows for the most part on the surface of the first leaves produced in 

 spring, forming white or pink patches, more or less extended and of vary- 

 ing thickness. Some of the fungus threads grow outwards, and become con- 

 stricted into a number of small cells like a string of beads (Fig. 14a). The 

 outermost of these cells or spores are being continually set free a- a fine 

 white powder; these spores germinate readily and diffuse the disease. 

 The fungus is often reddish-grey in appearance, and is then sometime* 

 mistaken for Bust. 



If the weather be favourable the fungus changes as summer approaches 

 into a greyish-white thick felt, embedded in which can be seen with the 

 naked eye numerous minute black or brown spore eases (Fig. 14b). Soon 



