W. J. DowsoN 97 



lady-bird beetle, the larva of which will devour the Icerya and helps to 

 check to some extent the damage caused by this insect. A resin soda 

 spray has been employed with good results against the Aspidiolus, but 

 it is now the endeavour of the dlovernment to destroy all trees infected 

 by the red scale and to replace them with healthy young stock. 



VII. Wheat. 



Wheat, like cofTee, affords another instance of the variable relations 

 existing between host and parasite under different climatic conditions. 

 For the purpose of the present paper it will prove useful to contrast the 

 relation of the rust fungi to wheat in three widely separated countries 

 such as England, East Africa and Australia. In all three countries the 

 same three rusts attack wheat, namely, the black stem rust {Puccinia 

 graminis Pers.), the yellow rust {Puccinia glumarum Eriks. & Henn.), and 

 the brown or leaf rust {Puccinia triticina Eriks.). In England Puccinia 

 glumarum is the commonest and most destructive ; in Australia Puccinia 

 triticina causes most damage because of its very early appearance in the 

 season, Avheat when only a few inches high in New South Wales being 

 attacked. In East Africa the greatest destruction is due to Puccinia 

 graminis, while in addition to this, Puccinia glumarum is very common 

 on certain wheats of Egyptian origin. These two rusts usually, and 

 Puccinia triticina nearly always, appear late in the season, generally 

 after the wheat has come into flower. Hence it will be seen that the 

 problem of controlling rust in wheat in East Africa differs from that in 

 the other two countries mentioned. Climatic conditions also influence 

 very considerably not only the growing of the wheat but also the spread 

 of the rust. Thus at Nairobi there are two rainy seasons in the year, 

 during both of which it is possible to grow early maturing varieties of 

 wheat. The fact that it is necessary to employ varieties which mature 

 early, that is, in four to five months, because of the shortness of the 

 seasons, makes it possible to grow with success such varieties as escape 

 the rust attack on account of this character. As an instance of this, the 

 Australian wheat "Florence" may be cited, which at Nairobi was found 

 to mature in four months after sowing. " Florence " is not a rust resistant 

 variety, but when sown early enough escapes the rust attack. This was 

 demonstrated at the Experimental Farm near Nairobi where, during 

 some of the trials, "Florence" and another Australian wheat, "Bobs," 

 were sown in adjacent plots and at the same time. "Bobs" takes 

 between six and seven months to ripen and always falls a victim to the 

 attacks of Puccinia triticina and Puccinia graminis which make their 



Ann. Biol, vm 7 



