94 



less still beset with difficulties, both in regard to the 

 permanence of the rust-resistance, and to the fact that 

 a wheat, which is resistant to one form of rust may be 

 equally susceptible to another. Further, wheats which 

 are resistant, say in England, are not necessarily so in 

 India or Australia. It has been essential therefore for 

 each country to establish its own rust-resistant varieties 

 of wheat. 



Many of our common garden plants are liable to 

 attacks by different rust fungi. The Mint rust caused 

 by Puccinia menthcE is one of the most prevalent of these 

 in some districts. In this example the three forms of 

 spore appear in succession, and there is no intervention of 

 a second host plant m the life cycle of the rust. In the 

 spring diseased plants send up shoots which are often' 

 swollen and distorted, and bear the cluster-cups of the 

 fungus. The aecidiospores are liberated as a bright yel- 

 low dust and infect the leaves of healthy shoots so spread- 

 ing the disease. The pustules produced by this infection 

 are brown in colour and are scattered, as minute dark 

 spots, over the leaves. Uredospores arise from these 

 pustules throughout the summer, and towards the end of 

 the season teleutospores are produced from similar disease 

 spots. The teleutospores remain in a resting condition in 

 the soil for some months, but germinate in the early 

 spring giving rise to sporidia, which infect the young buds 

 on the underground stem. Such infected buds are not 

 killed outright by the fungus, but grow out to produce 

 the distorted shoots bearing the aecidiospores, described 

 above. It has also been shown that once the underground 

 stem is infected, the fungus lives there perennially and 

 grows into the young buds as they are formed. This 

 renders the disease all the more difficult to eradicate. 

 Indeed, the best plan is to destroy all infected material 

 and only use for planting, rhizomes knowm to be free from, 

 the fungus. Care should also be taken to prevent infec- 

 tion by teleutospores from soil which has grown diseased 

 plants. 



As an example of a rust caused by a fungus with a 

 simpler life history, the Hollyhock rust produced by 

 Puccinia malvacearum may be considered. This disease, 

 occurs on a large number of the members of the Mallow 

 family ; it is widely distributed throughout the world and 

 is abundant in this^ country, both on wild mallows and on 



