940 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Plum Bust (Puccinia pruni). — Very similar remarks as to remedial 

 measures apply to this. The fungus is very common on Primus in the 

 hedges, and when Plum-trees in gardens are attacked the damage done is 

 often considerable in causing premature defoliation. Cherries, Almonds, 

 Peaches, and Apricots also suffer from the same pest.* 



Strawberry Mildew. — See Hop Mildew. 



Apple Mildew. — This is caused by Podosphceria oxyacantha, which is 

 found on Hawthorn and Primus avium and P. Cerastes. 



Pear-leaf Cluster-cup. — As the last of the long list of parasites 

 which attack the members of this natural order this Cluster-cup may be 

 referred to. It is an instance of a group of fungi which are known as 

 hetercecious : that is to say, for part of the year they live on one host and 

 for the other part they live on a perfectly different one. The habit of the 

 parasite on these two hosts is absolutely distinct, and in most cases one 

 could never guess that it was the same fungus in a different phase of its 

 existence. The grass rust, caused by Puccinia graminis, is the most 

 familiar example of a hetercecious parasite, and is the one described in 

 botanical text-books ; part of its existence is spent on cereals and grasses, 

 and the other part on the Barberry. 



The fungus which causes the Cluster-cup of Pear-trees is Gymno- 

 sporangium sabince (see Jouenal, xxvi. p. 724), and the other host which is 

 required for its existence is Juniperus Sabina. We see, therefore, that in 

 the case of hetercecious parasites the two hosts that are necessary are not 

 nearly allied to each other, but of widely separate orders. Thus, in looking 

 for wild plants which may afford a home for these particular garden pests, 

 we must not confine our search to plants of the same order, as we have done 

 for the most part hitherto, but extend it to plants as widely separated as 

 Barberry and Grass, or Pear-trees and Conifers. This, without the know- 

 ledge of the life-history of the fungus, would be utterly impracticable, 

 especially when one remembers that the Cluster-cup on Pear leaves has 

 no resemblance whatever to the teleutospore stage of the same fungus on 

 Juniper. The life-history of the fungus and the exact host-plant it 

 requires must be known, and the knowledge that botanists have gradually 

 acquired after years of labour is of all importance, if gardeners are to deal 

 successfully with such a parasite as Gymnosporangium. 



On Pear leaves the disease hardly looks like Cluster-cup to the naked 

 eye, but more like little horns arising from brown spots. The horns 

 are really, however, very elongated cups full of spores. The cups burst 

 when ripe, and the spores blow away and infect the Junipers. In some 

 parts of the country the disease is very prevalent and greatly affects the 

 health of the trees ; in other parts it is quite unknown. This, no doubt, 

 may be often accounted for by the Juniper necessary for its existence 

 being absent. 



On this latter plant the disease shows itself by producing swellings on 

 the branches, from which orange-coloured masses of spores protrude in 

 spring ; these are very conspicuous in damp weather, as the gelatinous 



* Attention need not be drawn here to what are known as biological species, as in 

 the light of the most recent research — e.g. "Grass Rust" (H. Marshall Ward, Ann. 

 BoL, xvi. 1902, p. 300) — it appears that the parasite can, by gradual transition from 

 one variety to another, secure a large range of hosts. 



