PLANT DISEASES. WILD PLANTS AS A SOURCE OF DANGER. 941 



matrix in which the spores are embedded swells, and often hangs to a 

 length of 4 or 5 inches ; but in dry weather it shrivels up and may easily 

 be overlooked. 



The spores produced on the Juniper give rise to minute secondary 

 spores which are capable of infecting the leaves of any Pear-tree in the 

 neighbourhood. 



The mycelium of the fungus is annual in the leaves of the Pear, but 

 perennial in the branches of the Juniper. Hence inoculation from the 

 latter to the former must take place each spring. The remedy, therefore, 

 is to remove Junipers if the Cluster-cup on Pear-trees is to be destroyed. 

 On the other hand, if it is wished to save the Junipers, all infected branches 

 should be cut out and the wounds tarred ; it is no use to remove the Pear- 

 trees alone, as the mycelium is perennial in the Junipers, and can go on 

 increasing without reinoculation each year. 



CUCUBBITACEJE.—Thc Vegetable Marrow Mildew.— This mildew 

 (Erysiphe C ichor acearum) is also found on many Composites, such com- 

 mon weeds as Groundsel and Sow-thistle being frequently attacked by it. 

 If these, therefore, are left to grow amongst Marrows, it is small wonder if 

 the disease gains ground. 



LABI AT JE. — The Mint Bust is a very troublesome pest to garden 

 Mint ; it is also abundant on the wild species of Mentha and other 

 Labiatce, hence they should be strictly watched. 



An additional difficulty in the extermination of this particular rust 

 (Puccinia Menthce) is found in the fact that the fungus mycelium is 

 perennial in the rootstock of the Mint ; thus, when once it has gained an 

 entrance, it is almost impossible to get rid of it. Cuttings which appear 

 quite clear of the pest often contain portions of mycelium, which sooner 

 or later shows itself by producing pustules. The best plan is to obtain a 

 fresh healthy stock, and plant it in a different part of the garden, but first 

 make sure that all infected Mint plants are dug up and burnt. As Basil, 

 Thyme, and Marjoram are subject to the same rust, diseased plants of 

 these herbs should be destroyed, if the fresh stock of Mint is to be kept 

 clean. 



UBTICACE^E. — The Hop Mildew, which is caused by Sphcerotheca 

 Humuli, sometimes does enormous damage in Hop-gardens. Humulus 

 japonicus is also attacked by it. As well as destroying Hops, it is very 

 common on many weeds of various orders, and of late years it has become 

 particularly troublesome to Strawberry-growers. A full account of this 

 disease is given by E. S. Salmon, Journal, xxv. p. 132. 



LILIACEJE. — Several plants of this order are disfigured by a fungus, 

 Urocystis colchici, which causes black streaks on the leaves. Colchicum 

 autumnale, Scilla bifolia, and species of Muscari are some of those 

 most commonly attacked ; the black streaks are masses of spores. Wild 

 liliaceous plants may also be infected, so care should be taken lest these 

 are propagating the disease. (M. C. Cooke, loc. cit. p. H99.) 



Many other cases might be mentioned of plant diseases growing on 

 weeds and wild plants, but the limits of this paper only permit of atten- 

 tion being drawn to the best known and most commonly occurring 

 examples ; every cultivator could doubtless extend the list by his own 

 personal observations. 



