450 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



IRIS LEAF BLOTCH. 



By M. C. Cooke, M.A., LL.D., F.R.H.S., &c. 



Foil some time past the foliage of 7m gcrmanica has been suffering in 

 many places from some disease, which ultimately destroys the plants, 

 find has caused considerable anxiety to cultivators. The leaves become 

 spotted with elliptical or oblong fuscous spots, about a quarter of an 

 inch in length, a little darker at the edge. At length these spots become 

 conlluent, until the whole surface of the leaf is brown and dead, whilst 

 the original fuscous spots remain of a paler colour than the surrounding 

 tissue. Often there is no external evidence of the presence of any 

 parasitic mould upon the spots, especially in the spots occurring in the 

 spring, but there is a plentiful fungoid mycelium in the tissues. Ultimately 

 some of the spots, towards the centre, are dotted with little black dots, 

 like pin-points, which represent the mature condition of the black mould 

 which is the source of the mischief. 



^licroscopic examination reveals the fact that these black dots consist 

 of a tuft of jointed smoky threads, which are unbranched, and proceed 

 direct from the mycelium, at length becoming erumpent, breaking through 

 the cuticle, and then appearing on the surface as black dots. These 

 hyplup, or threads, bear singly, at their apices, the conidia or fruit of the 

 mould. These conidia are long elliptical, at first colourless, with a single 

 transverse septum, but at length become more elongated, of a dusky olive, 

 with two, or rarely three, transverse septa, the whole surface being 

 rough with minute points. Except for the rough conidia, this mould 

 closely resembles the common Cladosporium, from which it has been 

 separated, under the name of Hetcrosporium. This particular species is 

 Hctcrosporium gracilc^ and has been found on Iris, Friesia, Antholyza, 

 and Hemcrocallis. A reference to its first discovery in Britain is 

 to be found in Gard. Chron. May 1894. In this particular species the 

 conidia measure from 85 to 70 micromillimetres in length and from 14 

 to 20 in diameter. 



In Europe this pest is recorded for Britain, France, Italy, and Germany, 

 and has been reported from New Zealand, Cape of Good Hope, and 

 North America. In its fullest and most strongly developed condition the 

 spots are quite blackened with velvety blotches of the threads and 

 conidia. 



There is no doubt that this and allied species are undoubted parasites 

 of a very destructive character. One species, Hetcrosporium cxasperatum, 

 is very destructive to Pinks and Carnations. Another species which I 

 described in 1888, under the name of Hcierosjyorium Auricuhe, was found 

 attacking the leaves of Auricula. Another species is found in this country 

 on Omithogalum. In California a species is destructive to Eschscholtzia, 

 and another is well known in Europe as a pest on the leaves of Spinach. 

 All of these are parasites, living upon and at the expense of living 

 tissues. 



