CONTRIBUTIONS FROM THE WISLEY LABORATORY 481 



CONTRIBUTIONS FROM THE WISLEY LABORATORY. 



XXIV. — Iris Leaf-blotch Disease (Heterosporium gracile Sacc). 



By J. K. Ramsbottom, R.H.S. Scholarship Student. 



For several years past the tall bearded section of the genus Iris has 

 suffered from the disease known as Leaf-blotch (Heterosporium 

 gracile). The disease was first recorded in this country on the Hving 

 leaves of Iris germanica by Dr. M. C. Cooke in 1893. Since then it 

 has become very troublesome and has excited inquiry among culti- 

 vators. With us the disease is noticed particularly in the late summer 

 and early autumn, during August and September, but it is often present 

 on the leaves in the spring. The attack in its earhest stages assumes 

 the form of very small discoloured spots, no bigger than pin-heads, 

 which later increase in size and become brown, with a rounded outhne. 

 As growth continues the margins of the spots become more irregular, 

 and usually two or more spots become confluent, until finally the 

 greater part of the leaf is covered with irregular yellowish-brown 

 blotches, bounded by a darker brownish margin (figs. loi, 102). At the 

 same time the leaves appear sickly, and later on lose their elasticity 

 and become shrivelled. The rhizomes are rarely killed, and in the 

 next year new growths from lateral buds make their appearance ; 

 the plants, however, are injured very greatly by the disease. They 

 may flower for a few years, but eventually become weak and die. 



On examining one of the diseased leaves with a hand lens a number 

 of very small black dots will be noticed on the diseased areas. Micro- 

 scopic examination reveals that these black dots consist of tufts of 

 3-5 — septate fungus threads, 150-200 /x in length, which push forth 

 one or many through the stomata of the leaf, bearing the spores 

 singly at their apices. These conidia are attached very loosely and 

 are readily dispersed by the wind. On the spots that are produced 

 early in the spring there is no evidence of the fruiting hyphae, but 

 there is plentiful mycelium, which can be easily made out in hand-cut 

 sections stained with an alkaline solution of azol blue. 



Not only is the disease known all over Europe but it has been recorded 

 from Asia (Lydia), Africa (the Cape), America (U.S.A.), and Australasia 

 (New Zealand). The species recorded in the most recent work on 

 Hyphomycetes * as affected in Europe are Iris bohemica, I. florentina. 

 I. germanica, I. Gueldenstaedtiana, I. pallida, I. Pseudacorus, I. spuria, 



* G. LiNDAU in Rabenhorst, Krypt. Fl. 2nd Edit., Pilze I. ix: (1907), p. 79. 

 t Mr. W. R. Dykes, in a letter to me, queries what plant is intended by 

 this name. 



