A DIEBACK, AND BARK DISEASE OF WILLOWS. 1 29 



Netherlands is Salix alba var. vitellina pendula — not Salix 

 babylonica. 



The disease appears first as dark blotches on the leaves, 

 covered with a velvety pile, the dark olive - coloured area 

 sharply contrasting with the green of the healthy leaf. These 

 spots resemble the spots on the apple leaf caused by apple 

 scab, and are similarly covered with the fungus producing 

 its fruiting stage. The upstanding conidiophores bearing at 

 the apex the conidial spore give the velvety appearance to 

 the diseased area. The spore is large, varying in size from 

 15/X X 5/x to 22/j, X lo/x, but usually about i8/x x 6/x. When first 

 formed this spore frequently appears non-septate, but when 

 mature is typically i-septate with a constriction at the septum, 

 hence the statement frequently made that the spore is shaped 

 like the sole of a shoe. The spores are exceedingly variable 

 in size and in shape. These early spots on the leaf give rise 

 to further infections in several ways : — 



(i) The spores are carried to and infect another leaf; 



(2) The spores are carried or blown to the tender tips of the 



young shoots, and infecting the soft tissue cause a 

 dieback, the shoot turning black, the tip sometimes 

 curling in a hook as in Monilia tip- wilt of plum ; 



(3) The parasite spreads over the leaf and travels down the 



mid-rib and down the petiole to the leaf-base and 

 infects the cortex of the stem at the junction of the 

 leaf, spreading above and below. The withered mid- 

 rib may be seen on several diseased twigs still 

 attached by the withered petiole to the stem. 

 The infection by the fungus causes the bark to turn black, 

 and the disease thus forms very striking long-shaped patches 

 on the bark. The black discoloration is sharply marked out 

 on the yellow bark of the rod, and an infected twig looks 

 strongly piebald. In a bad case the infected areas run 

 together until the whole rod is black. 



On these black areas small pustules are formed under and 

 pushing up the epidermis. They are often roughly set in an 

 oval, a half circle being found some way above the point of 

 infection, and a half circle some way below — others are 

 scattered. They are of varying size, but many are from 

 one-fourth to one-third of a millimetre. These pustules are 

 formed of a compact mass of fungal tissue which can go 



