150 DISEASES OF CROP-PLANTS 



thread blight of apple, pear and quince in the southern United 

 States. The fructification, which is rarely found, consists of a 

 reticulum or felty membrane, coloured dirty pinkish buff, cover- 

 ing the whole underside of a leaf and easily separable from it. 



The basidia are scattered along the hyphse on short lateral 

 branches ; they are simple, measuring ii X 7-8 microns, with 

 four short sterigmata ; the spores are hyaline, flattened or slightly 

 concave on one side, measuring 8-1 1 X 3-4 microns. 



Nature of the Attack. 



The strands of the fungus pursue a wavy but generally 

 longitudinal course on the thicker twigs. They may reach a width 

 of about a millimetre. The upper surface is somewhat romided, 

 while the lower is closely adpressed to the bark, to which it 

 adheres rather firmly ; the colour is white or cream in young, 

 deep brown in mature examples. The strands are made up of 

 apparently solid hyphse, which are closely adherent in the brown 

 integument, and in the body of the strand are loose, hyaline, 

 remotely branched, straight, of regular thickness, and mostly 

 following a parallel corurse. Adhesion to the bark seems to be 

 secured by short rhizoid-like hj^hae. There appears to be no 

 actual penetration of any twig upon which a cork layer has formed. 



A branch is given off from the strand at the base of each 

 leaf-stalk reached, and runs up the latter to the blade, where 

 it breaks up into successively finer branches with irregular 

 flat expansions and finally frays out into its ultimate threads. 

 The tissue of the leaf is penetrated by connected mycelium 

 of a different type, thin-walled and with numerous clamp con- 

 nections. The leaves become spotted where penetration occurs, 

 and finally dry up and remain hanging by the fungus strands. 

 The smaller twigs also die. Where two leaves touch the fungus 

 crosses from one to the other, often making a little knot or tuft 

 of mycelium which firmly joins the two. 



At intervals in the course of the strands along the twigs, and 

 sometimes on the leaf-stalk, prominent hemispherical sclerotia 

 are formed. They are creamy white in colour, rather smooth, 

 and reach 3-4 mm. diameter. They are made up of hyphse 

 similar to those composing the strands. 



Control. 



The dependence of the fungus, for its development to an 

 extent sufficient to be seriously harmful, on what may be regarded 

 as excessive humidity, suggests pruning and the reduction of 

 shade as the basis of control. The gathering of infected leaves 

 and removal of infected twigs may be expected to have some 

 influence in checking the disease and preventing its spread, but 

 in the case of trees upon which the infestation is at all general 

 it is ineffectual for its eradication. Spraying with Bordeaux 



