Plum. 341 



These swellings are caused by a fungus, and the insects find 

 them to be good breeding-places, which explains their presence 

 in many old knots. Although the fungus has long been known 

 to mycologists, its life history has not yet been entirely de- 

 termined; enough is known, however, to serve as a safe guide 

 in the treatment of the disease. Early in spring, when growth 

 starts, these swellings begin to appear. At first they are yellow- 

 ish in color, but later this changes to a darker shade. During 

 May and June a crop of spores is produced upon the surface of 

 the knots, causing them to appear as if coated with a thin layer 

 of velvet. This soon disappears, and then the knot becomes 

 darker until winter, when it is jet black. In November and De- 

 cember the surface of the knot may be seen to be thickly cov- 

 ered with minute black elevations, in which the winter spores 

 are borne. These are distributed during the latter part of 

 winter. The spores generally gain entrance into the trees at 

 the crotches of small limbs and at the junction of annual 

 growths. They cause swellings which extend along the younger 

 branches to a distance of four or five inches the first year. All 

 the mycelium does not die during the winter, so the following 

 spring the formation of new swellings may be seen at the edges 

 of the old knot. In this manner the disease may live from 

 year to year, or until the limb dies. 



Treatment. The general recommendation has been to cut out 

 the knots and destroy them as soon as they are discovered. It 

 should be done before a crop of spores is matured. If in addi- 

 tion to this the trees are thoroughly sprayed with the Bordeaux 

 mixture during the warm days of early spring before growth 

 starts, and again when the buds are about to burst, it is proba- 

 ble that the winter spores may be rendered harmless. If the 

 crop of spores produced during May and June is similarly dis- 

 posed of, no infection need be feared. Consequently the trees 

 should be sprayed for the third time with the Bordeaux mix- 

 ture during the latter part of May, and again about the middle 

 of June, These applications must be thoroughly made, and if 

 this is done the black knot fungus may be practically con- 

 trolled. In case a knot appears upon a large limb, or upon the 

 trunk of a tree where it cannot be easily removed, it should be 

 painted with pure kerosene oil. This will destroy the knot and 

 also the living tissue surrounding it ; care should therefore be 



