EXPERIMENT STATION BULLETINS. 



307 



DISEASES OF THE PLUM. 



THE BLACK KNOT (Plowrightia morbosa, [Schw.] Sacc). 



Although this disease is widely 

 disseminated and has been very 

 destructive to the plum orchards 

 wherever it has appeared, farmers 

 and fruitgrowers, as a rule, do not 

 appreciate the danger of allowing 

 it to get a start in their vicinity. 

 In many sections of New York and 

 New England, and in some portions 

 of Michigan, it has been allowed to 

 get a foothold and within two years 

 the otherwise best cared for and 

 profitable orchards have been 

 entirely destroyed. With these 

 terrible examples before us, every 

 fruitgrower should watch his trees 

 and endeavor to save them from the 

 scourge; 



The larvse of various insects are 

 often found in the knots, and it has 

 been claimed that they are the 

 cause of the knots; but very often 

 knots are found that do not contain 

 insects and, moreover, the micro- 

 scope and even the unaided eye 

 shows that it is of a fungous 

 nature. 



The swellings (Fig. 25) are 

 generally found upon the shoots of 

 the previous year's growth, but may 

 (Schw) SaTc*?^"^^^ ^^°^" ^^'"''^^''^''^ morbosa, appear upou the larger branches or 



1. Stem of" plum tree with knot npon it, as it the trunk. The spores enter the 



appears in the fall and winter. .. j i.i, i.i j £ i'u^ 



2. Perithecinm with mycelium, a a. between the tlSSUeS and tnO tlireaaS Ot tlie 

 cells of the stem, and covered with filaments bear- fn-nrmc a-nA flici /^ollnlnr o-rn'wI-Vi nf 

 ing spores, 6, at their extremities. Section made lUHgUS aUQ tlie COllUiar grOWlU OI 



in May, the stem, due to the disturbance 



maknfied"'"*' ^^ '^"^'^ ^'''^"'''''' ""'' ^'''' produced by the development of 



After^ariow*^''''°^^^*'^''*^*'°''*^^^^*^^°^^'''^^^" *^® fungus, cause the swelling, 



which often becomes several times 

 the size of the branch. As it grows, the bark cracks and the spongy 

 tissues below appear. The swellings begin to show during the month of 

 May, and early in June will be found covered with an olive- green inould, 

 which consists of the fruiting threads of the fungus, bearing at their tips 

 the minute spores. These are borne about by the wind and in various 

 other ways scatter the disease. 



Later on, the surface of the knots becomes hard and black, and a care- 

 ful examination will show it to be covered with minute pimples. These 

 are the pustules which contain the winter spores of the fungus, which 

 ripen during the winter and serve as an additional means of scattering the 



