PEAR DISEASES 



1621 



whicli means favorable to the blight 

 germs, the disease may run down into the 

 main limbs and branches, and in extreme 

 cases, may continue running downward 

 during the growing season, until it 

 reaches the root system. Often it has 

 progressed much further in the bark and 

 cambium of the branches than appears on 

 casual examination, for, as a general rule, 

 it works only in the bark and cambium 

 layers, leaving the mature wood un- 

 harmed. 



Sometimes a very small strip of cam- 

 bium is left unharmed and the sap, there- 

 fore, is able to continue to flow upwards 

 so that the leaves and branches may still 

 carry their foliage or often mature their 

 fruit. Very often, too, the bark and cam- 

 bium may be completely girdled, but the 

 young wood is still able to conduct 

 enough sap to keep the foliage and fruit 

 hanging, and often allowing the fruit to 

 mature. On microscopical examination of 

 the diseased tissues they are found to be 

 completely filled with countless millions 

 of minute germs. These germs are 

 mixed with a gummy, sticky material 

 formed by the sap of the tree. The 

 disease is therefore more in the nature 

 of fermentation, or perhaps we may say 

 a gangrene of the bark and cambium pro- 

 duced by the invasion of an immense 

 swarm of bacteria. The bacteria proceed 

 in all directions from the point of in- 

 fection; that is, when they go down a twig 

 or spur or a water-sprout into the 

 branches or bodies of the trees, there is a 

 development of the disease from this point 

 upwards and downwards and laterally. 

 On account of the vessels and fibres of 

 the bark the most rapid development is 

 lengthwise, that is to say, upward and 

 downward on the branches. 



The disease has a tendency to develop 

 in the green, fleshy or outer bark of the 

 branches. The branches of the pear tree 

 usually remain smooth up to eight or ten 

 years of age, sometimes lor a longer 

 period. During this time, the fleshy out- 

 er bark grows as fast as the limb grows, 

 but later on the outer layer dies and is 

 converted into firm, corky material, or the 

 characteristic rough bark of trees. This 



formation of rough bark often puzzles the 

 oixhardist in getting out pear blight, as 

 the dead spots resemble, to some extent, 

 the appearance of blight. However, a lit- 

 tle study will soon enable one to recog- 

 nize the rough-bark formation by cutting 

 through the dead layer to the fresh, 

 smooth bark which is found underneath. 



It may be stated here that where the 

 bodies of trees are covered with the rough 

 bark and these sunken spots of bark are 

 just beginning to die, the only method of 

 inspection should be by digging out a 

 small chip with a gouge, so as to expose 

 the inner bark, and thus be able to tell 

 whether or not there is any infection in 

 the tree. Sometimes the blight luns in 

 the green layer, leaving untouched the 

 slightly tougher inner bark, but such 

 cases can be easily recognized from their 

 connection with blighted bark above or 

 below. When blight is working in the 

 tissues it invades the vessels of the bark, 

 the intercellular spaces, and besides often 

 breaks down in its progress little pores 

 or channels, or sometimes large lenticular 

 spots which become filled with some of 

 the gummy matter. This very frequently 

 exudes in tiny drops not larger than a 

 pinhead, or sometimes forms on the bodies 

 or branches as copious gum masses flow- 

 ing downward on the bark. When this 

 exudate comes from newly invaded tis- 

 sues in the summertime it is milky white 

 from the numerous germs present in the 

 liquid. Later, the gum oxidizes into an 

 amber yellow or slightly brownish, then 

 finally into a dark brown or almost black 

 gum. Sometimes when it exudes from the 

 bark or larger branches it is reddish 

 brown from the beginning; occasionally 

 it is quite red in appearance, getting its 

 color from the red decomposing bark. 

 When the blight is running rapidly in the 

 tissues, the margin is at first merely 

 water-soaked, or very slightly water-soak- 

 ed in appearance. This is particularly to 

 be noted in the bark and infected young 

 fruits of apple, pear or quince. Later on 

 various stages of its discoloration follow 

 as the germs fully occupy and decompose 

 the tissues. 



