PEAR DISEASES 



1625 



vation to transfer the infection; pruning 

 tools are certainly a very frequent cause 

 of transmitting the disease, especially 

 during the growing season. 



Mr. Waite states that in Maryland he 

 saw a nursery block of 10,000 Bartlett 

 pear trees completely destroyed by blight. 

 This block carried actual cases of hold- 

 over blight in the stocks. When stocks 

 were cut off above the dormant buds in 

 the spring, the pruning tools became in- 

 fected and the disease was transmitted to 

 nearly every tree cut back by the pruner. 

 Instead of the buds starting growth, the 

 cut surface began to blight. The writer 

 has seen in certain nurseries in Nebraska 

 many cases where nursery infection has 

 been brought about through the use of 

 tools which had been used in cutting out 

 blight infection in large orchard trees 

 without previously disinfecting them. 

 Pear blight behaves in all sorts of irreg- 

 ular ways when it runs down the limbs 

 and branches. Occasionally a fruit spur 

 blighting causes the disease to spread in 

 a circular spot an inch or two in diam- 

 eter on the branches. More often it is 

 an elliptical spot extending lengthwise 

 of the branch. It may run down in a 

 long line from the lower edge, making it 

 very difficult to save the branch or even 

 the tree by cutting, on account of this 

 narrow strip of the disease. It is almost 

 impossible to anticipate the variations in 

 behavior of the disease, because it de- 

 pends upon so many different factors. It 

 may be well to point out some of the fac- 

 tors controlling the habits of the disease 

 in order that it may be seen how varied 

 are the influences controlling it. 



These factors may be divided more or 

 less completely into two sets. First, 

 those which govern infection, and second, 

 those which determine the spread of the 

 blight in the tree after infection. 



The first factor is the presence of the 

 bacillus. The pear-blight germ must be 

 present in the orchard or must be carried 

 there during the season in order to have 

 the blight. No matter how favorable the 

 conditions may be, unless the germ is 

 there the disease cannot develop. The 

 immunity of California and Oregon or- 



chards up to recent years, of course, is 

 attributed to the fact that the germ was 

 not there. The second factor is the num- 

 ber of insect visitors. We have pointed 

 out that insects carry the blight about. 

 The honey bee is one of the most active 

 in carrying the blight to the blossoms. 

 Other insects visit the pear and apple 

 blossoms and carry the blight very 

 widely. The presence of certain species 

 of insects, as already suggested, has been 

 the means of introducing the blight into 

 the twigs and branches or bodies of the 

 trees. Not only must the insect be pres- 

 ent and the germs there for them to carry, 

 but the weather conditions must be favor- 

 able for the activity of the insects and to 

 bring the trees into proper condition for 

 infection. 



Flower-visiting insects usually like sun- 

 shiny weather, especially sunny weather 

 following a moist season, which allows 

 many kinds to hatch out or develop from 

 the pupa. Young orchards are not 

 usually attacked by the blight, rather 



Body Infection of Bartlett Due to Water- 

 Sprouts. First attempt to eradicate the 

 blight unsuccessful, and it was necessary to 

 peel the bark and cambium at a greater dis- 

 tance. (Original.) 



