384 BACTERIAL DISEASES OF PLANTS 



Whatever you do, make full and legible notes. 



Transmission. — On account of its large dependence on ani- 

 mals (chiefly on insects) for distribution, this is one of our most 

 interesting diseases. It is quite easy for any student who has 

 the organism and a blossoming apple or pear tree to start blos- 

 som-infection and demonstrate transmission of the disease by 

 bees. This was discovered by Waite. According to O'Gara 

 the disease may be transmitted by at least 50 kinds of insects 

 visiting pear flowers. 



The disease is common, however, on nursery stock not in 

 blossom, and here bees and flies are probably not the common 

 agents of transmission. Sometimes birds carry the germs on 

 their bills (sap-suckers) or on their claws, which often break and 

 scratch twigs. O'Gara believes that the disease may be in- 

 troduced sometimes through growth cracks (see No. XIII). 

 D. H. Jones has shown that aphides by their punctures may 

 transmit the bacteria, especially on apple trees. Some beetles 

 also are carriers : Scolytus (D. H. Jones) ; and some bugs other 

 than aphides: Lygus (V. B. Stewart). More recently (1915), A. 

 C. Burrill also has proved the disease to be transmitted by 

 aphides {A. avenae), and by an apple leaf-hopper {Empoascamali) . 

 The disease may also be spread by means of pruning tools. 

 Waite saw a nursery block of 10,000 Bartlett trees destroyed by 

 pear blight which was transmitted in the Spring on pruning 

 tools. There was some "hold-over" blight in the nursery and 

 when the tops of the trees were cut down to the dormant inocu- 

 lated buds the tools were contaminated and the blight was 

 distributed to nearly every tree. In the West, on the apple, it 

 has been found to enter through wounds made by crown galls 

 (O'Gara). It may also enter and bHght trunks through soft 

 water-sprouts which for this reason should not be allowed to 

 grow around the base of the tree. According to O'Gara 80 per 

 cent of the initial fire-blight infections in California and South 

 Oregon were through water-sprouts and low fruit-spurs. Heald, 

 in Washington State, has found the disease entering the plant 

 (apple and pear) commonly through the leaves as if by water- 

 pore or stomatal infection (1915). Do apple and pear leaves 

 have water pores ? Section a leaf-tooth and see. V. B. Stew- 



