112 BACTERIAL DISEASES OF PLANTS 



the roots of some of the plants should be broken. In many 

 instances it is necessary to break them in order to obtain infec- 

 tions (see Part III, Nos. IV and V). 



Checks should be held in the same soil, or if not, and espec- 

 ially if the experiments are to be few, the earth should be auto- 

 claved in advance and the plants watered with sterile (auto- 

 claved) water. 



Infection by Means of Insects. — The common way is to 

 confine the plants in insect cages (like the infection cages but 

 with fine wire netting substituted for glass) into which the in- 

 sects are introduced after they have been allowed to feed upon 

 recently diseased leaves, stems, etc. Night-feeding insects should 

 be infected and liberated at night, being removed next morning 

 or earlier, especially if voracious. In case of trees, the insects 

 may be confined to special branches by means of fine mosquito 

 netting or surgeon's gauze. Under ground, the insects may be 

 confined to the roots of special plants by wire netting. 



TIME AND PLACE OF INOCULATION 



In studying a particular disease, the student will, of course, 

 seek to inoculate those parts of the plant which naturally de- 

 velop the disease — roots, tubers, stems, leaves, flowers or fruits, 

 as the case may be. He must remember, however, as he exam- 

 ines the plant, that the infection he has under observation took 

 place some days, weeks, or months ago when the diseased part 

 was in an earlier stage of growth and in making inoculations must 

 govern himself accordingly. 



In many instances, inoculations on very young, turgid, 

 rapidly growing stems, leaves and fruits offer best prospect of 

 success. Always some of the inoculations, whether by needle 

 puncture or by spraying, should be on such incompletely devel- 

 oped organs (see Part III, Nos. IV, VII, XII, and XIV). 



Full-grown leaves, stems and fruits are resistant to many 

 diseases (bean. No. VIII, pear, No. XII, and olive, No. XIII). 

 On the other hand in case of soft rots of roots (carrot) and tubers 

 (potato) the full-grown organs are quite susceptible provided 

 they have not become flabby (Fig. 167). Certain cankers may 

 also be inoculated through a wide range of months. 



