zi^S MANUAL OF METHODS FOR PURE CULTURE STUDY 



INSECT INOCULATION 



The translocation of microorganisms causing plant disease and 

 their introduction into susceptible plants by insects is a large and rela- 

 tively undeveloped field. The simplest technic with active insects 

 like cucumber beetles or leaf hoppers is merely to place the plant to be 

 inoculated in the same insect cage with an infested diseased plant. 

 (Leach, 1940.) 



For virus diseases, inoculation with slow-moving insects, like 

 aphids, is accomplished by placing a paper on a caged plant to be 

 inoculated, and by laying on this paper a portion of a diseased leaf 

 which carries aphids. As the new leaf tissue dries, the insects crawl 

 over the paper to the fresh leaf below. When insects are involved, a 

 variety of special cages (Leach, 1940) may be used. 



All stages in the life cycle of the insect employed must be considered 

 because inoculation capabilities often vary in this respect. The insect 

 should be identified by a competent authority, and if significant re- 

 sults are obtained a specimen should be deposited in a permanent 

 reference collection. 



A detailed discussion of methods for studying insect transmission 

 has been given by Leach (1940). Some knowledge of the mouth parts 

 of insects and of their feeding and breeding habits is necessary if 

 insects are to be used successfully in inoculating bacterial plant 

 pathogens. Insects are particularly important as carriers of virus 

 diseases. 



Before claims are made about insect transmission of a plant disease, 

 demonstrations of the following (Leach, 1940) seem a minimum for 

 proof: (1) close, but not necessarily constant association of the insect 

 with diseased plants; (2) regular visits by the insect to healthy plants 

 under conditions suitable for the transmission of disease; (3) presence 

 of the pathogen or virus in or on the insect in nature or after visiting 

 a diseased plant; (4) experimental production of the disease by insect 

 visitation under controlled conditions and with adequate checks. 



FUNGUS INOCULATION 



In general, inoculations with the spores or mycelia of fungi differ 

 only in detail from those made with bacteria. For pathogenic fungi, 

 variations in the mode of entrance and in other important characters 

 require modified procedures. Some of the more common methods 

 are discussed by Riker and Riker (1936). 



VIEUS INOCULATION 



Brief mention is given to inoculations with viruses without impli- 

 cation that they are microorganisms. Experimental inoculations are 

 more commonly accomplished by mechanical processes, insects (see 

 Insect Inoculations), and grafting. 



Mechanical inoculation of a virus is frequently made by grinding 

 diseased tissue in a mortar with a little water and by rubbing the juice 

 lightly over leaves of the host plant. With some viruses, the following 

 modifications may be helpful. A favorable reaction between pH 7.0 



