296 MANUAL OF MICROBIOLOGICAL METHODS 



workers and particularly graduate students are advised to employ such 

 cultures only after detailed plans have been made in conference with 

 their advisors. 



Various laws apply to the shipment of infected material. 



Relative efficiency in technic. The best methods of procedure for 

 making inoculations and for recording results have not always been 

 worked out and are not obvious from inspection. If the question is of 

 sufficient importance, the answer may be secured statistically. A doubt 

 may appear, with a leaf-spot organism, for example, whether to spray 

 or to make needle punctures. Likewise, when infection develops, the 

 question may occur whether the results should be recorded in terms of 

 total number of lesions, of total tissue involved, of the effect of the 

 disease on yield, or of some other criterion. Such possibilities may be 

 tested by means of the frequently described ''analysis of variance." 

 Thus the best method for making the trials and for recording the results 

 may be determined. In general, the method that gives the greatest 

 value for the variance ratio F is the most desirable. This value indicates 

 a greater uniformity in readings from different trials with the same 

 technique or a greater differentiation of the varieties used or treatments 

 employed without a proportional increase in error. 



Antibiotics. Numerous recent reports have shown that various higher 

 plants and saprophytic microorganisms produce substances that adversely 

 affect certain bacteria. Many instances occur in which various higher 

 and lower forms of plant life make chemicals that inhibit successful plant 

 inoculations. 



To secure the latest methods for plant diseases, one should consult 

 an active investigator in this rapidly developing field. 



RECORDS 



Taking notes on plant inoculations presents various problems depend- 

 ing upon the experiment in hand. To assist with such records, a tentative 

 protocol appears in Table 29. For some lines of work it is obviously 

 too complex, while for others it is clearly too simple. 



A number of the items listed for records may be critical factors for 

 the success or failure of an experiment. Since each one cannot be dis- 

 cussed, several examples are mentioned. (1) Infection may fail if the 

 incubation temperature is either too low or too high. Many plant 

 pathogens operate best between 18 and 30°C. (2) Plenty of mois- 

 ture is usually important for disease development, a deficiency of water 

 often being responsible for negative results. (3) The age of the plant 

 or of the part inoculated may influence the result. The relatively 

 young leaves are frequently more susceptible than old leaves to bac- 



