22 



S. E. A. McCallan 



The drops of spore suspensions cannot all be made exactly the same 

 size, nor will they encounter exactly the same conditions. Therefore, 

 in order to avoid magnifying small differences, the general rule has been 

 never to count more than one microscopic field of spores in a single drop. 

 Occasionally, because of a limited number of spores it may seem advisable 

 to count more than one field to a drop, but in this case the same number 

 of fields should be counted in each drop. Assuming 25 spores in one- 

 field, and one moist chamber to a test, 400 spores will have been counted 

 for one test. If possible, it is more convenient to use the low-power 

 objective in observing the spores. In this case a cover slip will not be 

 required. For convenience in counting, the field may be divided into 

 quadrants. To accomplish this, a cover slip on which thin lines are drawn 

 in ink at right angles to and bisecting each other, is placed on the ocular 

 diaphragm of the microscope. To facilitate and expedite the counting, 

 two Veeder Hand Tally Counters are used, one to record germinated 

 and the other ungerminated spores. Absolute germination percentages 

 should be recorded, and not relative germinations where all controls are 

 raised to 100 and the other readings elevated in proportion as has been 

 done by some workers. This latter method ignores the disproportionate 

 effect of treatment on spores of lower viability, discussed earlier, and 

 masks the variation between tests so that true comparisons cannot be 

 made. 



It is impossible to completely control all the conditions surrounding 

 the laboratory testing of fungicides, especially in view of the variability 

 and the sensitiveness of most spores (Duggar, 1901). An average of 

 400 or 500 spores to a test will tend to give, in the germination results, a 

 somewhat high probable error. A repetition of tests is therefore very 

 necessary. This may be accomplished by performing duplicate or tripli- 

 cate moist-chamber tests, or by repeating the experiment. One should 

 not expect to duplicate the figures of a former experiment. In table 9 



TABLE 9. Results of Sulfur Tests on Conidia of Venturia inaequalis 

 (Percentage germination at 21° C. in distilled water) 



Treatment 



Experiment 

 1 



Experiment 



Experiment 

 3 



Average 



Niagara 300-mesh sulfur 



Banks' colloidal sulfur diluted with Bentonite: 



0.3 per cent sulfur 



Banks' colloidal sulfur diluted with Bentonite: 



1.0 per cent sulfur 



Banks' colloidal sulfur diluted with Bentonite: 



10 per cent sulfur 



Niagara Kolodust (colloidal sulfur 10 per cent; 



dusting sulfur 90 per cent) 



Bentonite 



Control 



18.3 

 18.4 

 13.0 



9.2 

 39 . 2 

 47.2 



39.5 



34.5 



27.5 



9.6 



2.S 

 44.6 

 50.6 



44.6 



49.2 



49.2 



24.0 



23.8 

 64.0 

 69.5 



34.1 

 34.0 

 29.9 

 16.8 



11.9 



49 . 3 

 55.8 



