372 GROWTH OF PLANTS 



to substitute a known quantity of stimulant, such as ultra-filtered orange 

 juice,^^' ^® in order to obtain a consistent and high percentage of germina- 

 tion. Under these conditions fungicide tests replicated on the same day 

 using the same lot of spores in general do not vary more than is to be ex- 

 pected from their internal error, whereas tests replicated at different times 

 with different lots of spores vary considerably more than is to be expected.^^ 

 This is believed to indicate that the replicate test variation in the main is 

 due to the use of different lots of spores, rather than to errors of technique 

 in applying the chemical. These results have led to the conclusion that 

 when comparing fungicides the replicate count or internal error should not 

 be used as the error term but rather the compound X replicate test inter- 

 action. 



Time and temperature. The effect of time and temperature has been 

 determined for the germination of spores of Sclerotinia fructicola, Alternaria 

 solani, Glomerella cingulata, and Macrosporium sarcinaeforme in water and 

 in the presence of various chemicals.^^ No significant difference in precision 

 could be shown between counts made at 6, 12, 24, 48, or 96 hours. A linear 

 relation was found to hold between the reciprocal of elapsed time and 

 germination expressed as probits for the spores of all the fungi when ger- 

 minating in water at all temperatures from 10° to 35° C (50° to 95° F). 

 The results with Sclerotinia fructicola are showTi in Fig. 146A. A similar 

 relation held for germination in the presence of a given concentration of 

 chemical, provided that concentration permitted germination, as shoAvn 

 in Fig. 146B. A linear relation was also demonstrated for LD50 values when 

 the logarithm of concentration was plotted against reciprocal of elapsed 

 time. This curve is important in the estimation of the potency of a fungi- 

 static agent, since compounds are rated differently at various times on the 

 same fungus as in Fig. 146C; also, fungi may differ in their relative suscep- 

 tibility to a single compound, depending on the elapsed time before counts 

 are made, as may be seen in Fig. 146D. No significant difference in LD50 

 values could be demonstrated at 15°, 21°, or 27° C (59°, 70°, or 81° F) but 

 there was a temperature effect at 10° and 35° C (50° and 95° F), where 

 compounds were not rated in the same order. This temperature effect 

 may in part explain differences in field performance of fungicides in dif- 

 ferent seasons. 



Fungicidal vs. fungistatic. The fungicidal property of a material may be 

 defined in general terms as the ability to kill or inhibit the development of 

 fungus spores or mycelium. This is the sense in which it is commonly used 

 and has been employed previously in this chapter ; however, in the restricted 

 sense, "fungicidal" means the property of killing fungi, and ''fungistatic" 

 the property of inhibitmg. A clarification and comparison were under- 

 taken of the distinction between fungicidal (restricted) and fungistatic. ^^ 

 A series of water-soluble chemicals was compared as to their relative action 

 on four different species of fungi. Fungistatic action was determined by the 

 usual slide-germination tests where the fungus remains throughout in con- 



