with a manometric system where the growth of the 
organism is measured by the amount of oxygen ab- 
sorbed. McCallan, et al. (1954) and McCallan & 
Miller (1957) felt that measurement of oxygen con- 
sumption was not a particularly useful means of deter- 
mining fungitoxicity. 
Comparisons of Methods of Testing.—There have 
been relatively few published reports in which the 
methods for determining fungitoxicity have been com- 
pared. In observations of techniques involving spore 
germination, Frick (1964) found the test tube dilution 
test less variable than the dried deposit-slide germina- 
tion test. Gottlieb (1945) found the seeded, toxicant- 
agar method and the test tube dilution method equally 
sensitive. Manten, et al. (1950) found the roll culture 
method more useful than the seeded toxicant-agar, 
dried deposit-slide germination, and _ gravimetric 
methods. Himelick & Neely (1965) found the cello- 
phane transfer technique more sensitive than the 
seeded agar-toxicant spot method. Walker (1955) 
compared spore germination, spore respiration, and 
mycelia respiration techniques and found that, in gen- 
eral, the three methods gave the same ranking of 
fungicides. 
Measurements.—In establishing the value of fungi- 
toxicity tests we must not only know the method of 
testing but also the method of measurement and the 
means of expressing the resulting data. In bioassays 
this is a difficult task. McCallan and his co-workers 
have found that the amounts of fungicides absorbed 
by spores of different fungal species vary greatly and 
suggest that possibly fungitoxicity should not be based 
on the concentration of the external solution or sus- 
pension but on the weight of toxicant actually ab- 
sorbed by the fungus (McCallan & Miller 1958; Miller, 
et al. 1953). 
One problem encountered with the dried deposit- 
slide germination technique was whether to express 
the results in terms of weight of toxicant per unit area 
of slide or weight of toxicant per unit volume of water. 
Since the amount of toxicant in solution was not 
readily known, the results were usually expressed in 
weight per unit area. 
A second problem in giving the results with spore 
germination tests was how to express the percentage 
of germination at different fungicide concentrations. 
It could be shown graphically with ease using a 
dosage-response curve, but it was difficult to express 
verbally. McCallan & Wilcoxon (1938) used the term 
LD50 for that toxic concentration at which there was 
a reduction of 50 percent in spore germination. Later 
they used logarithmic paper to estimate the LD50 
(Wilcoxon & McCallan 1939), more appropriately 
called the ED50 (McCallan 1948), 
Interpretations of the role of the dosage-response 
curve in the evaluation of fungicides and the increas- 
ingly elaborate statistical methods that accompanied 
a 
use of the dosage-response curve were the subjects 
for numerous research papers from 1940 to 1960 
(Dimond, et al. 1941; Horsfall 1956; Litchfield & 
Wilcoxon 1949; McCallan, et al. 1959). In many re- 
ports written during this period the results were ex- 
pressed in a statistical language unreadable by many 
plant pathologists. Kundert (1956) commented on the 
exaggerated application of statistics in the assay of 
fungicides. Many researchers are now reporting re- 
sults as the lowest external concentration that com- 
pletely inhibits spore germination or mycelial growth 
expressed in parts per million or its equivalent 
(Guillemat & Lambert 1960; Pianka, et al. 1966; 
Luijten & van der Kerk 1961; Neely & Himelick 1966; 
Nisikado, et al. 1951; Pluijgers & Kaars Sijpesteijn 
1966). 
Fungal Selectivity 
Although phycomycetes, powdery mildew fungi, 
other ascomycetes, and basidiomycetes are not equally 
sensitive to fungicides and although certain fungi are 
sensitive or resistant to specific fungicides (Horsfall 
1951; Horsfall & Lukens 1966; Wellman & McCallan 
1943), most fungicides now in commercial use have 
a broad spectrum of activity against fungal species. 
The test organism commonly used in the laboratory 
will often rank candidate fungicides in the same order 
as a specific disease-causing organism. McCallan, 
et al. (1941a), in a spore germination test with 6 fungi 
and 20 compounds, reported the fungi similarly sensi- 
tive. Neely & Himelick (1966), in a test with 7 fungi 
and 24 compounds, in general confirmed these results. 
Casarini & Pucci (1957) in a test with 6 fungi and 6 
compounds felt the resulting differences between 
fungi sufficiently variable to recommend that the same 
fungus be used in in vitro trials as in field trials when- 
ever possible. 
Deposition, Redistribution, 
Tenacity, Stability 
To be successful a fungicide must prove itself on 
a natural surface and in a natural environment. This 
involves the physical and chemical factors of deposi- 
tion, redistribution, tenacity, and stability (Burchfield 
1960, 1967). Modifications of the basic fungitoxicity 
methods have been developed with the aim of learn- 
ing as much as possible about these additional factors 
in the laboratory. 
Redistribution of a fungicide is often essential for 
control of a disease because plant growth, poor 
tenacity, or poor coverage may leave the fungicide 
particles widely distributed. Powell (1961) measures 
the ability of fungicides to redistribute by placing a 
known fungicide concentration in one depression of a 
glass slide and a spore suspension in the second de- 
