pression, making a water bridge between the two with 
filter paper, and measuring the decrease in spore 
germination. 
The tenacity of fungicide deposists on glass slides 
was measured by Heuberger (1940) by dipping and 
swishing the slides several times in water, allowing 
them to dry, and seeding with a spore suspension. 
Chapman, et al. (1950) suspended the slides in circu- 
lating water and removed them at varying intervals 
of time before drying and seeding. Kovacs (1961) 
applied the fungicides on polyethylene discs, sprayed 
them with artificial rain, and placed the discs on 
seeded agar. 
The stability of fungicide deposits on glass slides 
was measured by Barratt (1946) by subjecting them 
to high humidity and seeding with spore suspensions. 
Serra (1964) exposed polyethylene discs dusted with 
fungicides to ultraviolet light for varying periods of 
time, then transferred the discs to seeded agar. 
Ciferri, et al. (1961) found that zineb frequently in- 
creased in fungitoxicity after 2 to 6 years storage al- 
though chemical tests showed a decrease in the 
amount of “active ingredient.” 
Predicting Field Performance 
In vitro fungitoxicity studies are often used as a 
means of predicting the field performance of candidate 
fungicides. The literature relating to predicting field 
performance from laboratory tests has been reviewed 
by Ciferri (1952), Rich, et al. (1953), McCallan 
(1959), Block (1959), and Torgeson (1967). 
Horsfall and his co-workers were among the first 
0 state that the protective value of fungicides could 
be predicted on the basis of laboratory results. 
Horsfall, et al. (1941) felt that fungitoxicity and 
enacity were the two fundamental components of 
protective value. Keil, et al. (1952) called attention 
‘0 the necessity of chemical stability. Waggoner, et al. 
(1952) later stated that a statistical prediction of 
‘field performance could be made on the basis of 
‘oxicity, tenacity, and stability of fungicides. 
The relationships between laboratory and field 
‘esults are not always clear cut. A hesitancy to predict 
ield results from laboratory tests often arises in con- 
versations or personal correspondence, but is infre- 
juently found in the literature. Martin (1942), how- 
ver, stated that to expect a simple correlation be- 
ween fungitoxicity alone and field performance is too 
unbitious. Smith & Read (1961), in studies with the 
ucumber powdery mildew fungus, found a great vari- 
ince between laboratory and field results. 
Yet, in vitro bioassays often correlate well with 
ield trials. Holloman & Young ( 1951) reported that 
aboratory assay was a reliable criterion for field 
erformance for control of Botrytis leaf spot on 
gladioli. Klomparens & Vaughn (1952) reported field 
and laboratory trials very consistent for control of 
Helminthosporium on bent grass. Brook (1957) 
found that laboratory and greenhouse experiments 
rated fungicides in almost the same order for control 
of Botrytis gray mold on tomatoes. Monroe (1963), in 
studies on bean powdery mildew and bean rust, stated 
that in vitro and in vivo methods generally ranked the 
fungicides in the same order. Misato (1963), in 
studies on rice blast, found the in vitro methods meas- 
uring inhibition of mycelial growth and spore forma- 
tion to be satisfactorily correlated with field results. 
Although a prediction based on in vitro tests may 
prove erroneous on a particular host with a particular 
set of environmental conditions in the field, the success 
or failure of the fungicide can often be explained with 
confidence if a sufficiently varied group of in vitro 
tests have been performed. Most researchers seeking 
protective fungicides at least agree with the statement 
that in vitro testing will result in the selection of 
chemicals worthy of greenhouse and field testing. 
FUNGICIDE-FUNGUS-HOST INTERACTJONS 
In vitro tests in the laboratory will not in the fore- 
seeable future replace greenhouse and field trials 
(Hamilton 1959 and McCallan 1966). An effort is 
being made, however, to introduce a plant or plant 
part into in vitro tests so that the reactions of fungi- 
cide, fungus, and plant are present. Leben (1949) 
sprayed leaf discs with an antibiotic, then transferred 
the leaf discs to seeded agar to determine if the 
material remained fungitoxic. Leben & Keitt (1949) 
applied artificial rain to fungicide-treated leaf discs to 
see if the material was tenacious. Chaves (1954) 
evaluated the effects of fungicide stickers in much the 
same manner. Adams, et al. (1951) devised a leaf 
punch so that numerous leaf samples with uniform 
areas could be easily obtained for use in fungicide 
bioassays. 
The ultimate step in in vitro testing is not simply a 
bioassay using a plant part, but the prevention of the 
disease itself. Schmidt (1951) treated the primary 
leaves of celery with fungicides, inoculated with 
Septoria apii, and obtained results on protective fungi- 
cides within 2 weeks. The laboratory fungicide trials 
of Hislop & Park (1962), using detached, treated, and 
inoculated pods of Theobroma cacao, correlated well 
with field tests for control of Phytophthora palmivora. 
Horn (1964) described a similar detached cucumber 
leaf method for screening fungicides for control of 
cucumber anthracnose. Niemann & Dekker (1966) 
described a method of evaluating compounds for con- 
trol of powdery mildew of cucumber by floating leaf 
discs treated with fungicides on water, inoculating, 
and observing disease symptoms. 
