competition for nutrients. Rapid growth of the 
competitive organism because of the presence 
of abundant glucose probably lead to the exhaus- 
tion of one or more other nutrients. 
Competition for nutrients, when further 
studied, may prove an important mechanism 
of biological control of soilborne plant patho- 
gens and nematodes, The intricate and diverse 
microecological conditions mediating the bal- 
ance among micro-organisms that are in 
competition with each other for energy-yielding 
materials will undoubtedly be of considerable 
practical and theoretical singificance. 
Hyperparasitism and Predation 
Plant pathogenic fungi and parasitic nema- 
todes may be directly parasitized by other 
hyperparasites, Hyperparasitism may be im- 
portant in the survival of fungi possessing 
dormant propagules, because it will slowly 
eliminate these propagules by attrition. A good 
example of this type of parasitism is that of 
sclerotia of Sclerotinia trifoliorum by 
Coniothyrium minitans (87), In inoculation 
experiments the hyperparasite killed more 
than 80 percent of the sclerotia of S, tri- 
foliorum within 11 weeks. Drechsler (25) 
described hyperparasitism of odspores of 
Pythium, Boosalis (2) found that R. solani, 
in unsterilized soil amended with green or- 
ganic matter andkept at 28° C, was parasitized 
by Penicillium vermiculatum and Trichoderma 
sp. Even under the most favorable con- 
ditions, however, only 18 percent of the 
Rhizoctonia hyphae were parasitized. Several 
other examples of hyperparasitism were sum- 
marized recently (4, 5, 16). 
Several fungi and bacteria are known to be 
parasitic on fungal pathogens attacking aerial 
plant parts. Darluca filum, for example, a 
hyperparasite, ramifies throughout pycnidia 
and aecial sori and urediospores and te- 
liospores of cereal rust fungi, This hyper- 
parasite, however, as well as other hyper- 
Parasites of rusts, was unable to reduce 
rust epiphytotics in the field (16), Ciccinobolus 
cesati, a hyperparasite that can be cultivated 
in artificial media and added to plants, para- 
sitizes hyphae and conidiophores of Erysiphe 
graminis (the cereal powdery mildew fungus). 
Ciccinobolus, however, has seldom reduced 
powdery mildew epiphytotics in nature because 
91 
of its inability to establish itself and build up 
large numbers (16). 
About 50 known species of predaceous fungi 
capture and kill nematodes in soil (26). There 
is great hope that nematode-trapping fungi may 
some day be successfully used for biological 
control of nematodes. This hope stems from 
the fact that, despite their remarkable mor- 
phological adaptation, nematode-trapping fungi 
are not obligate predators. Most of them exist 
in soils as saprophytes, and some are known 
to grow on agar media. Recently Pramer and 
his associates (75, 76) found that the trap for- 
mation in nematode-trapping fungi is induced 
by a metabolic product of nematodes, a 
product named "nemin" extracted from worm- 
free culture filtrates, It is, however, too early 
to evaluate the significance of this finding in 
the biological control of nematodes. 
Although hyperparasitism has been observed 
in some habitats, the evidence for its signifi- 
cance in biological control is circumstantial 
(4). As Boosalis (4) and Boosalis and Mankau (5) 
have pointed out in recent review papers, we are 
today faced with great difficulty toassess the 
importance of hyperparasitism and predation 
accurately. Itis even more difficultto evaluate 
the significance ofhyperparasitism and preda- 
tion on the biological control of plant pathogens 
andnematodes. Ourinabilityto determine the 
actual role taken by hyperparasitism and preda- 
tion for biologicalcontrolstems from the fact 
thatmostoftheworkon this subjecthas been 
performedin vitro, andalsofrom the fact that 
plant pathogens and nematodes livein a rather 
obscure environment difficult to observe. 
CONCLUSIONS 
It is clear that some of the important prob- 
lems and questions touched upon in this paper 
confront the plant pathologist and microbiolo- 
gist with attractive and challenging problems, 
the elucidation of which will have great practi- 
cal and theoretical significance. Only with 
extraordinary progress in microbiological and 
biochemical research methods, accompanied by 
thorough, integrated, and exhaustive research 
on fungal and nematode behavior, will our 
knowledge be widened. Only then will biological 
control measures cease to be merely empirical, 
unreliable, and inextensible and become sound 
and practical. 
