Indirect Biological Control 
Mediated by Plant Excretions 
In 1924, Noble (60) showed that spore 
germination of Urocystis tritici, the cause of 
flag smut of wheat, is stimulated by a variety 
of substances and by roots of several non- 
host plants, For over a quarter of a century 
this important discovery has become only a 
textbook curiosity. If nonsusceptible decoy 
crops could be found as a stimulus to propagule 
germination of plant parasites, an indirect par- 
tial or complete eradication of the parasite 
could be obtained before the advent of a sus- 
ceptible crop. During the last 25 years, many 
workers have attempted to control soilborne 
plant pathogens and nematodes by using the 
root exudates of nonsusceptible plants to 
promote resistant propagule germination in the 
absence of susceptible hosts. 
This kind of biological stimulation for spore 
germination (8), or egg or cyst hatching (44) 
by host-plant diffusates, is well known. White 
(90) reduced infection of potato tubers by 
Spongospora subterranea, the potato wart 
fungus, in field experiments by planting in a 
heavily infested soil a nonsusceptible decoy 
crop of Jimson weed (Datura stramonium), 
Macfarlane (49) reduced infections of test 
cabbage _ seedlings by Plasmodiophora 
brassicae, the club root organism, by growing 
short-term catch crops of ryegrass or cruci- 
fers in infested soil before planting the sus- 
ceptible host. Recently Schroth and Hendrix 
(80) found that chlamydospores of Fusarium 
solani f, phaseoli, the dry root-rot organism 
of beans, germinated in close proximity to 
seeds and roots of many nonsusceptible plants. 
After germination, the number of propagules 
increased in the rhizospheres of tomato, 
lettuce, and corn, but decreased in the rhizo- 
sphere of onions. Onion could therefore be 
used as a possible decoy crop to reduce 
survival of this important pathogen in soil. 


Selective Control Through 
Physiological Changes 
Very few cases of selective biological con- 
trol through changes in host metabolism have 
been reported (23). Research in our laboratory 
on the nutrition of Aphanomyces, the cause 
of a serious root rot of peas, radish, and 
87 
sugarbeets, indicated that the oxidation state 
of sulfur had a profound effect on the growth 
and sexual reproduction of the pathogen (19, 
64). Sulfur-containing amino acids, especially 
methionine, were most favorable for growth 
and sexual reproduction of the pathogen. Subse- 
quent studies (68, 69) on the effect of various 
sulfur sources on the development of 
Aphanomyces root rot of peas led to the 
discovery that methionine was able to prevent 
disease development completely, even though 
it was in no way detrimental to the fungus. 
Further studies were made (70) of the effect 
of methyl-containing amino compounds and 
related substances with and without sulfur in 
their molecules on the expression of disease 
symptoms. These studies showed (1) that the 
oxidation state of sulfur of the compounds and 
root rot severity were not related; (2) that 
all compounds partially or completely effective 
against the expression of disease symptoms 
possessed methyl and amino groups in their 
molecules; (3) that the position of the methyl 
group in the molecule was very critical with 
respect to control; and (4) that the effect of 
these methyl-containing amino compounds was 
not on the disease-causing organism, but 
rather on the expression of disease symptoms, 
These studies suggested a host response to 
the methyl group in conjunction with the 
amino group. They also suggested that trans- 
methylation in plants may be an important 
process related to disease resistance. The 
concept of transmethylation was further sup- 
ported by the demonstration (21) that pea 
plants were able to convert homocysteine, a 
precursor of methionine lacking a methyl 
group, to methionine, which possesses a 
methyl group and which, in turn, suppressed 
the disease. 
Transmethylation reactions occur not only 
in animals but also in several plants. The 
methyl group from methionine and other com- 
pounds was transferred intact to form the 
methoxyl groups of lignins in barley and 
tobacco plants (10) and the methyl esters of 
pectinic acid in radish plants (79). The middle 
lamella of pea roots contains pectinates with 
an unknown degree of esterification, whose 
breakdown by the fungus enzymes leads to 
maceration and root-rot development. Recent 
studies (1) on pectolytic enzymes of 
Aphanomyces showed that the parasite 
