Natural Organic Matter and Humic Substances Interactions

appeared more sensitive than the radial growth to all C-HAs. Apparently, any C-HA sample at
any concentration stimulated markedly sclerotial initiation and greatly increased the number
of sclerotia in the plates (Fig. 2). After 168 h from the fungus inoculation, the number of
sclerotia resulted generally much higher in C-HA treatments, especially in CC-HA, with
respect to the control. These results indicated that the presence of C-HA determined the
occurrence of adverse nutritional conditions that arrest the fungal activity, with evident
benefit for plant health.
Number of sclerotia
Number of sclerotia
35
30
25
20
15
10
5
0
35
30
25
20
15
10
5
0
MC-HA
136 144
Hours
160 168
CC-HA
136 144 160 168
Hours
C 10 mg/L 50 mg/L 200 mg/L
15th IHSS Meeting- Vol. 3
Number of sclerotia
35
30
25
20
15
10
5
0
GC-HA
136 144 160 168
Hours
Figure 2: Effects of C-HAs at concentrations of
10, 50 and 200 mg/L on the sclerotia formation
of S. sclerotiorum on PDA, with respect to the
control, as a function of time
Some significant relationships were found between C-HA properties and fungal inhibition. In
the case of P. ultimum, a significant positive correlation occurred between phenolic OH
content and mycelial growth inhibition. The number of sclerotia formed were positively
correlated with ash and phenolic OH content, total acidity and E4/E6 ratio, and negatively
correlated with COOH group content. These results, which are in good agreement with
findings of a recent study performed in vitro on the fungus F. oxysporum [3], apparently
confirm what already hypothesized in that work, i.e., the inhibitory action of humic acids on
the growth of the fungi tested might be related more to hydrophobic than hydrophilic
properties of HA.
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