Field Management of Late Blight of Tomatoes Caused by Phytophthora infestans Using Antibiotics from Streptomyces species

Several plant extracts were screened for
antibacterial activity against Pseudomonas syringae pv.
phaseolicola and Xanthomonas campestris pv. phaseoli in
vitro. Extracts from Eucalyptus citriodora Hook,
Nothoscordum inodorum L., Cupressus lucistanica Mill,
Tagetes minuta L. and Santolina chamaecyparissus L. were
found to be active on the two phytobacterial pathogens.
The volatile oil of E.citriodora produced a significantly (P
= 0.05) wider zones of growth inhibition on P. syringae pv.
phaseolicola and X. campestris pv. phaseoli when compared
to the other plant extracts. The growth inhibition zones
produced by the volatile oil of E. citriodora measured
32.83 mm and 24.50 mm on X. campestris pv. phaseoli and
P. syringae pv. phaseolicola respectively. There was an
interaction between the test pathogens and the extract
.- .
type. This was demonstrated by the significantly (P =
0.05) high sensitivity of X. campestris pv. phaseoli to the
different plant extracts when compared to P. syringae pv.
phaseolicola.
Further investigations were carried out to determine
the minimum inhibitory concentration of the volatile oils
from E. citriodora and T. minuta on P. syringae pv.
phaseolicola and X. campestris pv. phaseoli. T. minuta
extract caused total growth inhibition on the two
phytobacterial pathogens at 0.20% concentration while E.
citriodora extract caused complete growth inhibition on P.
syringae pv. phaseolico/a and X. campestris pv. phaseoli at
(xviii)
0.78% and 0.39% concentrations respectively.
Fractionation of the volatile oils from E. citriodora, T.
minuta, C. lucistanica and S. chamaecyparissus was
carried out and the resultant fractions tested for
antibacterial activity against P. syringae pv. phaseolicola
and X. campestris pv. phaseoli. The crude extracts of E.
citriodora and T. minuta had a significantly (P = 0.05)
higher activity than the hexane and ether fractions on P.
syringae pv. phaseolicola. The test for minimum inhibitory
concentration of the ether fraction of E. citriodora
indicated that it lies between 1.56% and 3.13%
concentrations. The ether fraction of T. minuta did not
produce a zone of growth inhibition when diluted beyond
6.25% concentration. Generally, the activity of ether
fraction of E. citriodora with respect to its activity
against X. campestris pv. phaseoli and P. syringae pv.
obeseoiicote was significantly (P = 0.05) higher than that
of T. minuta. A test was carried out to establish the
stability of the active compounds in T. minuta and E.
citriodora crude extracts when subjected to various
v 0
temperatures. All the temperature regimes viz., Dc 1Oc,
o o coo o 0 0 o 0
20c 30c 40c 50c 60c 70c 80c 90c 100c and 121c
tested did not affect the activity of the two plant
extracts under study with respect to their activities
against P. syringae pv. phaseolicola and X. campestris pv.
phaseo/i. The active fractions were not denatured by
o
temperatures as high as 121c . For instance the activity
o
of E. citriodora extract subjected to 121c on P. syringae
(x Ly )
pv. phaseolicola was not significantly (P=0.05) lower than
o
that subjected to Oc
In vivo test for antibacterial activity on P. syringae
pv. phaseolicola and X. campestris pv. phaseoli indicated
that streptomycin sulphate and copper oxychloride
reduced halo blight infection significantly (P = 0.05) when
compared to bean seedlings receiving foliar spray of E.
citriodora or T. minuta extract under green house
conditions. No significant (P = 0.05) difference was
observed in halo blight and common blight infection among
bean plants sprayed with streptomycin sulphate or copper
oxychloride and those seed dressed with 0.59%
concentration of crude extract of E. citriodora or 0.15%
concentration of crude extract of T. minuta.
Phytotoxicity tests for different concentrations of
ether fractions and crude extracts of T. minuta and E.
citriodora were conducted using fourteen day old bean
plants of cv. Rosecoco-GLP-2 in the greenhouse. Plants
which were sprayed with 6.25%, 9.38% and 12.5%
concentrations of ether fraction of T. minuta extract
showed defoliation two weeks after spraying. Bean plants
sprayed with 3.13% concentration of ether fraction of E.
citriodora and 0.20% concentration crude extract of T.
minuta showed some brown spots on the leaves. No signs
of phytotoxicity were observed on plants sprayed with
0.78%, 0.59% and 0.39% concentrations of crude extract of
E. citriodora.
Field trials for the control of halo blight and common
blight of beans was carried out using 0.59% concentration
crude extract and 1.17% concentration ether fraction of E.
citriodora. A significantly (P = 0.01) higher halo blight
infection was observed among treatments which had
artificially inoculated seeds when compared to those
obtained from small scale farmers receiving similar
treatments. Bean plants sprayed twice with 0.59%
concentration of crude extract of E. citriodora showed a
significantly (p=O.05)lower halo blight severity when
compared to the untreated control. Dressing bean seeds
and later spraying bean plants with E. citriodora volatile
oil at seedling stage and at pod filling stage reduced halo
blight severity when compared to bean plants which were
treated (dressed or sprayed) with the ether fraction from
the same plant extract. Halo blight incidence in dressed
inoculated seeds and foliar sprayed seedlings was not
significantly (P = 0.05) different from that of the
untreated control. Copper oxychloride and streptomycin
sulphate reduced infection by common blight significantly
(P = 0.05) compared to E. citriodora extracts. The ether
fraction and crude extracts from E. citriodora did not
significantly (PO.05) lower common blight infection
(applied as seed treatment or foliar spray) when compared
to the untreated control. Yield assessment indicated that
bean seeds which were dressed with the volatile oil from
E. citriodora produced a high yield. For instance, there was
a significantly (P=0.05) higher yield/hectare from plots
(xxi)
planted with dressed inoculated seeds when compared to
those from plots planted with similar seeds which were
foliar sprayed only.
Although the activity of these plant extracts was
lower than that of streptomycin sulphate, they can be
exploited as natural bactericides for controlling halo and
common bacterial blights of beans.
URI

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