Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

218j 11 Rhamnolipid-Producing PGPR and Their Role
zoospores in a water suspension are attracted toward the region of elongation behind
the tips of pea roots, to wounds in the epidermis and to the exposed stele at cut ends
of roots where they encyst and germinate. Factors that influence infection include
inoculum density, soil moisture, temperature, pH, cation composition, light intensity
and presence of other microorganisms. Soil temperature and moisture (or
experimentally tested environmental factors) are known to favor the seasonal activity
of Pythium spp. [68,69]. Generally, wet soil conditions (0 to 0.3 bar matric water
potential) are necessary for development of pathogen. Among Pythium species,
P. irregulase, P. spinosum and P. ultimum are more damaging at lower temperatures,
whereas P. aphanidermatum, P. arrhenomanes, P. myriotylum and related species
cause greater damage at higher temperatures. Available evidence suggests that high
soil moisture per se does not necessarily favor the activity of Pythium. High matric
water potential and accompanying poor aeration conditions indirectly favor disease
development by (i) decreasing host vigor and increasing host exudation and
(ii) providing a suitable environment for rapid diffusion [70,71] of host exudates
necessary for germination of dormant propagules and/or vegetative growth.
11.3.2
Control
Most species of Pythium produce oospores and chlamydospores and persist for
many years; therefore, field-level elimination of the disease is not only difficult but
also expensive. On a small scale, however, Pythium can be eliminated from soil by
steam treatment or pasteurization. Fumigation with chloropicrin or methyl bromide
has been a standard practice in nursery and horticultural operations [72].
Drenching the soil with suitable fungicides, such as chestnut compound, 1%
Bordeaux mixture, 0.1% ceresin, 0.3% Blitox-50 or 0.2% Esso Fungicide 406 has
been useful in eliminating soilborne infection. Crop rotation is another method of
reducing populations of soil pathogens; but on account of its wide host range,
Pythium spp. are difficult to eliminate through this approach. Biological control of
soilborne pathogens is yet another remedy. Microorganisms that can grow in the
rhizosphere are ideal for use as biocontrol agents, since the rhizosphere provides a
frontline defense for roots against attack by pathogens. Pathogens encounter antagonism
from rhizosphere microorganisms before and during primary infection and
also during the secondary spread on the root. In some soils described as microbiologically
suppressive to pathogens [73], microbial antagonism of the pathogen is
especially strong, leading to substantial disease control.
Greenhouse methods have been developed to screen antagonists of Pythium spp.
[74] on wheat and Phytophthora megasperma f. sp. glycinea on soybean [75]. Kloepper
et al. [76] demonstrated the importance of siderophore production as a mechanism
of biological control. Siderophores have been shown to be involved in suppression of
Pythium spp. [77,78]. Howell and Stipanovic [33,47] demonstrated that the purified
antibiotics pyoluteorin and pyrrolnitrin, obtained from P. fluorescens Pf-5, provided
the same protection of cotton against damping-off by P. ultimum or R. solanii as did
the bacterium.