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Abby Seaman, Area Extension Educator, NYS IPM Program
Plants respond to attacks by insects and diseases by mobilizing
an array of compounds that inhibit plant diseases, or reduce feeding
by insects. Often, plants in which resistance is induced by one
pathogen or insect will also be resistant to some other pathogens
or insects, but not necessarily the entire spectrum of potential
pests. It is also possible that mobilizing resistance to one pathogen
could increase susceptibility to another pathogen. Various approaches
to inducing resistance in plants are currently being studied as
possible pest management tools in the field.This article describes
some of the ways that plant resistance can be induced in the absence
of pests, rendering plants more resistant to future attacks by
insects or pathogens.
Composts
The application of mature composts to soil or potting mix has been
shown to induce a resistance response in above-ground parts in
several crops including cucumber and tomato. The exact mechanism
by which composts and compost extracts induce resistance is not
well understood, and not all composts are able to cause an induced
resistance response. One study that looked at 25 different composts
for induction of resistance to bacterial spot in radish found
that only two of the composts induced strong resistance (Krause
et al. 1998). The ability of a compost to induce resistance may
depend on the compost being recolonized by specific organisms
during the curing phase (Hoitink and Boehm 1999). Induced resistance
responses may also be variable from one batch of compost to the
next, and the response may be different when different types
of soils are amended (Abbasi et al. 2001).
Isolated Plant Growth Promoting Rhizobacteria
One of the hypothesized mechanisms for the induction of resistance
by composts is the presence of certain plant growth promoting
rhizobacteria (PGPR). Researchers have isolated a number of species
of bacteria that have plant growth promoting properties and tested
them in specific crop/pest situations. They have found that particular
species of bacteria work better in particular plant/pest situations.
Zehnder et al. (1997) found that plant growth was enhanced and
that striped cucumber beetle feeding and subsequent infection by
bacterial wilt were reduced in cucumber plants treated with a mixture
of species of PGPRs compared with untreated controls. PGPRs have
also been shown to protect cucurbits from anthracnose and angular
leafspot (Raupach and Kloepper 2000), and tomatoes from viral diseases
(Zehnder et al. 2001).
Other Microbials
Microbial products that are sold for biological control of soil-borne
root pathogens may also induce resistance to diseases of above
ground parts. We suspect that induced resistance is involved
because of examples in which the product was applied only to
the soil and the effect was seen in the above ground parts of
the plants. In trials conducted on an organic farm in western
NY during the summers of 2001 and 2002, tomatoes drenched with
a suspension of Plant Shield (Trichoderma harzianum) at transplanting
had reduced levels of early blight at the end of the season compared
with the untreated control. In a third season of this trial,
during which Septoria leafspot was the predominant foliar disease,
the Plant Shield drench did not reduce disease significantly
compared with the untreated control. In a greenhouse tomato trial,
Mycostop (Streptomyces griseoviridis) applied as a soil drench
provided control of gray mold comparable to foliar fungicide
(Bravo) applications (Lamboy et al.). Both Plant Shield and Mycostop
are OMRI approved.
Chemical induction
A number of compounds have been shown to induce resistance in plants
when applied to the foliage. Among the compounds demonstrated
to have this effect are salicylic acid, potassium phosphate,
a water solution of NPK fertilizer, certain plant extracts, and
extracts of microbial metabolites. Specific plant/compound combinations
seem to be necessary to induce resistance; e.g. a given compound
will induce resistance in some plants but not others. Commercial
products that act as resistance inducers are currently on the
market. One product is called Messenger. The active ingredient
of Messenger is a protein called harpin, which occurs on the
cell wall of the bacteria that causes fire blight in fruit trees,
and is recognized by some plants as a sign of pathogen attack.
Harpin application also has the beneficial side effect of increasing
plant growth. Another product, Actigard, is a synthetic chemical
that induces a resistance response in plants. A third product,
Milsana, is an extract from the giant knotweed plant. None of
these products is currently OMRI approved.
Nonpathogens or weak pathogens:
Weak or nonpathogenic strains of plant pathogens can induce a resistance
response if they have surface proteins detected by the plant
as those of pathogens. Weak strains of viruses have been used
to induce resistance that helps protect plants from later infection
by virulent strains. Cucumber plants inoculated with anthracnose
were found to have fewer striped cucumber beetles feeding on
them in cage studies (Zehnder et al. 1997).
Induced resistance on the farm
Although research on induced resistance has been conducted since
the mid 1970’s, there is still a lot to be learned before
it can be used predictably in the field. Organic growers, who
regularly add compost or other organic matter to their soil may
already be taking advantage of the benefits of induced resistance
because of the increased microbial activity that results from
additions of organic matter. It’s not clear at this point
how much additional advantage the use of resistance inducing
products would provide to plants growing in a very microbially
active soil. Those who are still working to build their soil
may get some benefits from the use of microbial or other products
that induce resistance. Microbial products have also been found
to be effective in a greenhouse situation where sterile potting
mixtures are being used. Products sold for biological control
of certain pathogens may also have more generalized induced resistance
effects. They include products such as Serenade (Bacillus subtilis
), Plant Shield (Trichoderma harzianum), and Soilgard (Giocladium
spp.), all of which are currently OMRI approved. These products
and the chemical resistance inducers listed above will not provide
complete control of plant diseases, but may delay or reduce disease
development in the field. Keep an eye on this emerging area of
research as specific practices and products are developed for
field use.
References
Abbasi, P.A., F. Sahin Al-Dahmani, H.A.J. Hoitink, and S.A. Miller.
2001. Effect of compost amendments on disease severity and yield
of tomato in conventional and organic production systems. Plant
Disease 86(2):156-161.
Hoitink, H.A.J., and M.J. Boehm. 1999. Biocontrol within the context
of soil microbial communities: a substrate-dependent phenomenon.
Annual Review of Phytopathology 37:427-46.
Krause, M.S., T.J.J. De Cuester, N.Y. Han, C. A. Musselman, and
H.A.J. Hoitink. 1998. Systemic acquired resistance induced by composts:
a highly specific phenomenon. Phytopathology 88(9):S49.
Lamboy, J. S., H. R. Dillard, and W. F. Lamboy. Microbial and
synthetic products for management of Botrytis grey mold in tomato.
http://www.nysipm.cornell.edu/publications/greymold.html
Raupach G. S. and J. W. Kloepper. 2000. Biocontrol of cucumber
diseases in the field by plant growth-promoting rhizobacteria with
and without methyl bromide fumigation. Plant Disease 84(10):1073-1075.
Zehnder, G., J. Kloepper, C. Yao, and G. Wei. 1997. Induction
of systemic resistance in cucumber against cucumber beetles (Coleoptera:
Chrysomelidae) by plant growth-promoting rhizobacteria. J. Econ.
Entomol. 90(2):391-396.
Zehnder G. W., J. F. Murphy, E. J. Sikora, and J. W. Kloepper
. 2001. Application of rhizobacteria for induced resistance. European
Journal of Plant Pathology 107(1): 39-50.
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