Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
community on the roots of transgenic Arabidopsis.
The root microbiome to the rescue: Microbiome changes upon defense activation i.e. the
interactions between a plant and its root microbiome might change when the plant is attacked.
Recently, it was demonstrated that infection of citrus by Candidatus Liberibacter asiaticus,
associated with Huanglongbing, drastically altered the composition of citrus rhizosphere
communities. Also, Verticillium dahliae infections affected the microbial composition of cotton
rhizospheres. Changes in rhizosphere composition upon infection might be the result of the
induced excretion of antimicrobial compounds by infected roots. However, infection does not only
lead to the secretion of pathogen deleterious compounds. It is also found that infected roots also
induced excretion of pathogen spore germination stimulators. For example infection of water
melon plants by F. oxysporum enhanced the stimulation of Fusarium spore germination by root
exudates.
In addition to this recent researches shown that plants recruit plant beneficial microbes to
their roots in response to the attack. For eg.- colonization of the roots of Arabidopsis by the plantbeneficial
soil bacterium Bacillus subtilis FB17 was greatly improved when aboveground plant
tissues were infected by Pseudomonas syringae pv. tomato. A mutually beneficial relationship
exists between Arabidopsis and FB17: FB17 is recruited to aid in plant defense, and the plant
provides the bacterium with malic acid. Activation of beneficials : inoculation of strawberry plants
with Verticillium dahliae stimulated the expression of cyanide biosynthetic genes in the biocontrol
bacterium Pseudomonas sp. LBUM300. These changes in gene expression could be a result of
nutrients leaking from damaged roots.
Future perspectives: Use of biocontrol agents in green houses is quite successful in
European countries. In Britain, although protected crops represent a small fraction of the total
area, they account for two thirds of all biologicals. At the same time, the use of pesticides in
greenhouses has declined from 4866 treated ha in 1981 to 2292 ha in 1995. A combination of
economic, political, and environmental factors has probably contributed to the transition to
biological due to the loss of insecticide registrations, insect resistance, and concern for worker
safety.
Limitations with the use of biocontrol: Because of the high value of the crop and emphasis on
quality in floriculture, vegetable crops, and ornamentals, there is less acceptance of damage and
thresholds for disease are very low. If biocontrol agents cannot perform with the consistency and
efficacy of fungicides in these crops, they may not be adopted. Control of microclimatological
conditions. The success of bioagents depends on the quality, timely availability and appropriate
release timing and methodology of bioagents. Given that a biological control strategy is
scientifically feasible, the successful grower is most likely to adopt the strategy if it is an
economically feasible decision for the greenhouse. The adoption decision is economically feasible
for the grower if expected profits are greater than zero (i.e. expected revenues are greater than
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