Effects Of Induced Resistance Research Articles

Role of induced resistance in interactions of Epilachna vigintioctopunctata with host and non-host plant species

The process of host/non-host determination was dissected in interactions of Epilachna vigintioctopunctata, a specialist herbivore of solanaceous plants, with various plant species. On host plants (tomato and egg plant) the ladybird beetle started feeding within 5 min. On red pepper, another solanaceous plant, it also started feeding within 5 min, but did not continue the feeding as vigorously as on tomato or eggplant. This result suggests that the ladybird beetle recognizes red pepper as a host plant but does not overcome its constitutive resistance. On Chinese cabbage, the ladybird beetle did not start feeding as quickly as on the host plants, but once started, it continued feeding as vigorously as on the host plants. This result suggests that the ladybird beetle does not recognize Chinese cabbage as a host plant but overcomes its constitutive resistance. Subsequently, the effect of induced resistance in a host (tomato) and non-hosts (Chinese cabbage and Arabidopsis) was evaluated. The treatment with methyl jasmonate (MeJA) showed no effects in tomato but decreased the damaged area in Chinese cabbage and Arabidopsis. A feeding test with Arabidopsis mutants supported the idea that induced resistance via the jasmonic acid (JA) pathway is effective against the ladybird beetle on the cruciferous plants. We suggest that a specialist herbivore has to overcome not only constitutive resistance but also induced resistance to utilize the non-host plant as a host, and that induced resistance is one of the factors that determine host specificity of the specialist.

INDUCED VS. CONSTITUTIVE RESISTANCE AND THE SPATIAL DISTRIBUTION OF INSECT HERBIVORES AMONG PLANTS

Because herbivores can move away from plants of low quality, induced resistance should affect the spatial distribution of herbivore damage (or herbivores) within or among plants. We used a spatially explicit simulation model and data from a field experiment to examine the effect of induced resistance on the distribution of herbivores and their damage among plants within a population. The model is appropriate only for rapid (as opposed to delayed) induced resistance, assumes that resistance affects herbivore behavior but not performance, and assumes no interplant communication. It is also appro- priate only for induced resistance that decays in the absence of herbivore damage. The simulation tracks herbivores on a linear array of plants or plant parts with reflecting bound- aries and allows manipulation of the timing of induced resistance and the initial distribution of herbivores. Our model suggests that increasing lags and thresholds for the production of induced resistance causes increasing aggregation of herbivores and their damage, but in the absence of either a lag or a threshold, induced resistance can lead to an even distribution of herbivores. The formation of even or aggregated distributions of herbivores depends not on initial distributions, but on the characteristics of the induced resistance. In the field experiment, we measured the degree of aggregation of Mexican bean beetle larvae among plants within experimental soybean populations consisting of soybean varieties that had no resistance (low constitutive, low inducible resistance), induced resistance (and low con- stitutive resistance), or constitutive resistance (and low inducible resistance). In general agreement with qualitative predictions of the model, we found that larvae on soybean varieties with induced resistance remained significantly more aggregated than larvae on varieties with no or high constitutive resistance.

Comparison of strains of Lysobacter enzymogenes and PGPR for induction of resistance against Bipolaris sorokiniana in tall fescue

Abstract Lysobacter enzymogenes C3, the only biocontrol agent previously known to induce resistance in tall fescue against Bipolaris sorokiniana, was compared in growth chamber experiments with other strains of L. enzymogenes, strains of plant growth promoting rhizobacteria (PGPR) that induce systemic resistance in dicot plants, and the synthetic elicitor 1,2,3-benzothiadiazole-7-thiocarboxylic acid-S-methyl-ester (BTH). The treatments were evaluated for induction of localized or systemic resistance against B. sorokiniana when applied to leaves and roots. In addition, the effects of induced resistance on pathogen conidial germination on the phylloplane were assessed. None of the bacterial or chemical treatments induced systemic resistance when applied to a leaf. Strains of L. enzymogenes differed in their ability to cause localized disease inhibition following foliage treatment and to induce systemic resistance in leaves when applied to roots. In contrast to C3, two other strains of L. enzymogenes were ineffective in inducing systemic resistance. PGPR strains varied in effectiveness in causing localized disease inhibition when applied to leaves. Most of the bacterial strains increased peroxidase activity in the treated leaves, providing evidence that localized disease inhibition may have been plant mediated. The involvement of localized induced resistance was confirmed in P. fluorescens WCS417r, which did inhibit B. sorokiniana conidial germination or hyphal growth in vitro. Soil drenches with nearly all PGPR strains resulted in systemic resistance in leaves, but the treatments varied as to the timing and strength of induced systemic resistance. BTH induced localized resistance when applied to leaves but did not activate resistance in leaves when applied to roots. All cases of induced resistance were associated with an inhibition of conidial germination on leaf surfaces and, thus, this reaction appears to be a hallmark of induced resistance in the B. sorokiniana-tall fescue pathosystem.

Understanding the involvement of rhizobacteria-mediated induction of systemic resistance in biocontrol of plant diseases

Specific strains of nonpathogenic rhizobacteria can induce systemic resistance that is effective against a range of plant pathogens. To exploit induced systemic resistance, detailed knowledge of the triggering bacterial traits involved and on signal transduction pathways in the plant is necessary. Possibilities to improve effectiveness of induced resistance by rhizobacterial strains are discussed.Key words: induced systemic resistance, lipopolysaccharides, Pseudomonas fluorescens, Pseudomonas putida, salicylic acid, siderophores, signaling pathways.

Low dose hypersensitivity and induced resistance of V79 cells after charged particle irradiation using 100 MeV/u carbon ions.

This work aimed to study the effects of low dose irradiation in order to detect possible deviations from the linear-quadratic dose dependence of survival after charged particle irradiation. Survival of V79 cells at low doses was studied using a semi-automated cell detection system similar to the DMIPS system. With such a technique, the precision of survival measurements is improved by avoiding the stochastic uncertainties of the conventional dilution assay. A significant hypersensitivity at low doses (0.1 Gy) has been detected for 100 MeV/u carbon ions, whereas for higher doses (up to 0.5 Gy) a transition to a plateau-like region is observed, which is consistent with the assumption of induced resistance. These results are apparently in contrast to earlier studies using high-LET radiation like, e.g. neutrons or negative pions, where the effect of induced resistance has been reported to be at least significantly reduced for single exposure experiments.

Bioassay versus chemical assay: measuring the impact of induced and constitutive resistance on herbivores in the field.

Studies of induced plant resistance usually either examine physiological/chemical mechanisms or explore the ecological and evolutionary role of induced resistance. To connect these two methods of study, data are needed that address the relationships between plant chemistry and effects of induced resistance on herbivores under field conditions. In this paper we combine the results of a greenhouse experiment and a field experiment to try to make such a connection. Levels of induced and constitutive resistance to Mexican bean beetles in several soybean genotypes were measured in a greenhouse experiment using a behavioral bioassay. In a field experiment, beetle performance and induced and constitutive levels of cystein proteinase inhibitor activity were measured for these same genotypes. Greenhouse bioassay ratings of induced and constitutive resistance and induced and constitutive levels of proteinase inhibitor activity agree closely for individual genotypes, suggesting that proteinase inhibitors (PIs) are involved in induced resistance. However, while greenhouse bioassay measures of constitutive and induced resistance were good predictors of beetle performance in the field, proteinase inhibitor activity did not predict beetle performance well. While PIs appear to be involved in induced resistance, they were not strongly correlated to Mexican bean beetle performance in the field in this study. These results suggest that measurement of single aspects of plant chemistry may not encompass all ecologically important aspects of plant resistance.

Effect of jasmonate‐induced plant responses on the natural enemies of herbivores

Summary Plant traits can act as defences against herbivores both by reducing herbivore performance directly and by increasing the effectiveness of the natural enemies of herbivores. Natural enemy performance and rate of parasitism can be affected by plant traits and/or changes in herbivore quality. Jasmonic acid is responsible for the induction of many changes in plant resistance that occur following herbivore attack. This study examines the effects of jasmonate‐induced defences on the abundance and performance of natural enemies, as well as the interaction between induced plant responses and parasitism in their ability to kill herbivores. In a tomato field containing plants induced with jasmonic acid and control plants, the abundance of natural enemies was counted using several census techniques. Induced resistance affected the abundance of natural enemies of herbivores differently. One predator of aphids, syrphid flies, was negatively affected by the decrease in herbivore abundance on induced plants. One parasitoid of caterpillars, Hyposoter exiguae Viereck, was not affected by induced resistance in this study although it was positively affected in a previous study. Two other natural enemies, a parasitoid of aphids and ladybeetle predators, were not affected by induced resistance. Induced resistance and parasitism both reduced the survivorship of herbivores. In a 2 × 2 factorial design with induced resistance crossed by parasitism, the interaction between the effects of induced resistance and parasitism on herbivore survival was tested. Two trials of this experiment were conducted; one where induced resistance affected both herbivore quantity and quality and one where induced resistance only affected herbivore quality (herbivore density was equalized). Parasitoids were more effective at killing herbivores feeding on control plants than herbivores on induced plants. This difference was due to a difference in herbivore quality, not herbivore quantity. Herbivores feeding on induced plants have lower mass than herbivores on control plants and this appears to be a major factor reducing the performance of developing parasitic wasps. Thus, jasmonate‐induced responses influence natural enemies species in differing ways, sometimes reducing the density of herbivores and sometimes reducing the quality of herbivores for natural enemies.

Induced Resistance in Cucumbers against Fusarium oxysporum f. sp. cucumerinum and Rhizoctonia solani AG2–2 by Infection of the Cotyledons

Localized infection in cucumber cotyledons with Colletotrichum lagenarium induced resistance against infection after challenge inoculation with Rhizoctonia solani AG2–2 and Fusarium oxysporum f. sp. cucumerinum in the roots. The plants were unprotected in soil that was infested heavily with R. solani or in contact with the mycelium, and induced resistance was not observed. Wounding of the root also negated the effect of induced resistance to F. oxysporum.

Pine wilt disease caused by the pine wood nematode: the induced resistance of pine trees by the avirulent isolates of nematode

Since the beginning of the 20th century, pine trees in Japan have been seriously damaged by the pine wilt disease. This disease is caused by the pine wood nematode, Bursaphelenchus xylophilus, which is transmitted by the Japanese pine sawyer, Monochamus alternatus. The control of disease depends to a large extent on chemicals, but the public is now demanding environmentally friendly control methods. The virulence of B. xylophilus varies very widely. Pre-inoculation of young pine trees in a nursery with avirulent B. xylophilus has induced systemic resistance of trees against a subsequent inoculation with virulent B. xylophilus. This induced resistance was considered a hopeful means for developing a biological control for the disease. The induced resistance by the avirulent nematodes was also expressed in mature pine trees in a forest where the disease was naturally epidemic. However, the effects of induced resistance were not satisfactory for practical biological control. Since the inoculation with higher concentrations of the avirulent B. xylophilus induced the resistance more effectively, the pre-inoculation method will need to be improved to develop the biological control. The induced resistance of pine trees by avirulent B. xylophilus should be one of the candidate biological control methods against pine wilt disease. This induced resistance also provides an experimental system to clarify physiological interactions between the nematodes and pine trees.

Resistance to 2-tridecanone in Tetranychus urticae: effects of induced resistance, cross-resistance and heritability.

Earlier studies of resistance to the tomato toxin 2-tridecanone revealed similar levels of resistance among tomato and cucumber strains of Tetranychus urticae, whereas we anticipated the tomato strains to have a higher resistance. Between-strain differences, however, may be concealed by toxin accumulation and/or cross-resistance. Mites do accumulate 2-tridecanone to a considerable extent, but we have shown before that this cannot explain the lack of difference in resistance. In this paper, we provide evidence against cross-resistance: selection for resistance to a homologous compound from cucumber, 2-nonanone, did not confer resistance to 2-tridecanone. Observed increases in resistance levels over generations may be due to induction or selection. To study induction, adult females from a cucumber strain and a tomato strain were exposed to a sublethal dose of 2-tridecanone. One day after exposure, the cucumber strain gained in resistance to 2-tridecanone (c. 8-fold), whereas the tomato strain showed a decrease in resistance. These changes in resistance disappeared within three days after exposure, the time the mites need to dispose themselves of the toxin. The results support earlier findings that resistance is induced in the cucumber strain, but not in the tomato strain. To assess the heritability of resistance to 2-tridecanone, offspring was obtained from individual females, and both the mothers and their offspring were tested for resistance. For the cucumber strain, offspring from susceptible or resistant mothers did not differ in resistance. For the tomato strain, however, offspring from resistant mothers were significantly more resistant to the toxin than offspring from susceptible mothers. This result provides unambiguous evidence that resistance to 2-tridecanone can be selected for.

Chemically-induced resistance against multiple pests in cotton

Plants can be treated with natural elicitors to induce resistance to herbivores. The effects of induced resistance to cotton aphids Aphis gossypii Glover (Homoptera: Aphididae), two-spotted spider mites Tetranychus urticae Koch (Acari: Tetranychidae), and western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in cotton plants were investigated using applications of the natural plant inducer, jasmonic acid (JA). Preference was reduced by more than 60% for aphids and spider mites, and more than 90% for thrips on JA-induced leaves compared with control leaves. Survival of aphid nymphs was reduced by two-fold and percentage of nymphs attaining the adult stage was reduced by about eight-fold on induced leaves compared with controls. Induction reduced survival and reproduction of adult aphids by about 40% and 75%, respectively. Induction did not affect survival in spider mites or thrips. However, induction reduced egg production in spider mites by more than 75% and feeding in thrips by up to 80% compared with controls. Induction reduced leaf area in cotton seedlings by about 28%. The implications of our results for cotton pest management are discussed.

Jasmonic acid induced resistance in grapevines to a root and leaf feeder.

We investigated the effects of induced resistance to the folivore Pacific spider mite, Tetranychus pacificus McGregor (Acari: Tetranychidae), as well as the root-feeding grape phylloxera Daktulosphaira vitifoliae (Fitch) (Homoptera: Phylloxeridae) in grapevines using exogenous applications of the natural plant inducer, jasmonic acid. Foliar jasmonic acid application at concentrations that caused no phytotoxicity significantly reduced the performance of both herbivores. There were less than half as many eggs produced by spider mites feeding on the induced leaves compared with control grapevine leaves. Induction reduced the numbers of phylloxera eggs and nymphal instars by approximately threefold and twofold, respectively, on induced compared with control grapevine roots. The negative demographic effects of jasmonic acid application appeared to be caused by changes in fecundity for the Pacific spider mite, and possibly changes in development rate and fecundity for grape phylloxera.

Specificity of induced resistance in wild radish: causes and consequences for two specialist and two generalist caterpillars

Inducible plant resistance against herbivores is becoming a paradigm of plant–herbivore ecology. Fundamental to understanding induced resistance and its evolutionary ecology is specificity of “induction” and “effects”. Specificity in the induction of resistance refers to whether plant damage by various herbivores causes the same response in plants. Specificity in the effects of induced resistance refers to whether induction has the same consequences (i.e., reduced preference or performance) for various herbivores. I examined both specificity of induction and effect employing four lepidopteran herbivores and wild radish plants, a system for which fitness benefits and costs of induction have been documented for the plant. Variation in the specificity of induction and effects of induced plant resistance was found; however, this variation was not associated with diet specialization in the herbivores (i.e., specialists vs generalists). Induction caused by Plutella (specialist) and Spodoptera (generalist) resulted in general resistance to all of the herbivores, induction caused by Pieris (specialist) induced resistance only to Spodoptera (generalist) and Pieris, and plant damage by Trichoplusia (generalist) failed to induce resistance and reduce the performance of any of the herbivores. To the contrary, plants damaged by Trichoplusia supported enhanced growth of subsequently feeding Trichoplusia compared to uninduced controls. These results add a novel level of complexity to interactions between plants and leaf chewing caterpillars. Within the same guild of feeders, some herbivores cause strong induced resistance, no induced resistance, or induced susceptibility. Similarly, caterpillar species were variable in the level to which induced resistance affected their performance. Such interactions limit the possibility of pairwise coevolution between plants and herbivores, and suggest that coevolution can only be diffuse.

Density dependence in induced plant resistance to herbivore damage: threshold, strength and genetic variation

The density‐dependent effect of induced plant resistance on herbivore populations depends on the relationship between the amount of herbivore damage and the level of induced resistance produced by the plant. This relationship should influence the interaction of induced resistance and herbivore population dynamics, and if the relationship varies among plant genotypes, it could be subject to natural selection by herbivores. In this study the relationship between percent leaf area damaged and level of induced resistance was characterized for four genotypes of soybeans grown in a greenhouse. Damage ranging from 8 to 92% of leaf area was imposed using Mexican bean beetle larvae, and induced resistance was measured by bioassay using Mexican bean beetle adults. The level of induced resistance was significantly affected by the amount of damage, and the level of induced resistance varied significantly among the four genotypes. There was also a marginally significant interaction of damage and plant genotype, suggesting that the form of density dependence varies among these four genotypes of soybeans. These results suggest that these genotypes of plants might affect herbivore populations differently. If this variation is heritable, the form of density‐dependent effects of induced resistance has the potential to evolve in this system.

What omnivores eat: direct effects of induced plant resistance on herbivores and indirect consequences for diet selection by omnivores

Summary 1. Omnivory, where an animal crosses trophic boundaries, is thought to be common in natural and managed communities. Foraging theory predicts that omnivores will balance their diet as a result of nutritional needs, food quality and availability of alternate foods. We investigated diet selection of the western flower thrips [Frankliniella occidentalis (Pergande)], a herbivore under some circumstances, and a predator under other circumstances. 2. We demonstrate that induced plant resistance can indirectly affect diet selection by thrips. The density of herbivorous spider mites is reduced on induced plants compared to controls and is also positively correlated with the number of mite eggs that the thrips consume. 3. In choice tests, mite eggs from induced plants were less preferred by thrips than eggs from control plants. Mite egg size was also smaller on induced plants compared to controls. However, this alone did not explain the diet selection by thrips. In a field experiment, thrips avoided colonizing induced plants, but they were attracted to induced plants that we inoculated with mites. Thus, the presence of herbivorous prey on induced plants attenuated the negative effects of induced resistance on thrips colonization. 4. Feeding decisions of thrips can be influenced by several factors. We showed previously (Agrawal, Kobayashi & Thaler 1999a) Influence of prey availability and induced host plant resistance on omnivory by western flower thrips. (Ecology, 80, 518–523.) that reducing plant quality can cause omnivores to shift towards relatively more predation than herbivory. We show now that on induced plants, reduced prey density and quality may antagonize this shift towards increased predation. 5. Induced plant resistance has negative effects on both the plant‐based food resource (direct effect) and the animal‐based food resource (indirect effect) of omnivorous thrips. Thus, variation in the quantity and quality of food items interact to determine the diet selection of omnivores.

How leaf domatia and induced plant resistance affect herbivores, natural enemies and plant performance

Predators and plant resistance may act together to control herbivorous arthropod populations or antagonistically, which would reduce the control of pest populations. In a field experiment we enhanced predation by adding simulated leaf domatia to plants. Leaf domatia are small structures that often harbor predaceous arthropods that are potentially beneficial to the plant. We also manipulated host plant quality by inducing resistance with controlled, early season exposure of seedlings to spider mite herbivory. Our manipulations had profound consequences for the natural community of arthropods that inhabited the plants. Leaf domatia had a direct positive effect on abundances of two species of bugs and one species of thrips, all of which are largely predators of herbivores. On leaves with domatia, each of the predators was found inside the domatia two to three times more often than outside the domatia. Eggs of predaceous bugs inside leaf domatia were protected from parasitism compared to eggs outside the domatia. The positive effects of leaf domatia on predator abundances were associated with reduced populations of herbivorous spider mites, aphids, and whiteflies. Plants with experimental leaf domatia showed significantly enhanced reproductive performance. Induced resistance also affected the community of arthropods. Of the abundant predators, all of which also fed on the plant, only minute pirate bugs were negatively affected by induced resistance. Populations of herbivorous spider mites and whiteflies were directly and negatively affected by induction. In contrast, aphid populations were higher on plants with induced resistance compared to uninduced plants. Effects of induced resistance and domatia were additive for each of the predators and for aphids. However, spider mite and whitefly populations were not suppressed further by employing both induced resistance and domatia compared to each strategy alone. Our manipulations suggest that plant defense strategies can have positive effects on some species and negative effects on others. Negative effects of “resistance traits” on predators and positive effects on some herbivores may reduce the benefits of constitutive expression of resistance traits and may favor inducible defense strategies. Multiple plant strategies such as inducible resistance and morphological traits that aid in the recruitment of predators of herbivores may act together to maximize plant defenses, although they may also be redundant and not act additively.

Induced resistance of host tree foliage during and after a natural insect outbreak

1. Plant resistance against insect herbivores often increases after experimental damage to foliage, but few studies have obtained field estimates of the effect of induced resistance on insect populations during and after a natural insect outbreak. 2. This study measured the effect of quality of the host tree, mountain birch (Betula pubescens ssp. czerepanovii), on the periodically fluctuating folivore Epirrita autumnata (Lepidoptera, Geometridae) during peak and postpeak years of an outbreak in Finnish Lapland. Comparisons were made both within and between study sites to assess host plant quality, and thereby the effect of delayed induced resistance (DIR). 3. In within‐site comparisons, a set of experimental trees was defoliated by wild larvae in the peak year of the outbreak, whereas control trees were protected from defoliation by spraying with an insecticide. The effect of host plant quality was quantified in the following year by measuring the pupal mass of E. autumnata larvae reared in enclosures on these trees. 4. In between‐site comparisons, the sizes of pheromone‐trapped males were measured at both outbreak and low density sites during the progress of the outbreak. The size of trapped males was subsequently used to estimate the corresponding fecundity of females at the same sites. 5. Pupal mass of E. autumnata reared on trees defoliated in the previous year was 0–10% lower than on those trees protected from defoliation by the insecticide. Field‐collected adults indicated a similar pattern: they were smaller at outbreak sites than at low‐density sites, and the size reached its minimum in the post‐peak year. However, the estimated loss of reproductive capacity of females resulting from DIR was too small to be the sole explanation for the termination of the outbreak. 6. Whether the weak DIR response in this system was a characteristic of the 1990s outbreak alone remains unclear, because different terminating agents may be important for different individual outbreak peaks. During this outbreak, larval parasitism and developmental asynchrony between larvae and birch were probably more important reasons for population collapse than DIR.

Specificity of resistance to pea seed-borne mosaic potyvirus in transgenic peas expressing the viral replicase (Nlb) gene.

Transgenic pea lines carrying the replicase (NIb) gene of pea seed-borne mosaic potyvirus (PSbMV) were generated and used in experiments to determine the effectiveness of induced resistance upon heterologous isolates. Three pea lines showed inducible resistance in which an initial infection by the homologous isolate (PSbMV-DPD1) was followed by a highly resistant state. Resistance was observed in plants in either the homozygous or hemizygous condition and resulted in no overall yield loss despite the initial infection. Resistance was associated with a loss of both viral and transgene RNA, which is indicative of a mechanism based upon post-transcriptional gene silencing. There was no correlation between the steady-state levels of transgene RNA and ability of the plants to show resistance. To test the specificity of the resistance, plants were also inoculated with the most distantly related sequenced PSbMV isolate, NY. PSbMV-NY varied between experiments in its ability to induce resistance, suggesting that the sequence identity in the NIb gene is borderline for the specificity required for triggering gene silencing. Upon challenge inoculation of virus-free recovered leaves, the specificity of the induced resistance varied between the two isolates and indicated that the virus and transgene additively determined the resistant state. These results suggest that the sequence requirements for triggering gene silencing may differ from those involved in the degradation process.

Induced Resistance to Apple Scab: Microscopic Studies on the Infection Cycle of Venturia inaequalis (Cke.) Wint.

AbstractApplication of Methyl 2,6‐Dichlorisonicotinate (DCINA) induced resistance in apple against Venturia inaequalis. The effects of induced resistance on the infection cycle of the fungus were investigated by fluorescence microscopy. Induced resistance interfered with all stages of infection except conidial germination and appressorium formation. The establishment of primary stromata after penetration of the cuticle was affected by induced resistance as indicated by a delay in the visualisation of these structures. Only 10% of the primary stromata further expanded and differentiated runner hyphae in induced resistant plants. Contrarily in unprotected plants, runner hyphae were formed at 100% of the successful penetration sites. Characteristically, circular fluorescing structures appeared associated with stunted penetration sites. The epidermal cells underneath these infection sites showed no hypersensitive response. On induced resistant plants the number and length of runner hyphae was reduced. In most cases neither colonisation nor sporulation occurred due to the inhibition of secondary stroma formation. Possible mechanisms of induced resistance in apple plants are discussed.

Induction of Systemic Resistance in Cucumber Against Cucumber Beetles (Coleoptera: Chrysomelidae) by Plant Growth-Promoting Rhizobacteria

Field studies were conducted in 1993 and 1994 to evaluate the effects of induced resistance in cucumber by plant growth-promoting rhizobacteria (PGPR) on numbers of the spotted cucumber beetle, Diabrotica undecimpunctata howardi Barber, and the striped cucumber beetle, Acalymma vittatum (F.). Cucumber plant growth and yields were significantly ( P <0.05) greater, and populations of cucumber beetles were significantly lower, on PGPR-treated cucumber than on nontreated cucumber. On dates when peak beetle populations were present, PGPR treatment resulted in significantly( P <0.05) greater cucumber beetle control than weekly applications of esfenvalerate insecticide. In no-choice greenhouse cage e:periments with 3 cucumber cultivars, beetles infected with the cucurbit wilt pathogen, Erwinia tracheiplila, were released and allowed to feed on PGPR-treated or nontreated cucumber plants. The incidence of cucurbit wilt disease was significantly( P <0.05) lower on PGPR-treated cucumber plants than on nontreated plants. These results indicate that PGPR-induced resistance may be more effective than insecticides for control of cucumber beetles and cucurbit wilt disease on cucumber. Possible mechanisms for PGPR-induced resistance against cucumber beetles are discussed.