Natural Infestation Conditions Research Articles

Evaluation of rice for resistance to pink stem borer ( Sesamianonagrioides Lefebre)

The pink stem borer ( Sesamia nonagrioides Lefebre) is one of the most important rice pests in the Mediterranean area. The objective of this study was to identify rices resistant to pink stem borer and to determine plant characteristics associated with resistance. A total of 257 rice lines was evaluated in the field under natural infestation conditions during 1997 and 1998. The comparison between years was based on nine rice cultivars (Baldo 363, Koral, Ariete, Loto, Senia, Thaibonnet, Cigalon, Makedonia and Dion) which were common in the experimentation of both years and were used as control. A randomized complete block design with two replications was used for evaluation. The number of stems and panicles with infestation symptoms by the pink stem borer was measured at maturity. The infestation percentage of the plants varied from 0 to 89%, and on the average, it was higher in 1998 compared to 1997. The lines exhibited considerable variation in heading day (HD, 50–110 days), plant height (HT, 50–170 cm) and stem diameter (SD, 3–8 mm). The infestation was significantly correlated with HD, HT and SD. The ratio of plants with stem damage to those with panicle damage varied from 0.2 (Harra) to 11 (CT-13) but in the majority of lines it was less than 3. Eleven resistant lines (CI 1992K15, YRL-196, Colina, Frances, Niva, SB 1086, SB 428, SBSS 82, Lampo, IR-28, and IR-30), with infestation percentage less than 10%, were identified. These might be useful sources of resistance for rice breeding programmes in Mediterranean countries.

Development and field performance of a broad-spectrum nonviable asporogenic recombinant strain of Bacillus thuringiensis with greater potency and UV resistance.

The main problems with Bacillus thuringiensis products for pest control are their often narrow activity spectrum, high sensitivity to UV degradation, and low cost effectiveness (high potency required). We constructed a sporulation-deficient SigK(-) B. thuringiensis strain that expressed a chimeric cry1C/Ab gene, the product of which had high activity against various lepidopteran pests, including Spodoptera littoralis (Egyptian cotton leaf worm) and Spodoptera exigua (lesser [beet] armyworm), which are not readily controlled by other Cry delta-endotoxins. The SigK(-) host strain carried the cry1Ac gene, the product of which is highly active against the larvae of the major pests Ostrinia nubilalis (European corn borer) and Heliothis virescens (tobacco budworm). This new strain had greater potency and a broader activity spectrum than the parent strain. The crystals produced by the asporogenic strain remained encapsulated within the cells, which protected them from UV degradation. The cry1C/Ab gene was introduced into the B. thuringiensis host via a site-specific recombination vector so that unwanted DNA was eliminated. Therefore, the final construct contained no sequences of non-B. thuringiensis origin. As the recombinant strain is a mutant blocked at late sporulation, it does not produce viable spores and therefore cannot compete with wild-type B. thuringiensis strains in the environment. It is thus a very safe biopesticide. In field trials, this new recombinant strain protected cabbage and broccoli against a pest complex under natural infestation conditions.

Screening of Barley Genotypes Against Corn Leaf Aphid, Rhopalosiphum maidis (Fitch.)

Twenty four genotypes of barley were screened for their resistance against corn leaf aphid, Rhopalosiphum maidis (Fitch.) under field as well as screen house conditions during the year 1994-95. The corn leaf aphid, Rhopalosiphum maidis (Fitch) population started appearing on plants of some genotypes during standard week 3(mid January) attaining a peak during 7th standard week (mid February) when maximum population of 61.67 and minimum of 14.60 aphids per iiller was observed on genotypes clipper and AR-4, respectively under natural infestation conditions. All the twenty four genotypes were grouped as tolerant (AR-4, AR-66, AR-5, AR-48, AR-14, AR-11, AR-17, AR-16 and AR-52), susceptible(AR-10, AR-7, AR-3, AR-30, AR-13, AR-2, AR-15, AR-12, AR-9 and AR-8) and highly susceptible (BH-75, AR-25, AR-19, AR-18 and Clipper) based upon their reaction against aphid infestation under field conditions. Under screenhouse conditions the genotypes AR-4 and AR-7 exhibited tolerant reaction, while 15 genotypes (AR-48, AR-5, AR-10, AR-14, AR-52, AR-30, AR-66,AR-16, AR-2, AR-13, AR-9, AR-17, AR-3, AR-11 and AR-8) were found to be susceptible and remaining seven genotypes (AR-25, AR-12, AR-15, AR-19, AR-18, BH-75 and Clipper) as highly susceptible.

Candidate gene analysis of quantitative disease resistance in wheat

Knowledge of the biological significance underlying quantitative trait loci (QTLs) for disease resistance is generally limited. In recent years, advances in plant-microbe interactions and genome mapping have lead to an increased understanding of the genes involved in plant defense and quantitative disease resistance. Here, we report on the application of the candidate-gene approach to the mapping of QTLs for disease resistance in a population of wheat recombinant inbreds. Over 50 loci, representing several classes of defense response (DR) genes, were placed on an existing linkage map and the genome was surveyed for QTLs associated with resistance to several diseases including tan spot, leaf rust, Karnal bunt, and stem rust. Analysis revealed QTLs with large effects in regions of putative resistance (R) genes, as previously reported. Several candidate genes, including oxalate oxidase, peroxidase, superoxide dismutase, chitinase and thaumatin, mapped within previously identified resistance QTLs and explained a greater amount of the phenotypic variation. A cluster of closely linked DR genes on the long arm of chromosome 7B, which included genes for catalase, chitinase, thaumatins and an ion channel regulator, had major effects for resistance to leaf rust of adult plants under conditions of natural infestation. The results of this study indicate that many minor resistance QTLs may be from the action of DR genes, and that the candidate-gene approach can be an efficient method of QTL identification.

Effect of vaccination with a recombinant Bm86 antigen preparation on natural infestations of Boophilus microplus in grazing dairy and beef pure and cross-bred cattle in Brazil

Current methods for the control of the cattle tick Boophilus microplus infestations are not effective and the parasite remains a serious problem for the cattle industry in tropical and sub-tropical areas. Recent advances have introduced the possibility for the immunological control of the parasite through the use of recombinant vaccines. Recently, it was shown that the recombinant vaccine Gavac™ (Heber Biotec S.A.) is able to control B. microplus populations in artificially infected grazing dairy cattle in Cuba. To assay the effect of the vaccine on a different B. microplus strain and under different ecological conditions, we conducted a trial in Brazil on grazing dairy and beef pure and cross-bred cattle under natural infestation conditions. A farm in the northeast of the state of Sao Paulo was selected and two groups of animals per breed were included in the experiment and were maintained grazing on separate but similar pastures. For each breed, one group was vaccinated with the vaccine Gavac and the second group was not vaccinated and was employed as a control. In vaccinated cattle, during 36 weeks of experiment, the average infestation rate was maintained below 78 ticks per animal while average infestation peaks (mean ± S.E.) of 144 ± 44 ticks per animal (for dairy cross-bred cattle) and 195 ± 42 ticks per animal (for beef cross-bred cattle) were recorded in the control groups. Tick infestation rates showed statistical significant differences ( p=0.04) between both experimental groups throughout the experiment. These results clearly showed, as in the Cuban study, that the vaccine controlled tick numbers in successive generations in the field.

Effects of Decis®and Systoate® on Megalurothrips sjostedti (Trybom) in Cowpea

The effects of Decis® (deltamethrin) and Systoate® (dimethoate) were assessed by the number of Megalurothrips sjostedti (Trybom) (Thysanoptera, Thripidae) recorded per cowpea plant at 40, 50, 60 and 70 days after planting (DAP), evaluated by the drop-cloth sampling method. The trial was carried out under natural infestation conditions during the 1985 early cropping season (from April to July). Systoate® was better than Decis® in controlling the flower thrips and its effect was significantly different from the unsprayed control (P = 0.01). The Decis® effect on the flower thrips was also effective in comparison with the control (P = 0.01). The effectiveness of two insecticides used in relation to pest management and the adequacy of the drop-cloth sampling method in relation to other methods are discussed.

Studies on the mechanism of resistance in sorghum accessions to the grain midge, Contarinia sorghicola Coquillet

Sixty-four sorghum accessions were screened for resistance to the grain midge, Contarinia sorghicola Coq., under natural infestation conditions at Tamil Nadu Agricultural University, Coimbatore during 1982. Absolute resistance was not found in any of them. Low grain damage was recorded in PVK 50 (6.61%), PVK 55 (11.58%), AF 28 (17.03%) and DJ 6514 (17.05%) while the susceptible accession, SPV 234 recorded high grain damage (92.72%) in the screening trial. The ovipositional preference studies at insectary revealed significantly reduced grain damage in the resistant accessions. Among them, the lowest damage (8.33%) was recorded in PVK 50 followed by AF 28, DJ 6514 and PVK 55 while the susceptible accession, SPV 234 recorded 21.57% grain damage. In the choice experiments, SPV 234 had 11.22-35.36% more infestation than in the resistant accessions evaluated along with it. Studies on development and biology of the midge revealed that antibiosis is one of the mechanisms of resistance in the lines, PVK 50, DJ 6514, AF 28 and PVK 55.

Artificial Versus Natural Infestation for Evaluation of Kernel and Stalk Damage in Maize by the European Corn Borer1

Abstract Artificial was compared to natural infestation for evaluating kernel damage and stalk tunneling on maize (Zea mays, L.) caused by European corn borer (Ostrinia nubilalis, Hiibner) larvae. Under conditions of natural infestation, differential damage levels among families were more closely correlated with silk date than oviposition frequencies on the host plants. The association between damage and silk date was reduced following artificial infestation. Artificial infestation, which permits infestation at a common stage of maturity (full silk), a uniform and greater insect population on host plants, and a uniform opportunity for larval development on the ear from time of infestation to time of evaluation, is preferred for differentiating maize genotypes for borer resistance.

Insect resistance in sorghum plants

Sorghum (Sorghum bicolor) is an important food crop, especially of the semi-arid areas of Asia and Africa. One of the major constraints affecting the popularity of high-yielding hybrids and varieties is their extremely high susceptibility to insect pests. Painter (1951) has reviewed in detail the work done on resistance in sorghum to the chinch bug (Blissus leucopterus), corn leaf aphid (Aphis maidis) and sorghum midge (Contarinia sorghicola). In the last decade and a half considerable work has been done on resistance to the shootfly (Atherigona seccata), stem borer (Chilo partellus) and sorghum midge (Contarinia sorghicola). By conducting multilo-cation screenings of world germplasm under natural and artificial infestation conditions, not only have promising sources of resistance been identified but a number of them have been utilised in crossing programmes to develop high-yielding resistant cultivars. Some sources with multiple resistance have also been identified. The mechanisms of resistance in sorghum to the major pests have been investigated. It is seen that the mechanisms are generally complex and the observed field resistance is the total reaction of several components. There are several leads available which justify further intensive work, especially on identifying naturally occurring plant products associated with antibiosis mechanism

Heritability Estimates for Resistances in Sweet Potato to Soil Insects1

Abstract Twenty-two sweet potato (Ipomoea batatas (L.) Lam.) breeding lines and 19 open-pollinated offspring from each were used to estimate the heritabilities of 7 measures of soil insect injury. Four measures of injury by the wireworm, Diabrotica spp., and Systena spp. (WDS) complex and h2 (± SE) were: percentage of roots injured, 0.45 ± 0.12; holes per root, 0.32 ± 0.09; severity index, 0.37 ±0.11; and damage score, 0.39 ± 0.17. Two measures of injury by the sweetpotato flea beetle, Chaetocnema confinis Crotch, and h2 were: percentage of roots injured, 0.40 ± 0.07, and tunnels per root, 0.25 ± 0.08. The h2 of percentage of roots injured by all insects was 0.51 ± 0.12. The percentage measures were more easily obtained and were as effective as the other measures under the conditions of natural infestation that occurred in this test. Further advances in selection for high levels of resistance to soil insects are possible within the breeding materials tested.

RE-EVALUATION OF TCA FOR GREEN FOXTAIL CONTROL IN RELATION TO SEED YIELD AND OIL QUALITY OF FLAX

The role of TCA (trichloroacetic acid) in green foxtail (Setaria viridis (L.) Beauv.) control and its effect on seed yield and oil composition of flax (Linum usitatissimum L.) cvs. Noralta, Raja, and Redwood were re-assessed under various weedy conditions from 1972 to 1975. When a mixture of weeds was present, TCA at 2.24 kg/ha controlled green foxtail but failed to control lamb’s-quarters (Chenopodium album L.) and redroot pigweed (Amaranthus retroflexus L.). However, TCA mixtures which included one or two broad-leaf herbicides gave good control of all weeds and a 40% increase in yield over the untreated check. Under natural infestation conditions where green foxtail was light but the predominant weed, TCA at 2.24–4.48 kg/ha gave partial weed control but did not increase the seed yield of flax. Cultivation consistently produced the highest yields and TCA alone generally produced the lowest yields. While the lower yields experienced with TCA were partially offset by cultivating before or after the application of TCA, in no cases were the yields from these combined treatments ever equal to that of cultivation alone. Plants treated with 3.36 and 4.48 kg/ha of TCA in the greenhouse showed leaf injury and a significant reduction in top weight. It is suggested that this type of injury is one of the factors contributing to the failure of TCA to significantly increase the yield of flax following suppression of green foxtail growth. TCA tended to increase protein content but has little effect on oil content, fatty acid composition, or iodine number.

Methods of Artificially Infesting Corn with the Corn Earworm and Factors Influencing Resistance1

Corn cars artificially infested with laboratory-reared 1-day-old corn earworm larvae, Heliothis zea (Boddie), produced nearly 100% infestation, and damage ratings were higher than under conditions of natural infestation. Although the application of 6 and 10 larvae per ear decreased the percentage of uninfested ears and increased damage grades, 3 larvae per ear appear adequate for differentiating resistance. Application of eggs and second and third-instar larvae did not appear adequate for artificial infestation of ears. Clipping the husk and silk of susceptible strains generally did not increase damage grades, indicating that larvae did not prefer the silks in these strains and moved rapidly to the grain. Under some conditions resistance of TIIS×T15S is probably due to silk balling in the silk channel. The very long husk of P.I. 186225 is the main reason for its high resistance under natural conditions. The resistance of P.I. 217413 (Zapalote Chico) is due to a tight, tough husk, and perhaps some form of silk resistance; a resistance factor in the grain was also indicated.