Sources Of Host Plant Resistance Research Articles

Monitoring of the diamondback moth (<i>Plutella xylostella</i> L.) on the <i>Brassica oleracea</i> L. collection in the vicinity of St. Petersburg

Background. Diamondback moth (Plutella xylostella L.) has now acquired the status of the most dangerous pest of plants from the Brassicaceae family in the world, including Russia. In order to identify genotypes resistant to the pest, cabbage accessions from the VIR global collection were assessed in the field according to plant infestation and damage by diamondback moth in the vicinity of St. Petersburg.Materials and methods. The infestation of plants by diamondback moth was assessed on model accessions when examining all plants in the plot by (1) the number of larvae and pupae, and (2) leaf damage, assessed using a standard scoring scale. The number of adults was monitored using sticky Delta traps of two designs: (1) cardboard traps equipped with commercial dispensers with synthetic sex pheromone, and (2) plastic LED traps designed at VIZR.Results. The results of field surveys attested to very high variation among cabbage accessions in the rates of infestation and damage to plants caused by diamondback moth. The data on the abundance of adults caught by pheromone traps correlated well with the estimates of larval and pupal density of the pest on plants. The capture rate of diamondback moths with LED traps varied greatly during the season. In June/early July, i.e., during the period of the so called “white” nights, LED traps caught diamondback moths much worse than pheromone ones, but during the second half of July and August their capture rate significantly exceeded that of pheromone traps. As a result, the relationship between numbers of diamondback moth larvae on plants and adults in LED traps turned out to be negative. Conclusion. The resulting materials indicate obvious prospects of studying the VIR global collection in order to identify sources of host plant resistance to diamondback moth. The abundance of this pest in northern regions of its spreading is recommended to be controlled with pheromone traps.

Solid Stems and Beyond: Challenges and Future Directions of Resistance to Wheat Stem Sawfly (Hymenoptera: Cephidae)

AbstractIn North America, the wheat stem sawfly (Cephus cinctus Norton) (WSS) is considered a key pest of wheat, causing significant damage to spring and winter wheat. Yield losses from WSS can be due to stem boring and/or stem cutting which causes plants to lodge. Cultural, chemical, and biological management strategies have not effectively controlled WSS. When available, host plant resistance (HPR) is an effective and environmentally friendly approach for pest management and is particularly important for controlling WSS. Solid stem genotypes are often considered the main WSS management option for producers. This review will highlight recent progress and future challenges for understanding HPR to WSS. In bread and durum wheat, we will highlight (1) the solid stem trait, (2) interactive effects of solid stem and biological control, (3) nonpreference trait, (4) novel sources of HPR, and (5) the molecular aspects of wheat–wheat stem sawfly interactions. We also highlight the available information on HPR in non-wheat crops. Future research should characterize other resistance traits that can complement solid stem genotypes to create more robust and predictable genotypes.

Evaluation of antixenotic and allelochemical traits of ber (Ziziphus mauritiana Lamk.) fruits as source of host plant resistance against fruit fly (Carpomyia vesuviana Costa) (Diptera: Tephritidae) in semi-arid region of India

The ber fruit fly, Carpomyia vesuviana Costa (Tephritidae: Diptera) is a well established notorious monophagous major pest of ber (Ziziphus mauritiana Lamk) in all the ber growing arid and semi-arid regions and may cause severe yield losses. Plants frequently display genetic variation within and between populations for traits that influence the host selection preference of herbivores. While analyzing the effect of various physico-chemical traits on the C. vesuviana infestation, significant differences in fruit fly incidence were observed in 39 cultivars of ber under field conditions. Fruit fly infestation had significant positive correlation with fruit size, average fruit weight, pulp: stone ratio, total soluble solids and Vitamin C, and was negatively correlated with fruit firmness, acidity content and total phenolic content. The maximum variation in fruit infestation was explained by total phenols (57.0%) followed by pulp: stone ratio (10.4%) and TSS (2.3%). Based on the varimax rotation method with Kaiser Normalization, two principal components (PCs) were extracted explaining the cumulative variance of 54.00% in fruit fly infestation. PC1 explained 29.91% while PC2 explained 24.09% of the total variance. Thus, the importance of genetic diversity is apparent and can be utilized to enhance the income of the farmers by reducing cost of cultivation by developing resistant cultivars which are major component of integrated pest management strategies.

Characterisation and mapping of adult plant stripe rust resistance in wheat accession Aus27284

A new adult plant stripe rust resistance gene, Yr80, was identified in a common wheat landrace Aus27284. Linked markers were developed and validated for their utility in marker-assisted selection. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is among the most important constraints to global wheat production. The identification and characterisation of new sources of host plant resistance enrich the gene pool and underpin deployment of resistance gene pyramids in new cultivars. Aus27284 exhibited resistance at the adult plant stage against predominant Pst pathotypes and was crossed with a susceptible genotype Avocet S. A recombinant inbred line (RIL) population comprising 121 lines was developed and tested in the field at three locations in 2016 and two in 2017 crop seasons. Monogenic segregation for adult plant stripe rust response was observed among the Aus27284/Avocet S RIL population and the underlying locus was temporarily designated YrAW11. Bulked-segregant analysis using the Infinium iSelect 90K SNP wheat array placed YrAW11 in chromosome 3B. Kompetitive allele specific PCR (KASP) primers were designed for the linked SNPs and YrAW11 was flanked by KASP_65624 and KASP_53292 (3cM) proximally and KASP_53113 (4.9cM) distally. A partial linkage map of the genomic region carrying YrAW11 comprised nine KASP and two SSR markers. The physical position of KASP markers in the pseudomolecule of chromosome 3B placed YrAW11 in the long arm and the location of markers gwm108 and gwm376 in the deletion bin 3BL2-0.22 supported this conclusion. As no other stripe rust resistance locus has been reported in chromosome 3BL, YrAW11 was formally designated Yr80. Marker KASP_ 53113 was polymorphic among 94% of 81 Australianwheat cultivars used for validation.

Sources of plant resistance to thrips: a potential core component in cotton IPM

AbstractThrips (Thysanoptera: Thripidae) are important pests of seedling cotton and their damage can delay crop maturity and/or reduce yield. Plant resistance to thrips in cotton would reduce the need to treat crops with insecticides for their control. This would support integrated pest management strategies by reducing the risk of selecting insecticide resistance in concomitant pests and of disrupting the natural enemy complex. Traits that reduce thrips abundance in cotton are poorly understood, but dense leaf hairs and high gossypol content are implicated to negatively affect thrips. Furthermore, some diploid cottons are resistant against thrips. Thrips abundance and damage on a range of Gossypium (Malvaceae) genotypes including hairy leaf, smooth leaf (glabrous), okra leaf shape, and diploid species was evaluated over four crop seasons. Thrips were significantly less abundant and they caused less damage on diploid cotton genotypes from Gossypium arboreum L. (BM13H, Roseum A256), Gossypium thurberi Tod. (GOS5310), and Gossypium trilobum (Moc. & Sess. ex DC.) Skov. (GOS5332) than on the standard commercial Gossypium hirsutum L. cv. Sicot 71. There was no significant relationship between thrips abundance or damage and leaf hairiness, leaf hardness, or leaf shape, though conclusions about the value of these traits must be made cautiously as they are confounded across genotypes that differ in a range of traits simultaneously. The diploid cottons had a lower damage per thrips ratio, indicating that they are more tolerant of thrips feeding damage and thus could be valuable sources of host plant resistance to thrips.

The Discovery of Resistant Sources of Spring Barley, Hordeum vulgare ssp. spontaneum, and Unique Greenbug Biotypes.

The genetic sources for host-plant resistance to the greenbug (Schizaphis graminum Rondani) in barley (Hordeum vulgare ssp. spontaneum) are limited in that only two single dominant genes Rsg1 and Rsg2 are available for the complex of greenbug biotypes. We evaluated four new barley lines from the Wild Barley Diversity Collection (WBDC) that previously showed potential for greenbug resistance. Three of those entries, WBDC 53, WBDC 117, WBDC 336, exhibited very dominant sources of resistance to older known biotypes B, C, E, F, H, I, and TX1, which also add to the host-plant differentials used to separate these greenbug biotypes. We also re-evaluated the earlier known set of greenbug biotypes that have been in culture for several years against the known host-plant differentials, and included seven newer greenbug isolates collected from Wyoming to the full complement of small grain differentials. This resulted in the discovery of five new greenbug biotypes, WY10 MC, WY81, WY10 B, WY12 MC, and WY86. Wyoming isolates WY4 A and WY4 B were identical in their phenotypic profile, and should be combined as a single unique greenbug biotype. These barley trials resulted in finding new sources of host-plant resistance, although more research needs to be conducted on what type of resistance was found, and how it can be used. We also document that the Wheatland, Wyoming area serves as a very conducive environment for the development of new greenbug biotypes.

Identification of Novel Sources of Host Plant Resistance to Known Soybean Aphid Biotypes

While soybean cultivars with resistance to the soybean aphid (Aphis glycines Matsumura) have been commercially released, the presence of virulent biotypes capable of overcoming plant resistance threatens the durability of host plant resistance as a stable management tactic. Novel sources of host plant resistance are needed to combat rapid biotype evolution. In this study, we screened 1,061 soybean plant introductions (PIs) for resistance to three known biotypes of A. glycines. Based on a series of growth chamber and field screenings, we identified 11 PIs that showed resistance to biotype 1 of A. glycines. Among these 11 PIs, 7 PIs were resistant to biotype 2 and 5 PIs were resistant to biotype 3. Further, two PIs (PI 606390A from Vietnam and PI 340034 from South Korea) showed resistance to all three biotypes of A. glycines. We also identified 11 PIs that were potentially tolerant to A. glycines, illustrated by no adverse impact on plant quality because of A. glycines infestation. As resistant PIs identified in this study belong to maturity group II-IV, they can be readily crossed to early maturing cultivars adapted to north-central states of the United States, where A. glycines is a major pest. The genetic characterization of resistance in these PIs and incorporation of novel resistant genes into elite soybean cultivars will broaden the defense against multiple biotypes of A. glycines.

Screening Exotic Sorghum Germplasm, Hybrids, and Elite Lines for Resistance to a New Virulent Pathotype (P6) of Peronosclerospora sorghi Causing Downy Mildew

A recent outbreak of sorghum downy mildew (SDM) in Texas has led to the discovery of both metalaxyl fungicide resistance and a new pathotype, P6, in the causal organism Peronosclerospora sorghi. New and alternate sources of host plant resistance are needed for successful management of SDM. To identify sources of resistance, a total of 333 (242 minicore lines representing diverse germplasm from India, 67 commercial hybrids from Kansas, and 24 elite breeding lines from Texas) were inoculated in the greenhouse. Using an established sandwich inoculation technique, artificial inoculation of test lines with P. sorghi conidia, resulting in < 10% infection, were scored as “resistant.” Fifty-two minicore and 20 accessions from Kansas exhibited ≤ 10% infection and were selected as resistant. Out of 52 resistant minicore accessions, 28 were photoinsensitive. Eleven of 20 commercial hybrids from Kansas showed zero percent infection. Thirteen of 24 elite breeding lines from Texas were also resistant. In this study, resistance sources for the new P6 SDM pathotype were identified. The diversity among these materials is expected to provide different single-gene sources as well as quantitative sources of SDM resistance for use in breeding programs. Accepted for publication 13 January 2011. Published 23 March 2011.

Field Assessment of Anthracnose Disease Response for the Sorghum Germplasm Collection from the Mopti Region

Problem statement: Sorghum anthracnose (Colletotrichum sublineolum) is a highly variable pathogen and new sources of host plant resistance are required for the development of resistant sorghum varieties. Germplasm collections are an important source of host plant resistance and screening germplasm will be essential to identify new sources of resistance. Approach: The sorghum (Sorghum bicolor) collection from the Mopti region of Mali was inoculated with Colletotrichum sublineolum and evaluated for foliar anthracnose disease response in Isabela, Puerto Rico during the 2004 and 2005 growing seasons using a partially balanced lattice design with three replications. Results: A resistant response was observed for 45 of the 97 accessions in the collection and mean infection severity for the 52 susceptible accessions was 27.6%. An association was observed between resistance and the administrative district where the germplasm was collected. More than 50% of the accessions from the Bandiagara and Bankass districts showed a resistant response. The lowest frequency of resistant germplasm was observed for the Mopti district with 25% of the accessions showing a resistant response. The susceptible accessions from the Mopti district, however, showed the lowest mean infection severity. Approximately 44% of the accessions from the Douentza district showed a resistant response with the susceptible accessions showing the highest mean infection severity. These results suggest an association between annual rainfall and anthracnose resistance, with sorghum accessions from drier regions showing greater susceptibility. Anthracnose resistance also showed an association with sorghum race classification and race guinea accessions were more frequently resistant as compared to accessions classified as race durra or durra-bicolor. Conclusion: The results indicated that anthracnose resistant sorghum germplasm is frequent in the Mopti region of Mali and that ecogeographic origin and sorghum characterization information can be used to aid in germplasm selection or germplasm acquisition to identify anthracnose resistant sources.

METHODS USED IN FRANCE TO STUDY STRAWBERRY POWDERY MILDEW IN LABORATORY CONTROLLED CONDITIONS

Strawberry powdery mildew caused by the ascomycota fungus Podosphaera aphanis (Wallr.) U. Braun and S. Takamatsu, reduces fruit production and quality. It is becoming the most damaging disease of strawberry in the soil-less systems which have been recently widely adopted in France. A collaborative project has been developed by several French technical and research institutes to address this issue, with four main objectives: better understanding of the epidemiology of the disease, evaluating the susceptibility of populations of the pathogen towards fungicides, identifying sources of host plant resistance, and assessing new methods for disease management. Several methods were developed and applied to study this obligate parasite. Monoclonal strains were obtained from deposition of single conidia on detached strawberry leaves. Strains were maintained for several months without manipulation on vitro plants at low temperature (7-9°C). One- to two-week-old leaves from one- to two-month-old strawberry plants provided the most susceptible material, and were used to produce inoculum. Methods have been developed to measure epidemiological characteristics of the asexual cycle of P. aphanis: conidia germination, infection efficiency, mycelium growth, latent period, and sporulating intensity. Leaf disks and seedlings from strawberry cultivars with varying levels of partial resistance were assessed according to these epidemiological characteristics. Resistance of the fungus towards fungicides was evaluated by inoculating leaf disks previously sprayed with several concentrations of fungicides. Inhibiting concentrations were calculated relative to untreated controls and suggested a range of resistance levels to fungicides.

Toward Breeding for Resistance to Fusarium Tuber Rot in Caladium: Inoculation Technique and Sources of Resistance

Fusarium tuber rot, incited by Fusarium solani, is the major cause of losses of tuber quality and quantity in caladium (Caladium ×hortulanum) during storage and production. To develop a reliable inoculation method for evaluating cultivar susceptibility to Fusarium tuber rot and identifying sources of resistance, the effect of temperature on the mycelial growth of F. solani in vitro and on tuber rot in vivo was examined. The optimal temperature was then used to study the aggressiveness of F. solani isolates. The effect of temperature (13, 18, 23, 28, and 33 °C) on radial mycelial growth of nine F. solani isolates in vitro was determined, and all responded similarly to temperature variables, with optimal growth predicted to be at 30.5 °C. The relationship of these temperatures to disease development was then determined for the most aggressive F. solani isolate 05-20 and it was found that disease development in inoculated tubers was greatest at low temperatures (13 and 18 °C). Cold damage to tubers was observed at 13 °C; therefore, 18 °C was chosen for all future disease screening. The aggressiveness of nine isolates was tested on two caladium cultivars. Significant differences among isolates were observed for the diameter of rotted tissue in both cultivars, indicating that choice of isolate was important for screening. Isolates 05-20 and 05-257 were highly aggressive on both cultivars. Tubers of 17 commercial caladium cultivars were inoculated with three isolates (04-03, 05-20, and 05-527) and incubated at 18 °C. The interaction between isolates and cultivars was highly significant (P < 0.0001), indicating that cultivars were not equally susceptible to different pathogenic isolates of F. solani. Lesion diameters differed significantly (P < 0.0001) among cultivars/isolates and ranged from 9.5 mm (‘Rosebud’ and ‘White Christmas’ for isolate 04-03) to 23.9 mm (‘Carolyn Whorton’ for isolate 05-20). The cultivars were ranked for susceptibility to tuber rot within each isolate and the normalized total rank for the three isolates was used to place cultivars into four categories: resistant (‘Candidum’, ‘Rosebud’, ‘White Christmas’, ‘Florida Sweetheart’, and ‘Aaron’), moderately resistant (‘White Wing’ and ‘Red Flash’), susceptible (‘Candidum Jr.’, ‘White Queen’, ‘Red Frill’, ‘Florida Cardinal’, ‘Miss Muffet’, and ‘Postman Joyner’), and highly susceptible (‘Fannie Munson’, ‘Gingerland’, ‘Frieda Hemple’, and ‘Carolyn Whorton’). The availability of these sources of host plant resistance, aggressive isolates, and resistance assessment techniques will facilitate the development of new Fusarium-resistant caladium cultivars.

Wild Allium spp. as Natural Hosts of Iris yellow spot virus.

The incidence of Iris yellow spot virus (IYSV) of genus Tospovirus, family Bunyaviridae in a commercial onion crop was first confirmed in Washington state during 2003 (1). First found in Adams County, IYSV has rapidly spread to all onion-producing counties in the state, affecting seed and bulb crops. The USDA-ARS Western Regional Plant Introduction Station (WRPIS) collects, maintains, and distributes various Allium (garlic and onion) accessions. As part of the regeneration process, accessions are grown under field conditions at the WRPIS farms in two locations: Pullman and Central Ferry, WA. Symptoms indicative of viral infection, now known to be caused by IYSV, first appeared in field-grown accessions in 1999. In June 2005, leaf and scape tissues were collected from WRPIS accessions of wild onions (Allium pskemense, A. vavilovii, and A. altaicum) in Central Ferry that had symptoms indicative of IYSV infection (2). IYSV infection was confirmed using enzyme-linked immunosorbent assay with a commercially available kit (Agdia Inc., Elkhart, IN). Virus infection was further verified using reverse transcription-polymerase chain reaction (RT-PCR) with primers derived from the small (S) RNA of IYSV. The primers flanked the IYSV N gene (5'-TAA AAC AAA CAT TCA AAC AA-3' and 5'-CTC TTA AAC ACA TTT AAC AAG CAC-3'). RT-PCR gave a PCR product of expected size (≈1.2 kb). The DNA amplicon was cloned and sequenced. Nucleotide sequence comparisons with known IYSV N gene sequences showed 95 to 98% sequence identity. The prevalence of the vector, onion thrips (Thrips tabaci), combined with the widespread incidence of IYSV in seed and bulb production areas of the state may have resulted in natural infection of wild relatives of cultivated onion. The potential role of wild Allium spp. in IYSV epidemiology remains to be determined. Information on the extent of IYSV infection of onion germplasm would be useful in identifying potential sources of host plant resistance to IYSV.

Expression of a Triticum turgidum var. dicoccoides source of Fusarium head blight resistance transferred to synthetic hexaploid wheat

AbstractYield and quality reductions caused by Fusarium head blight (FHB) have spurred spring wheat (Triticum aestivum L.) breeders to identify and develop new sources of host plant resistance. Four wheat synthetic hexaploids (×Aegilotriticum sp.) were developed, each having a quantitative trait locus (QTL), Qfhs.ndsu‐3AS, providing FHB resistance from Triticum turgidum L. var. dicoccoides chromosome 3A. Synthetics were produced by hybridizing a ‘Langdon’‐T. dicoccoides‐ recombinant chromosome 3A substitution line (2n = 4x = 28, AABB with two accessions of T. tauschii (2n= 2x = 14, DD). Synthetics were inoculated and evaluated for FHB resistance in two separate greenhouse seasons. One synthetic, 01NDSWG‐5, exhibited FHB severity ratings of 36% and 32% in the separate seasons, compared with ratings of 9% and 30% for ‘Alsen’, a FHB‐resistant spring cultivar, and ratings of 70% and 96% for ‘McNeal’, a susceptible spring cultivar, respectively. Synthetic × Alsen backcross‐derived lines were produced to initiate combining different sources of FHB resistance.

New Source of Southern Chinch Bug (Hemiptera: Lygaeidae) Resistance in a Diploid Selection of St. Augustinegrass

Over 400,000 ha of St. Augustinegrass, Stenotaphrum secundatum (Walt.) Kuntze, are managed as a turfgrass in the southern United States, and the southern chinch bug, Blissus insularis Barber, is its most important insect pest. New sources of host plant resistance to southern chinch bugs became necessary due to the development of virulent populations of chinch bugs which were able to feed on the only acceptable resistant cultivar, Floratam. Initial testing evaluated 14 lines for chinch bug resistance using insects collected from five locations from Palm Beach Co., FL. Host plant resistance was determined by mortality of adult chinch bugs held on a turfgrass for 14 d. A second study was conducted with five lines from the first test with southern chinch bugs collected from nine locations throughout Florida. Tests showed a high level of southern chinch bug resistance in NUF 76, NUF 216 and FX-10. Leaf blades of NUF-76 are significantly shorter and narrower than other tested St. Augustinegrass lines when evaluated 2 wks after mowing. NUF-76 is unique because for the first time, resistance to the southern chinch bug has been identified within a diploid line of St. Augustinegrass. Prior to this, southern chinch bug resistance was only associated with polyploid lines which generally have large leaves and reduced or no seed set due to sterility problems. This discovery will allow chinch bug resistance to be more easily bred into other St. Augustinegrass lines.

Effectiveness of Natural and Engineered Host Plant Resistance in Potato to the Colorado Potato Beetle

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is the most serious insect pest of potatoes throughout the eastern and north central United States. Host plant resistance to the Colorado potato beetle has been identified in wild Solanum species and Bt-transgenic potato lines. Detached-leaf bioassays (72 h) were conducted on insecticide-resistant, first instar Colorado potato beetles to study the effectiveness of individual and combined host plant resistance traits in potato. Potato lines tested include non-transgenic cultivars (`Russet Burbank', `Lemhi Russet', and `Spunta'), a line with glandular trichomes (NYL235-4), a line with high foliar leptines (USDA8380-1), and transgenic lines expressing either codon-modified Bt-cry3A or Bt-cry5 (Bt-cry1Ia1). Bt-cry3A transgenic lines, foliar leptine line, and foliar leptine lines with Bt-cry5 had reduced feeding compared to non-transgenic cultivars. Glandular trichome lines and glandular trichome lines with Bt-cry5 did not reduce feeding in this no-choice feeding study. Some Bt-cry5 transgenic lines, using either the constitutive promoters CaMV35s or (ocs)3mas (Gelvin super promoter), were moderately effective in reducing larval feeding. Feeding on Bt-cry5 transgenic lines with the tuber-specific patatin promoter was not significantly different than or greater than feeding on the susceptible cultivars. Mortality of first instars was highest when fed on the Bt-cry3A lines (68% to 70%) and intermediate (38%) on the Bt-cry5 `Spunta' line SPG3 where the gus reporter gene was not included in the gene construct. Host plant resistance from foliar leptines is a candidate mechanism to pyramid with either Bt-cry3A or Bt-cry5 expression in potato foliage against Colorado potato beetle. Without multiple sources of host plant resistance, long-term sustainability is questionable for a highly adaptable insect like the Colorado potato beetle.

Effects of planting pattern of collards on resistance to whiteflies (Homoptera: Aleyrodidae) and on parasitoid abundance.

Fourteen collard entries, Brassica oleraceae L., Acephala group, were evaluated for resistance to natural populations of Bemisia argentifolii Bellows & Perring in replicated field plots in Charleston, SC. Glossy-leaf phenotypes ('SC Glaze', 'SC Landrace,' 'Green Glaze') were the most resistant collard entries and had fewer whiteflies than the nonglossy, open-pollinated cultivars. Also, two F1 hybrid cultivars with normal, nonglossy leaves ('Blue Max' and 'Top Bunch') were resistant. In laboratory experiments, there were no differences in the intrinsic rate of growth (rs) of B. argentifolii populations on either glossy or nonglossy collard phenotypes. Over a 2-yr period, there were no differences in the abundance of whiteflies on the glossy phenotype of Green Glaze when it was planted in solid 20-plant plots or when it was alternated (every other plant) with the nonglossy phenotype of Green Glaze. In a similarly designed experiment, there was no difference in the resistance of Blue Max in either solid or mixed planting scheme compared with the susceptible 'Morris Heading'. Higher numbers of whiteflies and parasitoids (primarily Eretmocerus spp.) were collected on yellow sticky cards in the solid plantings of the nonglossy phenotype of Green Glaze than were collected in the solid plantings of the glossy Green Glaze phenotype. Counts on sticky cards in the mixed plots were intermediate. These data show that planting pattern of collard entries is relatively unimportant in the deployment of these sources of host plant resistance. The data also suggest that nonpreference is the primary mode of resistance to whiteflies for certain collard entries.

The Distribution of Fusarium Wilt of Blackeyed Cowpeas within California Caused by Fusarium oxysporum f. sp. tracheiphilum Race 4.

Fusarium wilt of blackeyed cowpeas has been known in California since the 1930s, and breeding for resistance to this disease pathogen has been a continuous effort. During the 1960s and 1970s, California Blackeye 5 (CB 5) cowpea (Vigna unguiculata L. Walp.), a widely grown cultivar of the time, became increasingly diseased by Fusarium oxysporum f. sp. tracheiphilum (Fot) Race 3 (2) throughout the growing regions of California. University of California cultivars CB 46 and CB 88 (1) were developed for resistance to Fot Races 1, 2, and 3. CB 46 is currently the principal blackeye cultivar grown on the majority of the acreage in the San Joaquin Valley. In 1989, a new race we designate "Fot Race 4" was isolated from wilted plants at a single field site in Stanislaus County. In years prior to identification, Fot Race 4 had caused severe wilt of CB 46 and CB 88 in this field. Even though the new Fot Race 4 remained confined to a small area for a number of years, sources of host plant resistance to Fot Race 4 were identified, hybridized, and screened, resulting in new progeny with desirable commercial agronomic characteristics. As observed in Stanislaus County, F. oxysporum f. sp. tracheiphilum Race 4 has the potential to cause serious crop damage, depending on virulence and soil inoculum levels, which may vary from year to year. In 1997 and 1998, an entirely different area in the southern San Joaquin Valley, about 140 miles from the original site in Stanislaus County, was found to have plants infected with Fot Race 4. Diseased plants were collected from patches in three separate CB 46 or CB 88 field sites in Tulare County. About 30 cultures were isolated from the diseased plants, which showed stunting, yellowing, and vascular discoloration. In greenhouse fusarium dip tests CB 46, CB 88, CB 5, and several Fot Race 4 resistant breeding lines were inoculated with all the collected isolates and evaluated. CB 46, CB 88, and CB 5 proved to be susceptible to these isolates, showing typical Fot Race 4 symptoms. The Fot Race 4 pathogen was then reisolated from greenhouse-grown, diseased stem tissue of CB 46, CB 88, and CB 5. These findings emphasize the importance of vigilance and necessity of continual disease surveys. They serve as an early alert for the University of California breeding program, and validate local cooperation with University of California Extension Farm Advisors. As a result of this effort new cultivar candidates with resistance to Fot Race 4 are in the final phases of multi-year commercial testing.

Evaluation of Peruvian Maize for Resistance to European Corn Borer (Lepidoptera: Pyralidae) Leaf Feeding and Ovipositional Preference

Partial control of the European corn borer, Ostrinia nubilalis (Hubner), in maize, Zea mays L., has been achieved through germplasm resistant to leaf feeding that was incorporated into breeding populations. Leaf-feeding resistance to European corn borer in maize had generally been attributed to presence of the chemical 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA). New sources of resistance, other than that mediated by DIMBOA, are desirable for developing new resistant cultivars. From 1990 to 1992, 1,601 accessions of Peruvian maize maintained in the U.S. National Plant Germplasm System were evaluated for leaf-feeding resistance to European corn borer. Eleven resistant accessions were identified, all of which originated from Peru's north coast. The 11 resistant accessions were then analyzed for 6-methoxybenzoxazolinone (MBOA), the degradation product of DIMBOA and an indicator of DIMBOA levels present in the plant. All 11 resistant accessions contained low MBOA concentrations, equivalent to that found in the susceptible inbred WF9, indicating that DIMBOA is not the basis of this resistance. The factor(s) underlying this source of host plant resistance remains to be determined. Six morphologically diverse Peruvian maize races were evaluated in the greenhouse for ovipositional nonpreference by the European corn borer. Some differences were noted, but morphological factors could not be confirmed as causing the differences.

Heritability of Resistance to Stored-Product Insects in Three Hybrid Populations of Rice

Three varieties of rice ( Oryza sativa L.) in the USDA World Collection were selected as potential sources of host plant resistance to the Angoumois grain moth, Sitotroga cerealella (Olivier) and crossed with the susceptible commercial cv. Vista. The resistant varieties were ‘Mutica’ (CI 12273), PI 160849, and CI 7097. The crosses were evaluated through three or five generations for resistance to the Angoumois grain moth and lesser grain borer, Rhyzopertha dominica (F.) in no-choice tests. In addition to these two species, rice weevil, Sitophilus oryzae (L.), red flour beetle, Tribolium castaneum (Herbst), and Cryptolestes spp. were included in free-choice tests. Two crosses were studied through the F3 generation. The cross with the most promising resistant parent (‘Mutica’) was studied through the F5 generation. In all cases, resistance was heritable (i.e., insect response in one generation tended to predict a similar response in the next generation). This tendency grew stronger as plants selected for resistance or susceptibility over several successive generations became more homozygous. Resistance to the Angoumois grain moth strongly predicted resistance to the lesser grain borer. The main criterion used to evaluate resistance was insect survival from larva to adult. This was related to the condition of the hulls, but the Angoumois grain moth generally required longer to develop on resistant lines than on susceptible ones.

Fifty-five years of research on the sorghum shootfly

The sorghum shootfly, Atherigona soccata Rondani, is the main species of Atherigona attacking cultivated and wild species of Sorghum in Africa and Asia. The past 55 years’ research on the pest has produced means for its management. The bionomics of A. soccata are well known and are being applied for its cultural control by adjusting planting dates. Natural enemies of the shootfly are few and their biology little known. The systemic insecticide carbofuran as a seed and seed furrow treatment provides effective fly control where the cost-benefit is favorable. Sources of host plant resistance to shootfly have been recognized for about 35 years. Incorporation of this resistance in high yielding varieties through plant breeding should be more rapid now that the mechanisms of resistance and their inheritance, through additive gene action, are better understood. These resistance factors include non-preference for oviposition, antibiosis and recovery through tillering.