Comparison of Crinipellis perniciosa isolates from Brazil by ERIC repetitive element sequence-based PCR genomic fingerprinting

Fifty isolates of Crinipellis perniciosa originating from Theobroma cacao , Heteropterys acutifolia and Solanum lycocarpum , from six states within Brazil, were characterized through ERIC-PCR, representing the first application of this method for molecular characterization within C. perniciosa . Phenetic analysis of banding patterns revealed a separation of isolates on the basis of host of origin, with T. cacao -derived isolates showing only a 0·2 similarity level to a cluster comprising the isolates from H. acutifolia and S. lycocarpum . Considerable intraspecific variability was observed within C. perniciosa isolates from T. cacao , with distinct groups observed correlating with geographical origin. Given that a number of isolates from T. cacao from the Amazon region grouped with isolates from Bahia state, this work discusses the possibility that current C. perniciosa populations pathogenic on T. cacao in Bahia originated from the Amazon region, rather than from alternative host plants.     

Genetic variation among asexual progeny of Phytophthora infestans detected with RAPD and AFLP markers

Genotypic variation among 32 single-zoospore isolates (SZI) of Phytophthora infestans , derived asexually from two hyphal-tip parental isolates (PI-105 and PI-1) of the US-8 genotype, was assessed with 80 random amplified polymorphic DNA (RAPD) primers and 18 amplified fragment length polymorphic DNA (AFLP) primer pairs. In previous investigations, the SZIs from parental isolate PI-105 showed high levels of virulence variability and were differentiated into 14 races, whereas the SZIs from PI-1 showed identical virulence to the parent. The purpose of this investigation was to determine if phenotypic variation observed among SZIs of P. infestans could be detected at the DNA level in these isolates. Polymorphism was detected with 51 RAPD primers and with all 18 AFLP primer pairs in PI-105 SZIs. In SZIs from PI-1, polymorphism was also detected with 25 RAPD primers and 17 AFLP primer pairs. Cluster analysis using the unweighted pair-group method with arithmetic averages (UPGMA) separated the SZIs from parent PI-105 into six virulence groups, 11 RAPD groups and three AFLP groups. Cluster analysis of PI-1 SZIs, which all belong to the same virulence group, differentiated them into four RAPD groups and six AFLP groups. No close correlation among RAPD, AFLP and virulence groups could be established within the two progenies of SZIs. Results of this study suggest that there is a considerable level of inherent genetic variability among SZIs derived asexually from the same parental isolate. The possible mechanisms and implications of this genetic variation are discussed.     

Predicting effective doses for the joint action of two fungicide applications

A function was derived to predict fungicide efficacy when more than one application of a single active ingredient is made to a crop, given parameters describing the dose–response curves of the component single-spray applications. In the function, a second application is considered to act on that proportion of the total pathogen population which was uncontrollable at the time of the first application (represented by the lower asymptote of the dose–response curve for the first treatment), plus any additional part of the population which survived the first application as a result of a finite dose being applied. Data to estimate the single-spray dose–response curve parameters and validate predictions of two-spray programme efficacy were obtained from separate subsets of treatments in four field experiments. A systemic fungicide spray was applied to wheat at a range of doses, at one or both of two times (t₁ and t₂), in all dose combinations. Observed values of the area under the disease progress curve (AUDPC) for septoria leaf blotch (Mycosphaerella graminicola) were used to construct response surfaces of dose at t₁ by dose at t₂ for each culm leaf layer. Parameters were estimated from single-spray and zero-dose treatment data only. The model predicted a high proportion (R² = 71–95%) of the variation in efficacy of the two-spray programmes. AUDPC isobols showed that the dose required at t₂ was inversely related to the dose at t₁, but the slope of the relationship varied with the relative timings of t₁ and t₂ in relation to culm leaf emergence. Isobols were curved, so the effective dose – the total dose required to achieve a given level of disease suppression – was lower when administered as two applications.     

Ovary colonization by Claviceps africana is related to ergot resistance in male-sterile sorghum lines

Ergot, caused by Claviceps africana , has emerged as a serious threat to sorghum hybrid seed production worldwide. In the absence of gene-for-gene-based qualitative resistance in commercial cultivars, varieties with high pollen production that can escape ergot infection are preferred. Recent demonstration of differences in ergot susceptibility among male-sterile lines has indicated the presence of partial resistance. Using chitin-specific fluorescin-isothiocyanate-conjugated wheat germ agglutin and callose-specific aniline blue, this study investigated the process of sorghum ovary colonization by C. africana . Conidia germinated within 24 h after inoculation (a.i.); the pathogen was established in the ovary by 79 h a.i., and at least half of the ovary was converted into sphacelial tissue by 120 h a.i. Changes in fungal cell wall chitin content and strategic callose deposition in the host tissue were associated with penetration and invasion of the ovary. The rate of ovary colonization differed in three male-sterile lines that also differed in ergot susceptibility. This work demonstrates a possible histological basis for partial resistance in male-sterile sorghum lines that could lay the foundation for variety improvement through further breeding and selection.     

Laboratory evaluation of the insect growth regulator lufenuron againstHelicoverpa armigera on cotton

An insect growth regulator (IGR), lufenuron (Match 5EC), was tested for its toxicity toHelicoverpa armigera on cotton. Potency of the IGR against the larval stage of the pest was demonstrated with respect to larval instars; the LC₉₀ values of 1st, 2nd, 3rd, 4th and 5th instar larvae were 5.63, 7.89, 8.03, 11.39 and 14.76 mg a.i.l ⁻¹, respectively. However, different larval instars did not differ significantly with respect to LC₅₀ and LC₁₀. IGR-treated larvae had swollen heads and were significantly smaller (1.5–2.3 mm) than the untreated control (2.9 mm). Larval weight was significantly reduced from 190 mg in the control to 50–70 mg in the lufenuron treatment. IGR treatment in the larval stage significantly affected both pupal length and pupal weight. Pupal duration of the test insect was significantly extended by IGR treatment. Pupal deformities, including an inability to shed the last larval skin and formation of larval-pupal intermediates, occurred following treatment. A significant reduction in adult emergence was recorded. In addition, abnormalities in the form of development of cavities in the forewings of adult were evident. A significant decline in fecundity was noted in the studies. http://www.phytoparasitica.org posting Feb. 3, 2003.     

Further suppression of<Emphasis Type="Italic">Botrytis cinerea</Emphasis> disease in cucumber seedlings by chitosan-copper complex as compared with chitosan alone

Chitosan-copper complex compared with chitosan alone enhanced suppression ofBotrytis cinerea rot development on four-true-leaf cucumber seedlings in controlled growth chambers. This paper constitutes the first report of such enhancement. The optimal concentrations for the most effective suppression ofBotrytis development were 0.2 gl ⁻¹ chitosan and 1.6 mmole copper. After 12 days’ incubation, marked and significantly better disease suppression was obtained with chitosan-copper complex (75% suppression) than with chitosan alone. The chitosan-copper complex could be a very promising decay control agent for use in both conventional and organic agriculture. http://www.phytoparasitica.org posting Dec. 18, 2002.     

Reduced paraquat translocation in paraquat resistant Arctotheca calendula (L.) Levyns is a consequence of the primary resistance mechanism, not the cause

Resistance to paraquat has been studied in detail in many weed species for more than a decade, with the precise mechanism of resistance still unclear. Several studies have indicated that reduced movement of the herbicide to the site of action in the chloroplast is at least partly responsible for endowing resistance. Although paraquat translocation studies have been performed in the past it has been rare for these studies to have been conducted on whole plants in the light, despite early observations which clearly showed that paraquat translocation is minimal unless treated plants are exposed to light. This study has addressed this issue in Arctotheca calendula by tracing the movement of ¹⁴C-paraquat in resistant and susceptible plants in both the dark and light. Differences in paraquat translocation between the resistant and susceptible biotypes of A. calendula were only observed when treated plants were exposed to light. It was observed that paraquat translocation was significantly reduced in the resistant compared to the susceptible biotype when plants were exposed to light but not in the dark. It is postulated that paraquat translocation is dependent on light mediated damage. As paraquat-induced damage is less severe in paraquat resistant plants, overall paraquat translocation is reduced in the resistant biotype.     

Immunocytochemical localization of the Bacillus sphaericus binary toxin components in Culex quinquefasciatus (Diptera: Culicidae) larvae midgut

This study describes the immunocytochemical localization of the Bacillus sphaericus 2362 binary toxin components, BinA and BinB, in Culex quinquefasciatus larvae that had been intoxicated with this entomopathogen. Ultrathin sections of C. quinquefasciatus larvae midgut embedded in the hydrophilic resin L.R. White were incubated with the antibodies anti-BinA or anti-BinB and then revealed with goat anti-rabbit IgG coupled to gold particles. Immunocytochemical detection demonstrated the presence of specific labeling in ultra-thin sections that had been incubated with the BinA antiserum. Gold particles were detected on the apical areas of cell membranes and inside the epithelial cell cytoplasm, particularly the mitochondria, of cells from the gastric caeca and posterior stomach in larvae exposed during 2 or 24 h to the entomopathogen. A similar labeling pattern was observed in ultrathin sections from both midgut regions when incubated with BinB antiserum.     

Association of a Geminivirus with a Leaf Curl Disease of Sunn Hemp (Crotalaria juncea) in India

Natural occurrence of a geminivirus causing severe leaf curl disease on sunn hemp (Crotalaria juncea) was recorded in India. The association of a geminivirus with the disease was demonstrated by whitefly transmission tests and polymerase chain reaction (PCR) amplification of DNA fragments of expected sizes with three pairs of degenerate geminivirus primers. The PCR-amplified viral DNA fragments were further characterized by Southern hybridization with a geminivirus probe consisting of the cloned coat protein (CP) gene of Indian tomato leaf curl virus (ITLCV). Restriction fragment length polymorphism analysis of a PCR-amplified CP fragment revealed that the geminivirus from sunn hemp was different than ITLCV.     

Evidence that a Leaf-Disk Test Allows Assessment of Isolate-Specific Resistance in Brassica oleracea Crops Against Downy Mildew (Peronospora parasitica)

A rapid resistance/susceptibility test for Peronospora parasitica (downy mildew) was established by inoculating leaf-disks of four Brassica oleracea accessions. Several conditions were tested: disk disinfection or not, agar medium with or without nutrients and with 50 or 100 ppm of benzimidazole. Using disinfected disks placed on agar (no nutrient and benzimidazole at 50 or 100 ppm), the responses of leaf-disks to four isolates were similar to those obtained using the classical cotyledon test, whereas undesired contaminations occurred in all other conditions. The possible effect of the particular leaf used for obtaining the disks was also studied. In each incompatible interaction tested, disks were resistant whatever the leaf used. In compatible interactions, susceptible phenotypes were observed on disks derived from the six lowest leaves, but disks from upper leaves were resistant. The genetic basis of resistance in a F1 hybrid broccoli was assessed, by testing six isolates on an F2 population derived from this hybrid. The cotyledon test only allows inoculation of two isolates per seedling, whereas many isolates can be tested on each plant by using leaf-disks. The segregation of the resistance to each of the six isolates was analysed: two dominant genes (tightly linked) control resistance to all isolates (one to five isolates; the other to only one isolate).