Effects of plant aqueous extract of Cymbopogon citratus (D.C.) Stapf. on sorghum seed germination and it efficacy in controlling Phoma sorghina (Sacc.) Boerema Dorenbosh and Van Kesteren transmission from naturally infected seed to sorghum plant organs and grains in field

The effect of aqueous extract of Cymbopogon citratus on sorghum seed germination and its efficacy against Phoma sorghina transmission to sorghum vegetative organs and seed were studied in field conditions in 2008–2010. During these years, we also examined the effect of panicle protection on the reduction of P. sorghina infection. The aqueous extract of C. citratus lowers sorghum seed germination compared to untreated seeds, seeds treated with water and those treated with fungicide calthio C. P. sorghina is transmitted to the whole vegetative organs and seed of all the treatments. However, the aqueous extract of C. citratus significantly reduces the transmission of P. sorghina compared to untreated seeds, to seeds treated with water and those treated with fungicide. The protection of sorghum panicles before flowering limits the infection by P. sorghina compared to the panicles not protected.     

First report of <Emphasis Type="Italic">Phoma sorghina</Emphasis> (Sacc.) Boerema Dorenbosch &; van Kest on wheat leaves (<Emphasis Type="Italic">Triticum aestivum L</Emphasis>.) in Argentina

A new disease caused by Phoma sorghina has been detected for the first time on wheat plants in the Province of Buenos Aires, Argentina. The pathogen was isolated from wheat leaves growing under field conditions, cultured on PDA and identified by its morphobiometric and cultural characters. The disease symptoms and morphological characters of the pathogen are described. Pathogenicity of the isolate was confirmed by inoculating 10 wheat cultivars under greenhouse conditions.     

Efficacy of plant extracts and effect of seed soaking duration on treatment of sorghum seed naturally infected by Colletotrichum graminicola and Phoma sorghina

The study was undertaken to evaluate the effectiveness of the aqueous extracts ofBalanites aegyptiaca (L.) Del., Cymbopogon citratus D. C. Stapf, Cassia occidentalis L. and Portulaca oleracea L. different durations of extracts maceration (6, 12, 24 and 48 h) and different durations of soaking seed in plant extracts (6, 12 and 24 h). All the plant extracts tested significantly reduced the infection rate by C. graminicola and P. sorghina except for the extract of C.occidentalis which was not efficient against P. sorghina. Extract of C. citratus and P. oleracea were more efficient on the reduction of the infection rate by C.graminicola. Those of B. aegyptiaca and C. citratus were the most efficient in reducing infection by P. sorghina. Long period of soaking and macerate were favourable in increasing of the antifungal efficiency of the extract.     

Genetic variation of <Emphasis Type="Italic">Phoma sorghina</Emphasis> isolates from Southern Africa and Texas

Genetic variability of Phoma sorghina, a ubiquitous facultative phytopathogen, was investigated on 41 isolates cultivated from surface-sterilized sorghum grains originating from South Africa and Texas; pearl millet isolates from Namibia were also included. Most of the isolates from Texas produced intense red pigments, especially on Czapek-Dox agar plates. Many African isolates formed conspicuous dark radial substrate hyphae with intercalated chlamydospores on oatmeal plates. Conidial dimensions and shape were very variable (mean lengths 4.5–5.7 μm). Haplotypes were defined based on 53 markers from banding patterns obtained with rep-PCR (primers: M13core, ERIC IR). The shared geographic origin was partially reflected in the clades of the haplotype phylogram. The values of G _ST were intermediate; 16–37 % of the variation was found between the populations. Nm values of gene flow were 0.84–1.15. Average gene diversity H _E was moderate (0.256). Sequences of ITS-rDNA were obtained from 21 isolates. Allele 1 was found in 9 isolates scattered throughout the clades, allele 2 occurred in 6 isolates (5 of them from the same clade), alleles 3 and 4 were shared by two isolates each and two isolates were unique. Alleles 1 and 2 were also found among highly related sequences from GenBank. All shared an 8-bp deletion near the 5′ end of ITS2 that was not found in any other Phoma/Didymella species and which may be a typical marker for P. sorghina. Among related species, members of legume-associated Ascochyta/Didymella complex, Epicoccum spp., D. applanata and P. glomerata were found.     

Control of fusarium wilt of <Emphasis Type="Italic">Solanum melongena</Emphasis> by <Emphasis Type="Italic">Trichoderma</Emphasis> spp.

Biological control of wilt of egg plant (Solanum melongena L.) caused by Fusarium solani was made with the application of five Trichoderma species, T. harzianum, T. viride, T. lignorum, T. hamatum and T. reesei. The effect of volatile and non-volatile antibiotics of Trichoderma origin on growth inhibition of the wilt pathogen was studied. T. harzianum showed maximum growth inhibition (86.44 %) of the pathogen through mycoparasitism. The non-volatiles produced by the Trichoderma species exhibited 100 % growth inhibition of the pathogen under in vitro condition. Production of siderophores and fungal cell wall degrading enzymes, chitinase and β-1,3-glucanase were found. Treatments with two most efficient Trichoderma species, T. harzianum and T. viride resulted in the decreasing population of Fusarium solani in soil thereby deterring disease incidence in field condition.     

Suppression by Zygorrhynchus moelleri of Growth and Sclerotium Production by Rhizoctonia solani and Sclerotinia sclerotiorum

As indicated by reduced cellulolysis, Zygorrhynchus moelleri suppressed mycelial growth in Rhizoctonia solani and Sclerotinia sclerotiorum. Sclerotium production by both pathogenic fungi was also reduced by Z. moelleri in dual sand-oatmeal cultures. The viability of sclerotia produced by S. sclerotiorum, but not those produced by R. solani, was greatly reduced. Sclerotium production by S. sclerotiorum on celery and tomato segments was reduced to a much greater extent when Z. moelleri was applied to the plant tissue 24 h before the pathogen than when applied at the same time or 24 h after the pathogen.     

Mycoparasitism of Trichoderma spp. in biocontrol of fusarial wilt of tomato

Wilt of tomato caused by Fusarium oxysporum is one of the most serious diseases posing a threat to its cultivation. As such a thorough search was made to evaluate the mycoparasitic potentiality of three species of Trichoderma, T. harzianum, T. viride and T. hamatum towards minimising the effect of the pathogen on ravages of the crop. All the experimental species of Trichoderma were able to produce lytic enzymes, β-1,3 glucanase and chitinase efficiently but their activity could be hastened up in the presence of cell wall material of the pathogen where T. harzianum was recorded to be the best in rank. Lysis of mycelium of the pathogen was achieved by treatment with metabolic filtrate of the antagonistic fungi. SEM micrographs correspondingly showed lysis of pathogenic mycelium due to overgrowth and penetration through hyphal pegs and coiling produced by T. harzianum. Application of the antagonistic fungi in the field showed their ability to reduce the incidence of the wilt disease to a reasonable extent where the performance of T. harzianum happened to be superior over the others.     

A role for β‐sitosterol to stigmasterol conversion in plant–pathogen interactions

Upon inoculation with pathogenic microbes, plants induce an array of metabolic changes that potentially contribute to induced resistance or even enhance susceptibility. When analysing leaf lipid composition during the Arabidopsis thaliana–Pseudomonas syringae interaction, we found that accumulation of the phytosterol stigmasterol is a significant plant metabolic process that occurs upon bacterial leaf infection. Stigmasterol is synthesized from β‐sitosterol by the cytochrome P450 CYP710A1 via C22 desaturation. Arabidopsis cyp710A1 mutant lines impaired in pathogen‐inducible expression of the C22 desaturase and concomitant stigmasterol accumulation are more resistant to both avirulent and virulent P. syringae strains than wild‐type plants, and exogenous application of stigmasterol attenuates this resistance phenotype. These data indicate that induced sterol desaturation in wild‐type plants favours pathogen multiplication and plant susceptibility. Stigmasterol formation is triggered through perception of pathogen‐associated molecular patterns such as flagellin and lipopolysaccharides, and through production of reactive oxygen species, but does not depend on the salicylic acid, jasmonic acid or ethylene defence pathways. Isolated microsomal and plasma membrane preparations exhibited a similar increase in the stigmasterol/β‐sitosterol ratio as whole‐leaf extracts after leaf inoculation with P. syringae, indicating that the stigmasterol produced is incorporated into plant membranes. The increased contents of stigmasterol in leaves after pathogen attack do not influence salicylic acid‐mediated defence signalling but attenuate pathogen‐induced expression of the defence regulator flavin‐dependent monooxygenase 1. P. syringae thus promotes plant disease susceptibility through stimulation of sterol C22 desaturation in leaves, which increases the stigmasterol to β‐sitosterol ratio in plant membranes.     

Evaluating a diverse panel of biocontrol agents against infection of blueberry flowers by Monilinia vaccinii-corymbosi

Seven microorganisms were evaluated for their biocontrol potential against Monilinia vaccinii-corymbosi which causes mummy berry disease through gynoecial (stigma-style-ovary) infection of blueberry flowers: the bacteria Bacillus subtilis QRD137, B. mojavensis RRC101, B. mycoides 7IIC4, and Pantoea agglomerans C9-1S; the yeast Wickerhamiella australiensis Y-27360; and the filamentous fungi Trichoderma harzianum KRL-AG2 and Gliocladium roseum H47. The epiphytic fitness of each organism was investigated by evaluating population dynamics or fungal growth on the stigmas of detached blueberry flowers, and such flowers, co-inoculated with M. vaccinii-corymbosi, were used to determine efficacy in reducing pathogen infection of the style. In addition, all organisms were tested in vitro for antibiosis using dual cultures and for nutrient competition (niche overlap) using Biolog microplates. The most promising antagonists were P. agglomerans, which exhibited high epiphytic fitness on the stigma and consistently reduced stylar infection by the pathogen; B. subtilis, which showed strong antibiotic activity in vitro and considerably reduced pathogen ingress into styles, but whose limited epiphytic fitness decreases its potential for field-use; and G. roseum, which exhibited complete niche overlap with the pathogen in vitro but produced more variable results in reducing stylar infection. Future work should evaluate combinations of these antagonists to determine whether there are additive effects and whether the variability inherent in biocontrol can be reduced.     

Genetic diversity analysis and development of SCAR marker for detection of Indian populations of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">ciceris</Emphasis> causing chickpea wilt

Genetic diversity of the isolates of Fusarium oxysporum f. sp. ciceris causing chickpea wilt collected from 12 states representing different agro-ecological regions of India was determined through randomly amplified polymorphic DNA (RAPD) markers. The UPGMA cluster analysis grouped the isolates into eight categories showing high magnitude of genetic diversity. Each group had the isolates from different states present in various agro-ecological regions of India. Therefore, the groups generated through the RAPD analysis were not corresponding to area of the origin of the isolates. The RAPD primers, namely, OPA 7 and OPA 11 produced Foc specific fragment of ≈1.3 kb and ≈1.4 kb, respectively in all the isolates. These fragments were eluted, purified, cloned in pGEM-T Easy vector and sequenced. Primers were designed with sequence information of these two fragments using primer.3 software. Two sets of sequence characterized amplified region markers (SC-FOC 1 and SC-FOC 2) developed from the sequences of these fragments were found to be specific to Foc and produced an amplicon of 1.3 and 1.4 kb, respectively. These set of markers were validated against the isolates of the pathogen collected from different locations of India representing various races of the pathogen. They are non-specific to the other Fusarium species, Rhizoctonia solani and R. bataticola.     

Characterization of magnesium and calcium tenuazonate from Phoma sorghina

Phoma sorghina Sacc., the fungus implicated in the aetiology of onyalai, a haematologic disorder, produces magnesium and calcium tenuazonate as toxic constituents.     

Latent infections of in vitro anthurium caused by Xanthomonas campestris pv. dieffenbachiae

Latent infections of tissue-cultured Anthurium andraeanum Lind. caused by the blight pathogen, Xanthomonas campestris pv. dieffenbachiae (McCulloch & Pirone) Dye, were examined. The pathogen survived in or on callus for over 4 months without producing symptoms in callus or turbidity in the medium. The pathogen survived for more than 1 year on or within stage II shoots without producing symptoms and was successively transferred three times as latently infected shoots were multiplied. The pathogen did not grow or survive for more than 2 weeks in Murashige and Skoog medium lacking plant material. The addition of coconut water enhanced bacterial growth and produced turbidity in culture media. Latently infected in vitro anthuriums may be inoculum sources for subsequent outbreaks of the disease.     

Plant Innate Immune Response: Qualitative and Quantitative Resistance

Plant diseases, caused by microbes, threaten world food, feed, and bioproduct security. Plant resistance has not been effectively deployed to improve resistance in plants for lack of understanding of biochemical mechanisms and genetic bedrock of resistance. With the advent of genome sequencing, the forward and reverse genetic approaches have enabled deciphering the riddle of resistance. Invading pathogens produce elicitors and effectors that are recognized by the host membrane-localized receptors, which in turn induce a cascade of downstream regulatory and resistance metabolite and protein biosynthetic genes (R) to produce resistance metabolites and proteins, which reduce pathogen advancement through their antimicrobial and cell wall enforcement properties. The resistance in plants to pathogen attack is expressed as reduced susceptibility, ranging from high susceptibility to hypersensitive response, the shades of gray. The hypersensitive response or cell death is considered as qualitative resistance, while the remainder of the reduced susceptibility is considered as quantitative resistance. The resistance is due to additive effects of several resistance metabolites and proteins, which are produced through a network of several hierarchies of plant R genes. Plants recognize the pathogen elicitors or receptors and then induce downstream genes to eventually produce resistance metabolites and proteins that suppress the pathogen advancement in plant. These resistance genes (R), against qualitative and quantitative resistance, can be identified in germplasm collections and replaced in commercial cultivars, if nonfunctional, based on genome editing to improve plant resistance.     

Antifungal effect of Trichoderma spp. β-1,3-glucanase on Phytophthora parasitica: Hyphal morphological distortions

Phytopathogenic fungi devastate agricultural crops worldwide. The biological agents, such as Trichoderma spp., antagonize phytopathogenic fungi by secreting various cell wall-degrading enzymes, for example, endochitinase and β-1,3-glucanase that target glycosidic linkages in β-glucan and chitin polymers of fungal cell walls, thus inhibiting pathogen growth. In this study, two antifungal genes endochitinase and β-1,3-glucanase cloned from local Trichoderma spp. were ligated in pET28a+ expression vector individually to generate two recombinant vectors. The vectors were mobilized into Escherichia coli host strain Rosetta-gami 2 for protein expression, and the 6xHis-tagged recombinant proteins were purified through Ni-NTA affinity chromatography. The purified proteins were individually confronted in vitro with pure cultures of Phytophthora parasitica (destructive pathogen affecting several hundred plant species worldwide) for analyzing their effect on pathogen growth. In vitro confrontation assay revealed P. parasitica growth inhibition by purified β-1,3-glucanase. The pathogen growth inhibition was due to hyphal morphological distortions, such as breakages, swelling, and holes evinced through electron micrography confirming direct role of β-1,3-glucanase in pathogen structural degradation.     

Entomopathogenic fungi stimulate transgenerational wing induction in pea aphids,Acyrthosiphon pisum (Hemiptera: Aphididae)

1. Aphid natural enemies include not only predators and parasitoids but also pathogens, of which fungi are the most studied for biological control. While wing formation in aphids is induced by abiotic conditions, it is also affected by biotic interactions with their arthropod natural enemies. Wing induction via interactions with arthropod natural enemies is mediated by the increase in their physical contact when alarmed (pseudo‐crowding). Pathogenic fungi do not trigger this alarm behaviour in aphids and, therefore, no pseudo‐crowding occurs. 2. We hypothesise that, while pathogenic fungi will stimulate maternally induced wing formation, the mechanism is different and is influenced by pathogen specificity. We tested this hypothesis using two entomopathogenic fungi, Pandora neoaphidis and Beauveria bassiana, an aphid specialist and a generalist respectively, on the pea aphid, Acyrthosiphon pisum Harris. 3. We first demonstrate that pea aphids infected with either pathogen and maintained in groups on broad bean plants produced a higher proportion of winged morphs than uninfected control aphids. We then show that, when maintained in isolation, aphids infected with either pathogen also produced higher proportions of winged offspring than control aphids. There was no difference between P. neoaphidis and B. bassiana in their effects on wing induction in either experiment. 4. Unlike the effect of predators and parasitoids on pea aphid wing induction, the effect of pathogens is independent of physical contact with other aphids, suggesting that physiological cues induce wing formation in infected aphids. It is possible that aphids benefit from wing induction by escaping infected patches whilst pathogens may benefit through dispersion. Possible mechanisms of wing induction are discussed.     

Association between the potato cyst-nematode, Heterodera rostochiensis Woll., and Verticillium dahliae Kleb. in the early-dying disease of potatoes

Investigation of a possible association between Verticillium dahliae and H. rostochiensis (pathotype E, British notation) was based on field observations and an examination of disease development in single-stemmed potato plants grown in pots. An association was found in the distribution of the nematode and the fungus in the field, and the disease was far more severe with a combined infection than with either pathogen alone. Studies on leaf-area development and yield reduction suggest there is synergism between fungus and nematode, the reductions produced by the combined infection exceeding the sum of those produced by either pathogen alone. Fungal mycelium and the extent of host colonization by V. dahliae were greatly increased by the presence of the nematode. The possible benefits to V. dahliae in the fungus-nematode complex are discussed.     

Biocontrol Activity and Plant Growth Promotion Exerted by Aureobasidium pullulans Strains

The most common leguminous plants’ diseases are caused by soil-borne pathogens leading to important economic losses worldwide. Strains L1 and L8, belonging to Aureobasidium pullulans species, were tested in vitro and in vivo as biocontrol agents (BCAs) against Rhizoctonia solani (Rs1) (AG-4) and as plant growth promoters (PGPs). The non-volatile metabolites produced by L1 and L8 strains inhibited the pathogen mycelial growth by 87.9% on average, with no significant differences between the two strains. The lower pathogen diametric growth inhibition was displayed by both yeasts’ volatile metabolites (VOCs) that significantly reduced the colony growth of R. solani, and similarly to the control, with an average of 10.5%. By in vivo assay, L1 and L8 strains showed the ability to control the pathogen virulence probably through the biofilm formation around the bean and soybean plant roots, as confirmed by scanning electron microscope (SEM) analysis. The spectroscopic analysis highlighted the composition of non-volatile compounds: complex carbohydrates (pullulan), degrading enzymes, siderophores and antifungals (aureobasidins). Moreover, the ability of L1 and L8 strains to stimulate the bean and soybean plant roots, stems, and leaves growth was investigated, showing that these yeasts could have an application not only as BCAs but also as plant growth biostimulator.

     

Density-Dependent Growth and Life History Evolution of Polycyclic Leaf Pathogens: A Continuous Time Growth Model

A density-dependent growth model of a polycyclic leaf pathogen is analysed. Viability and fecundity of the pathogen are regulated by the current population density. Spores are produced continuously from a sporulating infection until death of the infection, and consequently, all age classes of infections are present at a certain point in time. The leaf area of the host varies with time. Evolution of life history strategies are studied by letting different pathogen genotypes compete with each other. Evolution of life history strategies and evolution of impact of disease are discussed in relation to the ecology of the host-pathogen system. The model is exemplified by Erysiphe graminis f. sp. hordei growing on Hordeum vulgare.     

The insecticidal activity of beauvericin and the enniatin complex

The insecticidal activity of a complex mixture of enniatins, cyclohexadepsipeptides produced by Fusarium lateritium, a pathogen of the scale insect Hemiberlesia rapax, is compared with that of enniatin A and beauvericin in two bioassays against Calliphora erythrocephala and Aedes aegypti.