Cross Inoculations Research Articles

Infection of papaya (Carica papaya) by four powdery mildew fungi

Papaya (Carica papaya L.) is an important fruit crop in many tropical and subtropical countries. Powdery mildew commonly affects this host, causing premature leaf loss, reduced yields and poor fruit quality. At least fifteen different fungi have been identified as the causal agents of papaya powdery mildew. Powdery mildew symptoms were detected on potted papaya plants growing in two locations in Hungary. This study aimed to identify the causal agents. Morphology of powdery mildew samples was examined, and sequences of two loci were used for molecular taxonomic identifications. Only anamophs were detected in all samples, and four morphological types were distinguished. Most samples had Pseudoidium anamorphs, while some were of the Fibroidium anamorph. Based on morphology and molecular taxonomy, the Fibroidium anamorph was identified as Podosphaera xanthii. The Pseudoidium anamorphs corresponded to three different Erysiphe species: E. cruciferarum, E. necator and an unidentified Erysiphe sp., for which molecular phylogenetic analyses showed it belonged to an unresolved species complex of E. malvae, E. heraclei and E. betae. Infectivity of P. xanthii and E. necator on papaya was verified with cross inoculations. A review of previous records of powdery mildew fungi infecting papaya is also provided. Podosphaera xanthii was known to infect, and E. cruciferarum was suspected to infect Carica papaya, while E. necator was recorded on this host only once previously. No powdery mildew fungus belonging to the E. malvae/E. heraclei/E. betae species complex is known to infect papaya or any other plants in the Caricaceae, so the unidentified Erysiphe sp. is a new record on papaya and the Caricaceae. This study indicates host range expansion of this powdery mildew fungus onto papaya.

Soil management legacy alters weed-crop competition through biotic and abiotic pathways

Agricultural practices often have persistent effects on soil physicochemical properties and soil biota, which can feedback to influence plant performance. We investigated management-induced differences in soil fertility and soil microbiota for their legacy effects on plant nutrient acquisition and crop/weed competition. Sorghum bicolor (L.) Moench (sorghum), Ambrosia artemisiifolia (L) (ragweed), and Amaranthus powellii S. Wats. (pigweed) were grown as monocultures and mixtures in the greenhouse in soils with distinct management histories. Cross inoculations of sterilized soils were used to quantify biotic vs. abiotic influence on plant growth, nutrient uptake and competitive outcomes. Differences in management history led to a two-fold difference in soil organic matter (OM), and a 37% increase in N acquisition in all species in the high OM soil. The increase in N availability preferentially benefitted pigweed growth at the expense of sorghum. However, microbiota from the high OM soil alleviated the negative effects of pigweed competition on sorghum. Soil management legacy also affected sorghum tiller production and mycorrhizal colonization. Management-induced differences in soil biotic and abiotic factors altered plant performance in a species-specific manner. Management legacy effects on soil microbiota have the potential to increase crop competitiveness against some weed species.

Occurrence and Distribution of Races 713, 733 and 734 of Sunflower Downy Mildew Pathogen in the Russian Federation

AbstractThe samples of Plasmopara halstedii (the causal agent of sunflower downy mildew) were collected on sunflower fields in south of the Russian Federation: in the Krasnodar, Rostov and Stavropol regions, and in the Republic of Adygea in 2016 and 2017. Virulence code of 545 isolates was identified; i. e. 280 isolates originating from 24 fields in 2016, and 265 isolates from 17 fields in 2017. Races 330, 334, 710 and 730, widespread within the region in previous years, were found in all mentioned areas. In addition, races 713, 733 and 734 have been detected in several districts of the Krasnodar region since 2016. This is the first report of these races in Russia and above that the first record throughout Europe and Asia for the races 733 and 734. The identification of new P. halstedii races was confirmed by the method of cross inoculations from individual differential lines. The phenotype corresponding to virulence code 734 was in some cases disclosed as a mixture of race 334 with 710 or 730. The presence of several P. halstedii races on an individual plant was also confirmed for the first time in the territory of the Russian Federation.

Diversity and virulence of Diaporthe species associated with wood disease symptoms in deciduous fruit trees in Uruguay

Several Diaporthe species are recognized as causal agents of many plant disease symptoms, including twig and branch cankers, dieback, shoot blight, and root and fruit rots. In Uruguay, the proximity between apple, pear and peach orchards offers the possibility to study the presence of different Diaporthe spp. associated with wood cankers across different deciduous fruit trees. Symptomatic twigs and branches of these orchard species were sampled, and isolates of Diaporthe were obtained. Selected isolates were used for cross inoculations in the three hosts. Seven Diaporthe spp. were identified, based on the internal transcribed spacer (ITS) region of rDNA and the translation elongation factor 1-alpha gene (EF1-α) phylogenies. The species were: Diaporthe amygdali , D. foeniculina , D. infecunda , D. eres , D. terebinthifolii , D. oxe and D. phaseolorum, while two isolates Diaporthe sp. 1 and Diaporthe sp. 4 could not be assigned to any species. Diaporthe infecunda, D. eres, D. terebinthifolii, D. phaseolorum and D. oxe on Pyrus communis and D. foeniculina on Malus domestica represent new records in these hosts in Uruguay, while D. oxe isolated from Prunus persica is a new record for this species. Diaporthe eres and D. phaseolorum were the most virulent species, posing the greatest risk due to their wide distribution and virulence in apple and peach trees. Although pear trees showed less symptomatic tissues and were less susceptible than peach and apple trees in the pathogenicity tests, they harboured seven of the species, and therefore should be considered as reservoirs of Diaporthe in Uruguayan orchards. Trees of the three hosts could be considered potential reciprocal sources of pathogenic Diaporthe spp.

Further Support of Conspecificity of Oak and Mango Powdery Mildew and First Report of Erysiphe quercicola and Erysiphe alphitoides on Mango in Mainland Europe.

Mango leaves and inflorescences infected by powdery mildew in southern Spain were analyzed using multigene sequencing (ITS + 4 single-copy coding genes) to identify the causal agent. Erysiphe quercicola was detected in 97% out of 140 samples, collected in six different orchards in the Malaga region. Among these, a small proportion also yielded E. alphitoides (8% of all samples) and E. alphitoides was found alone in 3% of samples. A phylogenetic approach was completed by cross inoculations between oak and mango, which led to typical symptoms, supporting the conspecificity of oak and mango powdery mildews. To our knowledge, this is the first report of E. quercicola and E. alphitoides causing powdery mildew on mango trees in mainland Spain, and thus mainland Europe, based on unequivocal phylogenetic and biological evidence. Our study thus confirmed the broad host range of both E. quercicola and E. alphitoides. These results have practical implications in terms of the demonstrated ability for host range expansion in powdery mildews. They also open interesting prospects to the elucidation of molecular mechanisms underlying the ability to infect single versus multiple and unrelated host plants since these two closely related powdery mildew species belong to a small clade with both generalist and specialist powdery mildews.

Eucalypt powdery mildew caused by Podosphaera pannosa in Brazil

Eucalypt powdery mildew is an important disease in greenhouses and clonal hedges of Eucalyptus spp. in Brazil, which can cause leaf and shoot distortion, shoot discoloration, and growth reduction that results in production losses. Because reliable information regarding the causal agent of the disease is lacking, this study used ITS and 28S rDNA sequencing and morphological analyses to identify the powdery mildew pathogen that occurrs in eucalypt nurseries within different regions in Brazil. Based on the results of morphological characteristics and phylogenetic analyses, the pathogen isolates were identified as Podosphaera pannosa, also known as the rose powdery mildew pathogen. Cross inoculations with pathogen isolates from rose and eucalypt demonstrated that P. pannosa can infect both host species. The ITS sequence-based phylogeny showed that 42 sequences generated in this study were comprised within a single clade containing P. pannosa, which was supported by a posterior probability of 88%. Identical ITS sequences were obtained from all 42 pathogen isolates, which suggests a clonal population.

Ceratocystis tiliae sp. nov., a wound pathogen on Tilia americana

Species in the North American clade (NAC) of the Ceratocystis fimbriata complex are mostly weak pathogens that infect native tree hosts through fresh wounds. Isolations from discolored tissue of wounded Tilia americana (basswood) in Iowa and Nebraska yielded a Ceratocystis species that was similar to but distinct from isolates of C. variospora from other hosts. Sequences of 28S rDNA showed that isolates from basswood did not differ from C. variospora, but there were minor differences in ITS rDNA sequences. The DNA sequences of a portion of the Cerato-platanin gene and TEF1α showed the basswood fungus to be a unique lineage. Cross inoculations in two experiments showed that the basswood isolates and C. variospora isolates from Quercus spp. were most aggressive to their respective hosts. Isolates from basswood grew slower and were less pigmented than C. variospora isolates from Quercus spp. The basswood fungus thus is distinguished from C. variospora based on phylogenetic analyses and phenotype and is herein described as C. tiliae sp. nov.

Taxonomy of two host specialized Phakopsora populations on Meliosma in Japan

Phakopsora meliosmae, a macrocyclic autoecious rust fungus, is reported to occur on several Meliosma species widely distributed in Asia. Despite the apparent broad host range, a recent molecular phylogenetic study indicated that two rust populations on Meliosma myriantha and Meliosma tenuis respectively in Japan were biologically distinct. To clarify the biological and taxonomic relationships of these populations, cross inoculations and comparative morphological examinations were carried out. Cross inoculations using basidiospores and aeciospores confirmed the macrocyclic, autoecious nature of the life cycle in both rust populations and showed that the two populations were distinct in their host specificity. Furthermore, they were found to be distinct in the structure of the aecial peridium surface, the size and wall thickness of uredinial paraphyses, and the urediniospore size and shape. Consequently, the fungal population on M. tenuis is taxonomically separated from P. meliosmae originally proposed for the fungus on M. myriantha. A new name, Phakopsora orientalis, is proposed for the fungus on M. tenuis.

Effect of crop rotation on specialization and genetic diversity of Macrophomina phaseolina

Charcoal rot, caused by Macrophomina phaseolina, is one of the most important diseases of soybean. Genetic variability among soybean isolates has been observed but the effect of host specialization on genetic variability has not been reported. In this work, isolates from soybean, corn and sunflower were evaluated based on cross inoculations and number of microsclerotia/g of roots. The highest and lowest significant (P<0.005) production of microsclerotia was obtained in soybean (863 microsclerotia/g of roots) and sunflower (578 microsclerotia/g of roots), respectively, regardless of the origin of the isolate. Additionally, the effect of a 20-year crop rotation on genetic variability based on RAPD was studied. Eighty-nine isolates from five populations were obtained from soil samples under four crop rotation systems and an uncropped soil. Seven clusters were obtained considering a similarity level of 85%. Analysis indicated that M. phaseolina is a highly diverse species and also revealed a strong effect of the rotation system on genetic diversity. AMOVA was conducted for the RAPD data. From the total genetic variability, 21% (P<0.0001) could be explained by the differences between populations while 79% could be explained by differences within populations ( Φst = 0.2110; P<0.0001). The mean coefficient of gene differentiation (GST) estimated among the five populations indicated 27% of differentiation between populations similar to the AMOVA results where Φst= 0.2110. Total gene diversity estimated indicated high levels of variability (HT =0.3484). Results suggest that genetic differentiation of M. phaseolina can be altered by crop rotation.

Podosphaera balsaminae on Impatiens balsamina and Impatiens × hawkeri

Podosphaera balsaminae was recorded for the first time in Argentina and apparently in the Americas, causing powdery mildew on Impatiens balsamina (balsam) and Impatiens × hawkeri (New Guinea impatiens). This is also the first record of P. balsaminae on New Guinea impatiens. In a preliminary test, results of cross inoculations suggest that a pathogenic specialisation of P. balsaminae for these two hosts is present.

Suscetibilidade do cupuaçuzeiro e outras espécies vegetais a isolados de Crinipellis perniciosa obtidos de quatro hospedeiros diferentes no sul da Bahia

Visando avaliar as interações de suscetibilidade do cupuaçu (Theobroma grandiflorum) e de outros hospedeiros ao fungo Crinipellis perniciosa, plântulas de cupuaçu , cacau (Theobroma cacao), cacau-do-peru (Theobroma bicolor) e jurubeba (Solanum paniculatum), com idade entre seis a oito semanas, foram inoculadas com basidiósporos provenientes de vassouras secas e/ou frutos infetados destes hospedeiros coletados no sul da Bahia. As inoculações foram feitas depositando-se uma gota de 20 µl da suspensão de 5,0 x 10(5) basidiósporos/ml de C. perniciosa, obtidos de cada um dos hospedeiros, na gema apical e no hipocótilo (cupuaçu) de cada muda. Após a inoculação as plântulas permaneceram por 24 h em câmara climatizada, com temperatura em torno de 25 ºC e aproximadamente 100% de umidade. Realizou-se a avaliação final dos sintomas 60 dias após a inoculação. O delineamento experimental utilizado foi o inteiramente casualizado, com 20 tratamentos e quatro repetições de dez plantas. O cacau e o cacau-do-peru foram suscetíveis ao inóculo obtido dos quatro hospedeiros. A jurubeba apresentou reações de suscetibilidade somente aos inóculos dela própria e de cacau. O cupuaçu apresentou sintomas quando inoculado com basidiósporos obtidos dele próprio, de cacau e de cacau-do-peru. O inóculo proveniente de cacau foi o mais infetivo a todos os hospedeiros.

First Report of Ascochyta spp. on Soybean in South Africa.

During February 2000 soybean fields over a wide area in South Africa were affected by a previously unreported disease. A typical target spot developed on leaves and many became blighted. Elongated dark brown to black lesions developed on the stems, often resulting in wilting of young shoots. Petioles and leaf axils were colonized, resulting in premature leaf drop. The most severe manifestation was on pedicels where infection suppressed podfill and plants remained green as ripening was delayed. Two distinct groups of isolates were obtained from lesions. In both groups an Ascochyta anamorph was present, while from some lesions collected in Kwazulu-Natal and Mpumalanga, a teliomorph (Mycosphaerella) was also present. Since dry beans are produced in these areas and M. phaseolorum is common on this crop, it was believed that the pathogen might have moved to soybean. In cross inoculations, both soybean and dry bean isolates were pathogenic to both hosts. However, M. phaseolorum isolates were more aggressive to both hosts than the Ascochyta sp. Morphologically the anamorphs of two types of isolates were indistinguishable. DNA was isolated from freeze dried mycelia using a modified version of the CTAB-method described by Graham et al. (1). The DNA concentration and purity were estimated by measuring absorbances at A260 and A280. Genetic difference between both isolates were determined using amplified fragment length polymorphism (AFLP) technique. The AFLP analysis was performed following the protocol described by Vos et al. (2) and the product manual supplied by Life Technologies Inc. (Gaithersburg, MD) with minor modifications. Five randomly selected primer pair combinations were tested for their ability to reveal polymorphisms between the isolates. The gel electrophoresis for AFLP products was as described by Vos et al. (2). AFLP gels were silver stained following the protocol described by silver sequence DNA sequencing system manual (Promega, Madison, WI). All five primer pairs revealed only polymorphisms between isolates. No corresponding bands between the two isolates were detected using these five primer pair combinations. It is concluded that both M. phaseolorum and an unidentified Ascochyta sp. were the cause of the epidemic. Ascochyta spp. have not previously been reported on soybean in South Africa.

Virulence Pattern of Venturia inaequalis Field Isolates and Corresponding Differential Resistance in Malus x domestica

AbstractAll commercially important apple cultivars are susceptible in the field to scab caused by Venturia inaequalis. The scope of this study was to investigate variation in virulence in Venturia inaequalis populations towards commercial apple cultivars. For this purpose, primary lesions were sampled in orchards with different varietal compositions and diversities. The virulence pattern of monosporic isolates, obtained by isolation of single conidia, was assessed by cross inoculations of the cv. Ananas Reinette, Boskoop, Glockenapfel, Golden Delicious, Gravenstein, James Grieve, Jonathan, Maigold, Reinette de Champagne, Spartan and Yellow Transparent. All cultivars were susceptible to some isolates and resistant to others. No cultivar behaved the same way, which suggests the presence of differential resistance in each cultivar and corresponding virulence in some isolates. Isolates from a monoculture of Golden Delicious consisted mainly of a pathotype that was virulent to Golden Delicious but not to other cultivars. In the samples from cultivar mixtures, virulence pattern variation was considerable. The results give further evidence of the existence of a large pool of differential and ephemeral resistances in Malus, which were overcome by the local scab populations during co‐evolution.

Host specialisation ofRhynchosporium secalisin New Zealand

Abstract Cross inoculations demonstrate that 15 New Zealand isolates of Rhynchosporium secalis (Oud.) J. J. Davis from various hosts show at least two degrees of host specialisation. All of four isolates from Hordeum vulgare L., and isolates from Secale cereale L. and Agropyron repens (L.) Beauv., were pathogenic to their original host only. Four isolates from Hordeum murinum L. were pathogenic to both H. murinum and H. vulgare but were most aggressive on H. murinum. Four of six isolates from A. repens were not pathogenic on any hosts. In culture, isolates from H. vulgare, H. murinum, and S. cereale were similar in growth characteristics, with conidia of most isolates characteristically 2-celled and beaked. Isolates from A. repens were slower-growing and sporulated less than those from other hosts, with conidia frequently short and bulbous, especially in culture.

Occurrence of bacterial leaf spot of pepper in Pakistan

Abstract Bacterial leaf spot of pepper, Capsicum annum L., characterized by small dark moist spots on the underside of the leaflets which eventually become brown or black and dry scabby, is reported for the first time from Pakistan. Isolations from diseased plants collected from Islamabad yielded two isolates: Xcv1 and Xcv2 of Xanthomonas campestris var. vesicatoria (Doidge) Dawson from bell pepper, C. annum var. grossum and ornamental pepper, C. annum var. minimum respectively. The cultures were pathogenic in cross inoculations to both hosts and gave hypersensitive reactions in tobacco plant. The isolate Xcv1 produced a copious yellow slime on yeast extract dextrose CaCO3 at 25°C and 48 h incubation. These cultures were biochemically and physiologically similar.

Control of Winged Waterprimrose (Jussiaea decurrens) and Northern Jointvetch (Aeschynomene virginica) with Fungal Pathogens

An indigenous, host-specific, pathogenic fungus that parasitizes winged waterprimrose [Jussiaea decurrens(Walt.) DC.] is endemic in the rice growing region of Arkansas. The fungus was isolated and identified asColletotrichum gloeosporioides(Penz.) Sacc. f.sp. jussiaeae(CGJ). It is highly specific for parasitism of winged waterprimrose and not parasitic on creeping waterprimrose (J. repensL. var.glabrescensKtze.), rice (Oryza sativaL.), soybeans [Glycine max(L.) Merr.], cotton (Gossypium hirsutumL.), or 4 other crops and 13 other weeds. The fungus was physiologically distinct from C.gloeosporioides(Penz.) Sacc. f. sp.aeschynomene(CGA), an endemic anthracnose pathogen of northern jointvetch[Aeschynomene virginica(L.) B.S.P.], as indicated by cross inoculations of both weeds. Culture in the laboratory and inoculation of winged waterprimrose in greenhouse, growth chamber and field experiments indicated that the pathogen was stable, specific, and virulent in a wide range of environments. The pathogen yielded large quantities of spores in liquid culture. It is suitable for control of winged waterprimrose. Winged waterprimrose and northern jointvetch were controlled in greenhouse and field tests by application of spore mixtures of CGJ and CGA at concentrations of 1 to 2 million spores/ml of each fungus in 94 L/ha of water; the fungi did not damage rice or nontarget crops.

Some pathological studies on botryodiplodia theobromae pat. causing banana rot

B. theobromae Pat. was found to be pathogenic on banana (Musa paradisiaca L.) variety “Bhusaval”. Its symptoms and morphology were studied. Inoculation tests were performed and the pathogenicity of the organism on its host was confirmed. It was also found that injury to the fruits enhanced the chances for infection. Percentage rot of banana fruits was found to be maximum at 25° C and 30° C and minimum at 7° C and below. An external supply of humidity always favoured the advancement of the disease on the fruits. Mode of germination of the spores was studied. Highest percentage germination ofB. theobromae was obtained at 25° C. 1 % host extract solution also gave the maximum results. Cross inoculations revealed that the pathogen was capable of infecting other varieties of banana and fruits of other plants as well. The longevity of spores and their thermal-death point have been determined.

Studies on the indicator plants for citrus viruses. II

1. Twenty-seven species of non-leguminous herbaceous plants known to be susceptible to virus diseases were selected from twelve families including Cruciferae, Chenopodiaceae, Cucurbitaceae, Solanaceae, Compositae, Amarantaceae, and others. They were inoculated with the sap from the young shoots of Satsuma orange affected by Satsuma dwarf. Among them only sesame (Sesamum indicum L.) proved to be susceptible to this disease, showing local lesions on the inoculated leaves, and also vein clearing, vein necrosis, curling and malformation on the upper leaves.2. Cross inoculations between sesame and Blackeye cowpea or between sesame and Chashiroingen, a variety of kidney bean were positive. Therefore the virus in the infected sesame plants is apparently the Satsuma dwarf virus which has been transmitted by sap-inoculation to leguminous plants as previously reported by the authors.3. Sesame proved to be susceptible to Satsuma dwarf virus, but nonsusceptible to tristeza and vein enation viruses which are commonly carried in Satsuma orange plants.4. Among three varieties of sesame tested, White sesame (Shiro-goma) was the most susceptible. Brown sesame (Cha-goma) was somewhat less susceptible whereas Black sesame (Kurogoma) was infected systemically showing no local lesions.5. No significant difference in susceptibility to the present virus was shown at different growing stages of individual sesame plant. However, plants inoculated at the first or second true leaf stage developed the most distinct symptoms and such young plants proved to be the most suitable indicator for Satsuma dwarf virus.6. Sesame plants are much more heat-tolerant than most of the Leguminous plants, and the former is useful as an indicator for the present virus even in the midsummer season.7. When sesame plants were exposed in a higher temperature than 34°C immediately after inoculation, they usually showed no symptoms, but sometimes systemic infection appeared ten days after inoculation. On the other hand, when the inoculated sesame plants were kept more than eight hours at 25°C, they showed striking symptoms even if they were later exposed to temperatures higher than 36°C.8. When sesame seedlings were inoculated with the sap from shoots of affected Satsuma orange, adjusted to different pH by McIlvaine buffer, infection was obtained with inocula at pH 5-8, with the optimum at 7-8.

Some pathological studies of Alternaria tenuis causing leaf spot disease of areca catechu

Pathogenicity ofAlternaria tenuis has been established on the leaves ofAreca catechu. Laboratory evaluation of fungicides showed that phygon, zerlate, tillex, cereson, dithane, spergon and isothane could inhibit the growth of the organism but repeated applications of phygon could control the disease in field. Cross inoculations were made and it was found that it could infect the leaves ofDracaena sp.,Solanum melongena, S. nigrum, S. lycopersicum, Dhatura metal and the fruits ofPyrus communis andM. sylvestris.

An ecological study of pasmo disease (Mycosphaerella linorum) On linseed in Canterbury and Otago

Abstract Pasmo, caused by the fungus Mycosphaerella linorum, is one of the commoner diseases of linseed in New Zealand, and occasionally causes severe losses. The most recent was during the 1960-1 season. Laboratory tests and field trials during the following season showed that seed transmission, though possible, was a minor factor. Infected cotyledons were found in one commerciai crop only. From mid-January_1962, however, the incidence of the disease steadily increased, until by harvest it had become general. The probable sources of infection were Australian flax (Linum marginale) growing on the roadsides, and litter remaining Irom the previous linseed crops. The fungus was isolated from both sources, and cross inoculations between linseed and Australian flax were successful. Both in the Ashburton crops and in all plots grown in the Dunedin district, the disease was evenly distributed, which suggests that the infection arose from wind-blown spores picked up over a large area. The disseminants were probab...