Sanitation practices that inhibit reproduction of Monosporascus cannonballus in melon roots left in the field after crop termination

Ascospores of Monosporascus cannonballus function as primary inoculum for infection of melon roots. Previous studies demonstrated that pathogen reproduction (i.e. ascospore production) occurs on infected melon roots primarily after the crop has been terminated. Therefore, the key to maintaining low soil population densities of the pathogen is to destroy the hyphae of the pathogen in infected roots as soon as possible after crop termination, thereby inhibiting ascospore production. Results from a 3-year field study demonstrated that, relative to the nontreated controls, an immediate postharvest application of metam sodium (applied via the drip irrigation system at 187 L ha⁻¹) or cultivation (which lifts roots onto the surface of the soil for rapid desiccation) significantly inhibited pathogen reproduction in infected melon roots, as shown by the number of roots subsequently bearing perithecia. Additionally, ascospore populations in plots that received either the metam sodium or cultivation treatment were significantly lower ( P  < 0·05) than populations in the nontreated control plots at the end of the 3-year study. These results demonstrated the efficacy of these postharvest treatments in the inhibition of pathogen reproduction and retardation of inoculum build-up in soil.     

Effect of water potential on mycelial growth and perithecial production of Monosporascus cannonballus in vitro

The effects of osmotic water potential (Ψ_s) on mycelial growth and perithecial production of Monosporascus cannonballus , the cause of root rot and vine decline of melons, were examined at 25°C on potato dextrose agar (PDA) amended with KCl, NaCl or sucrose. Patterns of the growth responses of four isolates to decreasing Ψ_s were similar for each of the osmotica. Compared with growth on nonamended PDA (−0·3 MPa), growth of all isolates increased as Ψ_s was reduced to −0·8 MPa. Maximum growth occurred at Ψ_s values of −0·6 to −0·8 MPa. Growth was not reduced below that on nonamended PDA until Ψ_s was reduced to −1·8 MPa, and a 50% reduction in growth did not occur until Ψ_s was reduced to < −2·5 MPa. Reproduction was much more sensitive to reduced Ψ_s than was mycelial growth, and perithecia were produced only at Ψ_s ≥ −0·7 or −0·8 MPa on PDA amended with KCl or NaCl, respectively. Three isolates produced perithecia on PDA amended with sucrose only at Ψ_s ≥ −0·6 MPa, but the fourth isolate produced perithecia at ≥ −1·9 MPa. Colonization of the xylem early in disease development may provide an essential source of water for subsequent reproduction in the root cortex during plant senescence. Postharvest cultivation to expose and desiccate roots may prevent reproduction even when temperatures lethal to hyphae are not attained.     

Infection and colonization of melon roots by Monosporascus cannonballus in two cropping seasons in Arizona and California

Although canopy collapse of melons (one of the above-ground symptoms of vine decline caused by Monosporascus cannonballus ) occurred late in the growing season, the onset of root infection occurred much earlier. In three early winter-spring and two late winter-spring crops, the onset of root infection occurred 47–65 and 35–36 days after planting, respectively. In contrast, in four summer-autumn crops, the onset of root infection occurred within 9–17 days after planting. Vine decline occurred commonly in winter-spring crops, but did not occur in any of the summer-autumn crops. Following the onset of root infection, the percentage of plants infected increased at rates of 0·031–0·036 and 0·038–0·070 per unit per day for winter-spring and summer-autumn crops, respectively, based on the monomolecular disease progress model. Root lesions were first observed 14–42 days after the onset of infection in winter-spring crops, and 14–28 days after the onset of infection in summer-autumn crops. Pathogen reproduction occurred primarily at the end of each growing season.     

Effect of Irrigation Regimes on Disease Expression in Melon Plants Infected with Monosporascus cannonballus

The effect of irrigation regimes on disease expression in melon plants infected with Monosporascus cannonballuswas studied during two summer growing seasons (1998 and 1999) in the Arava region of southern Israel. Less frequent and reduced irrigation postponed the onset of plant collapse and lowered disease incidence. Delaying disease development in infested fields by reducing irrigation frequency allowed crop harvest before plant collapse. However, reduced irrigation regimes reduced yields, as shown in methyl bromide fumigated plots. Fruits from melon plants grown under reduced irrigation in the infested plots were also of lower quality due to water shortage. The delay in plant collapse under the reduced irrigation treatments was attributed to a combined effect of reduced fruit load and the development of a deeper root system, which could support the increased water demand of the mature, fruiting plant.     

Quantitative detection of <Emphasis Type="Italic">Monosporascus cannonballus</Emphasis> in infected melon roots using real-time PCR

A method was developed for the specific detection, identification and quantification of Monosporascus cannonballus in infected melon roots based on real-time PCR (SYBR® Green chemistry) targeting the ITS1 region of the rDNA conserved between different strains of the pathogen. The specificity of the reaction was assessed using a number of fungi taxonomically and ecologically related to M. cannonballus. The method was highly sensitive and M. cannonballus was first detected in the roots of a susceptible Piel de Sapo cultivar 2 days after inoculation, before symptom appearance. Although conventional PCR methods could also provide such a specific and sensitive detection, real-time PCR was also able to produce reliable quantitative data over a range of 4 orders of magnitude (from 5 ng to 0.3 pg). The method allowed the quantitative monitoring of fungal growth from the very first stages of infection, and was successfully employed in the early screening of resistance. The assessment of disease progress and severity obtained with real-time PCR was more accurate than that obtained with the visual scoring of root lesions or root biomass losses. Therefore, there exists a great potential for its implementation in those steps of breeding programmes where high accuracy is required.     

Effect of soil matric water potentials on germination of ascospores of Monosporascus cannonballus and colonization of melon roots by zoospores of Olpidium bornovanus

Results document, for the first time, the role of soil moisture on a unique, tripartite, host-specific rhizosphere interaction (i.e., Cucumis melo-Monosporascus cannonballus-Olpidium bornovanus). Specifically, colonization of cantaloupe roots by zoospores of O. bornovanus and germination of ascospores of M. cannonballus were highest at a soil matric potential of ?0.001 MPa but significantly inhibited at a matric potential of only ?0.01 MPa. Matric water potentials of ?0.01 MPa or drier are characteristically inhibitory to the motility of zoosporic microbes but not hyphal growth of filamentous fungi like M. cannonballus. These results support our previous conclusion that germination of ascospores of M. cannonballus, a destructive root pathogen of cantaloupe is mediated by O. bornovanus, an obligate, zoosporic fungus.     

Effect of potassium silicate on epidemic components of powdery mildew on melon

The effects of silicon (Si) supplied in the form of potassium silicate (PS) were evaluated on epidemic components of powdery mildew of melon under greenhouse conditions. The PS was applied to the roots or to leaves. In the first experiment, epidemic components were evaluated after inoculation with Podosphaera xanthii. In the second experiment, the disease progress rate was evaluated on plants subjected to natural infection. The area under the disease progress curve was reduced by 65% and 73% in the foliar and root treatments, respectively, compared to control plants, as a consequence of reductions in infection efficiency, colony expansion rate, colony area, conidial production and disease progress rate. However, root application of PS was more effective than foliar application in reducing most of the epidemic components, except for infection efficiency. This can be explained by the high Si concentration in leaf tissues with root application, in contrast to the foliar treatment where Si was only deposited on the external leaf surfaces. The effects of PS reported in this study demonstrated that powdery mildew of melon can be controlled, and that the best results can be achieved when PS is supplied to the roots.     

Brown spot caused by Sphingomonas sp. on yellow Spanish melon fruits in Spain*

Brown fruit spot symptoms were observed on yellow Spanish melons ( Cucumis melo var. inodorus ) grown in greenhouses at Almeria in Spain. Nonsporing, motile, rod-shaped bacteria were isolated from diseased fruits, which on nutrient agar produced small yellow colonies. Two bacterial isolates, used for further investigations, were pathogenic on fruits but not on cotyledons of Spanish melon plants. They provoked disease symptoms similar to those observed in the greenhouse. Both isolates were Gram-negative, catalase-positive, weakly oxidase-positive and phenylalanine deaminase-negative. They hydrolysed esculin but not gelatin and they utilized glucose oxidatively. Fatty acid analysis revealed that both isolates belong to the genus Sphingomonas . In addition, 16S rDNA sequence analysis, performed on one isolate, demonstrated that it had a significant sequence similarity (more than 98%) with Sphingomonas pruni and Sphingomonas mali , nonphytopathogenic bacteria isolated from plants. Although enterobacterial repetitive intergenic consensus PCR and repetitive extragenic palindromic PCR seem to indicate that the Sphingomonas isolates from Spanish melon fruits may belong to a new species, DNA–DNA hydridization analysis is necessary to verify this hypothesis.     

Evaluation of biological control agents for managing cucurbit powdery mildew on greenhouse-grown melon

An evaluation was made of the ability of two mycoparasite-based products AQ10® ( Ampelomyces quisqualis ) and Mycotal® ( Lecanicillium lecanii ), as well as three strains of Bacillus subtilis , to manage powdery mildew disease, caused by Podosphaera fusca on melon seedlings maintained under different regimes of relative humidity and on plants grown under greenhouse conditions in Spain. In every case fungal and bacterial biocontrol agents (BCAs) performed better under conditions of high relative humidity (90–95% RH). In greenhouse experiments, the effectiveness of the mycoparasites to manage powdery mildew was absolutely dependent on mineral oil. The strains of B. subtilis provided disease control similar to that achieved with the mycoparasites or the fungicide azoxystrobin. Microscopic analysis showed the ability of these bacterial strains to efficiently colonize leaf surfaces and revealed the occurrence of antagonistic interactions between biological agents and P. fusca structures. These results confirmed the usefulness of these BCAs for managing powdery mildew on greenhouse-grown cucurbits either as single products or as a component of integrated control programmes.     

Transmission of the agent causing a melon yellowing disease by the greenhouse whitefly Trialeurodes vaporariorum in southeast Spain

The agent causing a yellowing disease of melon (Cucumis melo), which results in severe losses in crops under plastic on the coastal plains of southeast Spain, was shown to be transmitted in a semipersistent manner by the greenhouse whitefly (Trialeurodes vaporariorum Westwood). The agent was transmitted by grafting, but not by mechanical inoculation or through seeds. The agent was acquired in the minimum period tested (2 h) and could infect plants in an infection feeding interval of 6 h. Capsella bursa-pastoris, Cucumis melo, C. sativus, Cucurbita moschata, Cichorium endivia, Lactuca sativa andTaraxacum officinale were found susceptible.Results suggest that the yellowing disease affecting melon crops in the southeast of Spain is due to a pathogen similar to beet pseudo yellows virus, but this has to be confirmed by serology.     

Colonisation and histological changes in muskmelon and autumn squash tissues infected by <Emphasis Type="Italic">Acremonium cucurbitacearum</Emphasis> or <Emphasis Type="Italic">Monosporascus cannonballus</Emphasis>

Muskmelon (Cucumis melo cv. Temprano Rochet) and autumn squash (Cucurbita maxima) seedlings were inoculated either with Acremonium cucurbitacearum or Monosporascus cannonballus, two of the soil-borne fungi implicated in ‘melon collapse’. Inoculation was achieved in two different ways: by growing the plants in pots containing infested soil to study the histological changes produced in the infected tissues using light microscopy and by growing seedlings in Petri dishes together with fungal colonies in order to observe the colonisation of the plant tissues using scanning electron microscopy. Both muskmelon and autumn squash roots infected with A. cucurbitacearum showed a suberised layer in the epidermis and the outermost layers of the parenchymatic cortex, but these symptoms developed earlier in the muskmelon plants. Muskmelon plants infected by this fungus also presented hypertrophy and hyperplasia, which led to a progressive separation of the vascular bundles in the lower stems of the affected plants. This response was not observed in autumn squash during the study. On the other hand, few histological changes were observed in tissues infected with M. cannonballus and only a slight increase in the size of cortical intercellular spaces was noted in the lower stems of muskmelon plants, and infected autumn squash tissues remained free of these symptoms throughout the study. The scanning electron microscope observations revealed that both fungi were able to colonise the tissues of the two host plants which were studied. A. cucurbitacearum colonised the epidermis and cortex of both muskmelon and autumn squash. The hyphae grew both inter- and intracellularly, and the density of the colonisation decreased within the endodermis. The same colonisation of host plants was observed as a result of M. cannonballus infection. The xylem vessel lumina of both muskmelon and autumn squash showed hyphae and tylose formation as a result of both fungal infections. However, non-fungal structures were detected in the hypocotyl vascular tissues. The present study demonstrates that both fungi are capable of infecting the tissues of a species which is resistant (autumn squash) and a species which is susceptible (muskmelon) to melon collapse.     

Differential expression of resistance to powdery mildew incited by race 1 or 2 ofsphaerotheca fuliginea inCucumis melo genotypes at various stages of plant development

Thirty-nine genotypes ofCucumis melo (plant introduction entries, open-pollinated cultivars and F₁ hybrids) were evaluated for resistance to powdery mildew under either natural field conditions or artificial inoculation in growth chambers at the cotyledonary stage and the 2-true-leaf stage. Results confirmed that susceptibility in cotyledons was not necessarily associated with susceptibility in either true leaves in growth chambers or adult plants in the field. However, resistance at the 2-true-leaf stage in growth chambers was highly correlated with resistance of field-grown plants. Results also showed that 20 muskmelon genotypes resistant to race 1 at the cotyledonary stage were also resistant at the 2-leaf-stage and as adult plants in the field. The same was true for ten genotypes with race 2 inoculations. Because muskmelon genotypes expressing resistance in cotyledons were also resistant in true leaves in growth chambers or the field, the use of plants at the cotyledonary stage is recommended for screening for powdery mildew resistance caused by race 1 or race 2 ofS. fuliginea. When cotyledons are susceptible, screening should be done at the 2-true-leaf stage.     

Identification, occurrence and pathogenicity of Rhizopycnis vagum on muskmelon in Spain

Rhizopycnis vagum is a recently described coelomycetous fungus that contributes to vine decline of muskmelons in Honduras, Guatemala, Texas and California. This fungus has been associated with roots of muskmelon plants affected by vine decline in most Spanish muskmelon production areas. Isolates were collected from 1996 to 2000 and identified from their cultural and morphological characteristics and by sequencing the ITS region of the ribosomal coding nuclear DNA (rDNA) and phylogenetic analysis. A few isolates from muskmelon with growth characteristics similar to R. vagum were identified as Phoma terrestris . Watermelon, Cucurbita hybrids used as rootstocks for watermelon production, Amaranthus sp. and grapevine were also hosts for R. vagum . Based on disease reaction in muskmelon roots, the pathogenicity of 10 isolates of R. vagum from different hosts and geographical origins was verified. The fungus caused root discoloration, corky lesions, and eventually the presence of pink coloration on the roots. Rhizopycnis vagum appears to be a minor pathogen that contributes to muskmelon vine decline complex in Spain through infection of roots.     

秸秆覆盖对农田黑土春季地温的影响

通过黑土区田间长期定位试验,研究了秸秆覆盖度对太阳反射和土壤含水量的影响,进而探讨了其对土壤温度的调控作用。在作物出苗前,秸秆覆盖度免耕(覆盖度70%)>少耕(覆盖度10%)>旋松(覆盖度0%)。研究结果表明:免耕、少耕和旋松的累计反射强度依次递减,反射辐射强度越大土壤温度越低;免耕具有较高的质量含水量,质量含水量均与地温存在显著的负相关关系。     

Effect of temperature on peroxidase activity,isozyme patterns and concentration of threadlike particles in tristeza—infected citrus plants

Peroxidase activity (PA) in various tristeza-infected citrus varieties was significantly higher than in healthy controls. In leaves and bark of Egyptian sour lime and Palestine sweet lime, PA was correlated with symptom severity and content of threadlike particles (TLP). In infected roots of Egyptian sour lime, there was an increase in PA but TLP content was minimal. Temperatures above 22°C caused a gradual decrease in PA in both healthy and infected leaves of Egyptian sour lime trees. At 36°C, no differences in PA between infected and healthy samples were observed. In these plants TLP content was minimal and almost no symptoms were observed. No new isozymes were found in various host tissues infected with tristeza, psorosis, exocortis or impietratura. One isozyme appeared earlier in tristeza-infected plants than in healthy controls, suggesting that isozymes associated with senescence are activated at an earlier stage in infected plants than in healthy ones.     

Effect of temperature and host genotype on the production of inoculum by Phytophthora fragariae var. fragariae from the roots of infected strawberry plants

A bioassay was used to monitor the release of inoculum in drainage water from strawberry plants inoculated with zoospores of Phytophthora fragariae var. fragariae. The fungus was detected in drainage water from plants that had been held at temperatures between 2 and 20 C. but not from plants held at 26°C. The lag phase before secondary inoculum was first released, the maximum and total amounts of inoculum released, and the length of time over which inoculum was released were all greater at the lower temperature regimes, especially those below 10 C. The results were consistent with observations on the effect of temperature on zoospore production from agar discs and on zoospore motility: more zoospores were produced at lower temperatures and they remained motile for longer. From this it is concluded that the inoculum detected consists mainly of motile zoospores. In most experiments with standardized suspensions c. 10-15 were sufficient to initiate infection of the plants in the bioassay. In general, more inoculum was produced by host genotype/fungal isolate combinations in which there were marked root rot symptoms than in combinations in which the host was resistant.     

Effect of seed bacterization on peroxidase activity in wheat plants when infected with Bipolaris sorokiniana under high temperature and low moisture

European Journal of Plant Pathology - Sorghum anthracnose caused by the fungal pathogen Colletotrichum sublineolum, is one of the most economically important diseases of sorghum globally and in...     

Bacterial fruit blotch of melon: screens for disease tolerance and role of seed transmission in pathogenicity

Bacterial fruit blotch (BFB) of cucurbits, caused by Acidovorax avenae subsp. citrulli, is a serious threat to the watermelon and melon industries. To date, there are no commercial cultivars of cucurbit crops resistant to the disease. Here we assessed the level of tolerance to bacterial fruit blotch of various commercial cultivars as well as breeding and wild lines of melon, using seed-transmission assays and seedling-inoculation experiments. Selected cultivars were also tested in a greenhouse experiment with mature plants. All tested cultivars/lines were found to be susceptible to the pathogen, and most of them showed different responses (relative tolerance vs. susceptibility) in the different assays; however, some consistent trends were found: cv. ADIR339 was relatively tolerant in all tested assays, and cv. 6407 and wild lines BLB-B and EAD-B were relatively tolerant in seed-transmission assays. We also provide evidence supporting a strong correlation between the level of susceptibility of a cultivar/line and the ability of the pathogen to adhere to or penetrate the seed. To the best of our knowledge, this is the first attempt to assess melon cultivars/lines for bacterial fruit blotch response.