Resistance to Leaf Scald Disease Is Associated with Limited Colonization of Sugarcane and Wild Relatives by Xanthomonas albilineans

ABSTRACT A streptomycin- and rifampicin-resistant mutant of Xanthomonas al-bilineans was used to study symptom expression of leaf scald disease (LSD) and colonization of sugarcane (Saccharum spp.) and its wild relatives by this bacterial pathogen. A total of 40 sugarcane cultivars and 15 clones from the Saccharum complex that differed in resistance to LSD were inoculated by a decapitation technique in both field and greenhouse experiments. In the plant crop, disease severity varied between 0 for the most resistant genotypes and 100 for the most susceptible ones. Resistance to LSD was characterized by limited colonization of the host plant by X. albilineans. Although almost all genotypes were colonized by the pathogen, the greatest bacterial population densities were found in the susceptible cultivars. There was a high correlation between disease severity and pathogen population in the apex. Several genotypes exhibited no or slight symptoms even though they were highly colonized in the upper and/or basal nodes of stalks. Two mechanisms, therefore, may play an important role in resistance to LSD: resistance to colonization of the apex, which is characterized by absence of symptoms, and resistance to colonization of the upper and lower parts of the stalk. In contrast, disease severity and pathogen population densities in the first ratoon crop in the field were nil or very low in the stalks, except for the highly susceptible cv. CP68-1026. Sugarcane ratoons, therefore, may recover from the disease after plant cane infection. Nevertheless, because low levels of the pathogen were still detected in some stalks, it is possible that LSD could develop from latent infections if favorable environmental conditions occur.     

Rehabilitation Experiment at Menzel Habib,Southern Tunisia

This long - term rehabilitation experiment is located near Menzel Habib, in the arid zone of southern Tunisia in a site with unambiguously degraded vegetation and soil, caused by a decades - old practice of overexploitation (grazing and plow agriculture). The fundamental premise is that by reintroducing an appropriate combination of native perennial plants, a simplified version of a previously occurring vegetation (and ecosystem) could be reinstated that would utilize the sparse and irregular water resources available in an efficient and complementary fashion, while also showing considerable resistance and resilience to moderate livestock grazing. The long - term goal is, on the basis of such a simplified system, to progressively achieve, through additional reintroductions and other manipulations, a state similar to that of the "ecosystem of reference," an example of which is provided in the nearby Bou Hedma National Park. In this paper we report various aspects of the experiment related to germination and establishment of four supposed keystone species (Rhanterium suaveolens, Plantago albicans, Stipa lagascae, and Cenchrus ciliaris) sown in field conditions, and compared to the ecosystem of reference for a number of parameters related to the structure and functioning of the ecosystems under study. Over a 4 - year period, distinct changes in the desired direction were observed in the experimental plots, and positive effects of controlled grazing were also recorded.     

Protective Effects Against Erwinia amylovora Induced by hrp Mutants in the Whole Plant are Only Partially Mimicked in Cultivated Cells

A virulent strain of Erwinia amylovora, the causal agent of fire blight of Maloideae, and two of its non-virulent hrp mutants (a secretory and a regulatory mutant) were inoculated into apple cell suspensions either alone or in mixed inoculations. In single inoculations, death of 4- to 5-day-old apple cells occurred only when the concentration of the virulent strain of E. amylovora reached a threshold inoculum concentration of 10⁴CFUml^–1, while high concentrations of the hrp mutants were unable to kill apple cells. When mixed inoculated with the virulent parental strain, both hrp mutants protected apple cells from death caused by the virulent strain. The protective effect was associated with a decrease in the population level of the virulent strain and it was dependent on the non-virulent to virulent inoculum concentration suggesting a competition between the non-virulent mutant and the virulent strain. However, no differential protective ability between the two types of mutants could be noticed, contrary to previous results obtained with apple seedlings or apple flowers in which the regulatory mutant was significantly more effective than the secretory mutant. In conclusion, inoculation of apple cell cultures with E. amylovora does not seem to be a model suitable for investigating mechanisms leading to protection.     

Etiology of potato soft rot and blackleg diseases complex in Egypt

Journal of Plant Diseases and Protection - Recent observations of increasing incidences of potato plants showing blackleg symptoms have caused serious concern for potato growers and traders in...     

The bleaching induced by the phenylpyridazinone herbicide Metflurazon in Chlorella is a metabolic and reversible process

The treatment of synchronized cells of the green alga Chlorella fusca under photoautotrophic conditions with metflurazon (SAN H 6706; 4-chloro-5-dimethylamino-2-(α,α,α-trifluoro-m-tolyl-3(2H)-pyridazinone)) induces a bleaching process and results in white-appearing cells. This process of bleaching was followed by quantitative analysis of cell growth and cell division, photosynthetic oxygen evolution, respiratory oxygen consumption, and of pigment pattern at 0, 6, 12, 18, 24, 48, and 72 hr after incubation with different concentrations of metflurazon. Increasing concentrations of metflurazon gradually affected cell growth of Chlorella measured as increase in cell diameter. Cell division was inhibited completely with 1, 10, and 100 μM metflurazon. Photosynthetic oxygen evolution and respiratory oxygen consumption were not inhibited by 1 μM metflurazon during the first 6 hr; after this time a gradually increased inhibition was observed. Both parameters were inhibited by 100 μM metflurazon immediately after herbicide addition. A detailed analysis of the pigment content during the bleaching process revealed that: (a) The bleaching of Chlorella cells by metflurazon is not a simple photochemical process like the photobleaching of boiled cells, but is directed by the active metabolism of Chlorella itself. (b) The bleaching process is characterized by two phases: an accumulation of pigments followed by their degradation. The accumulation phase extends to 6 hr after herbicide addition. (c) During the accumulation phase, chlorophyll is accumulated to 380 and 106% in cells treated with 1 and 100 μM metflurazon, respectively, compared to the initial pigment content. The breakdown of chlorophyll, however, during the degradation phase is 5 times faster in the 1 μM treatment than in the 100 μM treatment. This difference resulted in the faster appearance of white cells with the low metflurazon concentration. (d) During the accumulation phase in the 1 μM treatment, the biosynthesis of chlorophylls, xanthophylls, and carotenes is inhibited by 56, 74, and 78%, respectively, when compared to a nontreated control. When related to the initial amounts, chlorophylls, xanthophylls, and carotenes are accumulated to 380, 230, and 153%, respectively. However, the synthesis of violaxanthin is specifically inhibited, followed by α-carotene. During the degradation phase, violaxanthin and α-carotene again, are the most rapidly disappearing pigments. Continuous culturing of white Chlorella cells resulted in a regeneration to green cells after 96, 240, 384 hr for 1, 10, and 100 μM metflurazone, respectively. The bleaching of Chlorella by metflurazon is evidently dependent on a functioning metabolism and is itself a regulated disassembly of the photosynthetic apparatus, which is reversible and not lethal.     

An immunocytochemical procedure for protein localization in various nematode life stages combined with plant tissues using methylacrylate-embedded specimens

Plant-parasitic nematodes possess a large number of proteins that are secreted in planta, allowing them to be successful parasites of plants. The majority of these proteins are synthesized mainly in the nematode subventral and dorsal glands as well as in other organs. To improve the immunovisualization of these proteins, we adapted a methacrylate embedding method for the localization of proteins inside nematode tissues, and extracellularly when secreted in planta or within plant cells. An important advantage is that the method is applicable for all nematode stages: preparasitic as well as parasitic stages, including large mature females. Herein, the method has been successfully applied for the localization of four nematode secreted proteins, such as Mi-MAP-1, Mi-CBM2-bearing proteins, Mi-PEL3, and Mi-6D4. In addition, we could also localize 14-3-3 proteins, as well as two cytoskeletal proteins, by double-immunolabeling on preparasitic juveniles. Superior preservation of nematode and plant morphology, allowed more accurate protein localization as compared with other methods. Besides excellent epitope preservation, dissolution of methacrylate from tissue sections unmasks target proteins and thereby drastically increases antibody access.     

Induction of defence related enzymes and phenolic compounds in lupin (Lupinus albus L.) and their effects on host resistance against Fusarium wilt

Two different biotic inducers [Pseudomonas fluorescens and Pseudomonas putida] and three different abiotic inducers [copper sulphate, indole butyric acid and potassium chloride] were tested for their efficacy in inducing resistance in lupin plants against Fusarium wilt disease caused by Fusarium oxysporum f. sp. lupini. Application of the biotic and abiotic inducers as seed treatments significantly reduced wilt disease incidence under greenhouse and field conditions. Potassium chloride and Pseudomonas fluorescens were superior. A time course of defence-related enzymes showed substantial increases in enzyme activities in induced infected seedlings compared with untreated healthy plants or infected controls. However, the magnitude of the increase varied among treatments. The maximum increases in chitinase and ??- glucanase activities were recorded at 12 and 8?days after inoculation with the pathogen, respectively. Also, the activity of phenylalanine ammonia lyase increased dramatically 8?days after inoculation. Greater accumulation of phenolic compounds and specific flavonoids upon infection with the pathogen was found in induced and/ or infected seedlings compared with healthy plants. In addition to inducing disease resistance, the treatments were accompanied by significant increases in crop parameters and seed yield compared with untreated controls.     

Potentiality of Soil Mulch and Sorghum Extract to Reduce the Biotic Stress of Weeds with Enhancing Yield and Nutrient Uptake of Maize Crop

Two field trials were conducted in 2018 and 2019 to develop practical and economically efficient weed control programs in corn. The experiment included six treatments (cowpea, rice straw, sorghum extract, hoeing, foramsulfuron herbicide and weedy check). The treatments were arranged in randomized complete block design with four replicates. Findings showed that reduction in total weed number was evident with application of hoeing, cowpea and rice straw in 2018 and 2019 seasons, in addition to foramsulfuron herbicide and sorghum extract in 2019. Cowpea, hoeing, foramsulfuron herbicide and rice straw recorded higher reduction in dry biomass of grassy weeds in both seasons. Reductions in N, P and K uptake by weeds because of rice straw and cowpea treatments were similar to hoeing treatment in both seasons. The increases in ear grain weight and grain yield ha^?1 due to cowpea and hoeing treatments were similar to rice straw one. In 2019 season, cowpea treatment was the superior practice for improving N, P and K uptake of maize, significantly leveling hoeing for K uptake. The values of gross returns and benefit/cost ratio of cowpea and sorghum extract, respectively, were higher than rice straw application. In conclusion, cowpea as a live mulch achieved acceptable weed control in maize by reducing weeds growth and lowering their ability to deplete the soil nutrients. Also, cowpea treatment improves growth and yield of maize with better utilization of nutrients. Thus, cowpea could be exploited as an eco-friendly method of weed management programs in maize cultivation.

     

Molecular and Pathogenicity Characteristics of Phytophthora nicotianae Responsible for Root Necrosis and Wilting of Pepper (Capsicum annuum L.) in Tunisia

Isolates of Phytophthora from pepper, produced in Tunisia, were characterised according to molecular and pathogenicity criteria. Polymerase chain reaction amplification of the ITS1 region in the ribosomal DNA resulted in different sized fragments. The pepper isolates and P. nicotianae yielded a fragment of 310bp that distinguished it from P. capsici with a fragment of 270bp. The ribosomal RNA gene amplicons of both internal transcribed spacers and the 5.8 S of the pepper Phytophthora and P. nicotianae were digested with 8 endonucleases. The patterns generated, with the 2 enzymes that cut, were identical for both taxa. This molecular analysis corroborated the morphological and biological characteristics and suggests strongly that the isolates of Phytophthora from pepper belong to the species P. nicotianae. Inoculation of pepper, tomato, eggplant and tobacco plants with the isolates of P. nicotianae from pepper showed they were highly pathogenic on pepper but not on tobacco, while their pathogenicity was weak on tomato and eggplant and was associated with atypical symptoms not observed in the field. These pathogenicity tests suggest that pepper isolates of P. nicotianae are particularly adapted to their host and may thus constitute a forma specialis of P. nicotianae.