Antifungal activity of metalaxyl and furalaxyl

The systemic acylalanines metalaxyl and furalaxyl proved in vitro highly selective to fungi of the order of the Peronosporales, the former being somewhat more active than the latter. Various other fungal, and bacterial, species did not show sensitivity to the toxicants. Metalaxyl was strikingly more active on Pythium splendens and Phytophthora cinnamomi at low than at high temperature. With Phytophthora species metalaxyl displayed improved activity in combination with sodium dodecylsulphate and Triton X-100. Neither the composition of the medium nor its pH influenced activity. None of many compounds tested could antagonize their effect. The fungitoxicity caused by metalaxyl in the gas phase was strikingly better than that of furalaxyl against P. splendens. The toxicants had little effect on germination of the hyphal swellings of P. splendens, but had significant influence on the subsequent hyphal development. This included reduced growth, abnormal variation in the fungal thickness, and inhibition of the formation of hyphal swellings. Metalaxyl and furalaxyl were fungistatic at minimal inhibitory concentration (MIC) values during the first 24 hr. Thereafter, the toxicants became fungicidal. Metalaxyl and furalaxyl did not inhibit respiration of P. splendens at MIC values or somewhat higher concentrations. Neither did metalaxyl inhibit the [¹⁴C]carbon dioxide evolution from externally supplied radiolabeled glucose.     

Control of crown rot (Phytophthora cactorum) of apple trees with the systemic fungicides metalaxyl and fosetyl-aluminium

Four fungicides were evaluated as combined soil drench and trunk treatments for the control of crown rot, caused by Phytophthora cactorum, in naturally infected apple trees. In a 3-year preliminary study, metalaxyl at 1 and fosetyl-aluminium at 8 g a. i. per tree, applied twice a year, prevented death of infected trees showing only initial, but not more severe, symptoms of crown rot. This indicates that these fungicides have the ability to arrest further symptom development in naturally infected trees if they are treated as soon as symptoms appear. Healthy trees remained free of infection if they were treated with these fungicides, suggesting a preventative action. Repeated bi-annual application of metalaxyl or fosetyl-aluminium should be sufficient to prevent the loss of trees to crown rot.     

After-effects of systemic fungicides in the control of soil-borne pathogens,and the interaction between systemic fungicides and growth regulators

After benomyl treatment of mustard seedlings, grown in Rhizoctonia-infested soil, root remainders left in the soil significantly suppressed Rhizoctonia infection in two successive, later ‘crops’ of mustard seedlings. This effect was even larger with the combination tomato — Verticillium albo-atrum.Antagonistic interactions between growth-regulating substances (chlormequat, dimas; chlorflurenol and ethephon) and benomyl were shown upon treatment of Phialophora cinerescens-infected carnations and of Verticillium albo-atrum-infected snapdragons. Application of growth regulators before fungicide treatment reduced the effect of the fungicide on Phialophora and Verticillium wilt. When applied after fungicide treatment, these growth substances did not interfere with benomyl activity.Also, observations on stem growth pointed to an interaction between growth regulators and benomyl. Growth substances, in themselves slightly inhibitory to growth, diminished the growth-promoting effect of benomyl.     

Bioassay and transcuticular movement of systemic fungicides

A sensitive bioassay method was developed using bidirectional diffusion of fungicides in agar bands instead of diffusion through 360°. This method permitted the measurement of fungicide movement through isolated cuticular disks into agar bands underneath. Oil dispersible formulations of oxycarboxin and pyracarbolid penetrated through cuticles of coffee much more efficiently than wettable powder formulations.     

Mode of action of oxathiin systemic fungicides : III. Effect on mitochondrial activities

Carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide) was tested for its effect on the activities of mitochondria from several fungi, pinto beans and rat liver. Succinate oxidation by mitochondria from the sensitive fungus Ustilago maydis was inhibited by low concentrations of carboxin, the K_i being 0.32 μM. The inhibition was of a noncompetitive nature. Succinate oxidation was also inhibited in the mitochondria from other sources but not to the extent as in those from U. maydis. Carboxin had little effect on the oxidation of reduced nicotinamide adenine dinucleotide. The dioxide of carboxin, oxycarboxin, was not as effective in inhibiting succinate oxidation as was carboxin, but was more effective than the monoxide. Carboxin did not appear to uncouple oxidative phosphorylation in the presence of succinate in tightly coupled rat liver mitochondria but did decrease the respiratory control ratio. Carboxin was ineffective in releasing oligomycin inhibition in coupled rat liver mitochondria while dinitrophenol and salicylanilide were effective in this respect. It is believed that carboxin inhibits mitochondrial respiration at or close to the site of succinate oxidation and does not greatly affect the remaining portion of the electron transport system or the coupled phosphorylation reactions.     

Uptake of systemic fungicides by maize roots

The absorption of nine benzimidazole derivatives and four thiophanates by excised maize roots was shown to be due to passive mechanisms because a. the temperature coefficient was low, b. anaerobic conditions or metabolic inhibitors had little or no effect, c. the rate of uptake was a linear function of external concentration, d. compounds of similar structure did not compete for absorption.For benzimidazole derivatives, we observed a very rapid initial uptake which reached a maximum after one or two hours; this saturation did not occur with thiophanates.The effect of pH on the root absorption of benzimidazole derivative indicates that molecular (or unionized) forms of these amphoteric compounds were taken up preferentially; the behaviour of thiophanates was different.As observed with other pesticides, root absorption was greater for lipophilic than for lipophobic substances and in our experiments the rate of uptake of fungicides was well correlated to their partition coefficient in an octanol/ water system at pH 6.5.Generally, the various fungicides were taken up to the same extent all along the length of the roots.     

Recent progress in the field of N-acylalanines as systemic fungicides

The influence of structural variations in compounds of the N-acylalanine type, on their fungicidal activity towards Peronosporales, was examined. Within a series of methyl N-(acyl)-N-(2,6-xylyl)alaninates ( I ), a quantitative structure-activity relationship was given for the curative action against Plasmopara viticola, through a Hansch-type equation. A deviation from this equation is discussed with reference to benalaxyl ( I ; acyl=phenylacetyl). Syntheses and qualitative indications of activity are reported for structural changes in the xylyl and in the alanine moieties. Progress in the latter approach is briefly outlined.     

Comparison of cuprous oxide and metalaxyl with mixtures of these fungicides for thecontrol of Phytophthora pod rot of cocoa

Experiments were conducted using small-scale, detached pod evaluation methods to compare the likely effectiveness in controlling black pod of cuprous oxide-metalaxyl mixtures with these fungicides alone at approximately equivalent cost doses. All fungicide treatments were about equally effective and persistent except when rain fell soon afterspraying. Treatments containing cuprous oxide then gave very poor control of artificialinoculations with Phytophthora palmivora zoospores . Further laboratory' scale tests confirmed that cuprous oxide was readily leached by washing before the deposit completely dried whereas metalaxyl was very rapidly adsorbed on to ihe pods. A black pod control strategy is proposed using metalaxyi during critical wet periods and cuprous oxide for the rest of the year to reduce the possibility of metalaxyl resistance becoming apractical problem.     

Analytical methods for the detection and determination of residues of systemic fungicides

A review of the analytical methods for the detection and determination of residues of systemic fungicides in foodstuffs is presented.     

Experiments with some of the newer systemic fungicides on apple and marrow

In field trials against apple scab (caused by Venturia inaequalis) and powdery mildew (caused by Podosphaera leucotricha), improved control of one or other disease has been given by the newer systemic fungicides benomyl, triarimol and the thiophanates. However, in tests on apple seedlings in the greenhouse, biological evidence of translocation from individual deposits was generally limited to movement within the treated leaf. In greenhouse tests with cucumbers and marrows, using Oidium sp., there was again little evidence for movement of toxicant from a treated leaf, although effective disease control was readily obtained by root application of several compounds at low dosage. These results suggest that the systemic properties of the compounds evident when used as soil or seed treatments are of little account when they are applied as foliar sprays.     

Fungi in stored barley treated with systemic fungicides

Six fungicides (benomyl, carbendazim, fuberidazole, thiabendazole, thiophanate and thiophanate-methyl) were applied as dusts to barley in concentrations of 100, 1000 and 10 000 μg g^?1 and the barley then stored at 20°C and 90% relative humidity for 60, 105 and 150 days, respectively. Fungal development during storage was assessed by counting the visible patches of developing mould and by dilution plating. None of the fungicides completely prevented fungi developing but for the period up to 60 days, all provided a degree of control that was generally greatest for the highest concentration of fungicide. Benomyl was the most effective of the fungicides but no clear pattern was established for the others. Of the fungi that developed, species of Aspergillus and Penicillium accounted for 94% of the total count.     

Joint action of fungicides in Mixtures: Theory and practice

Fungicide mixtures aimed at the control of multiple target organisms have been used since the middle of this century. In the 1970s synergistic mixtures of site-specific (high resistance-risk) fungicides with multi-site (low resistance-risk) fungicides became available with the aim to provide adequate disease control while maximizing evolutionary problems for the pathogen. In the 1980s there were introduced mixtures of negatively correlated fungicides, in which each ingredient is capable of controlling pathogen genotypes resistant to the other ingredient. Such fungicides are currently deployed in the form of homogeneous mixtures (tank mixes, prepacked mixtures). A new strategy is under development in which different fungicides are applied to different adjacent plants. Diversification of cultivars and fungicides in a crop may minimize the buildup of resistance and maximize disease control.     

Ultrastructural changes induced by the systemic fungicides triadimefon,nuarimol and imazalil nitrate in sporidia of Ustilago avenae

The effects of the systemic fungicides triadimefon, nuarimol, and imazalil nitrate (5 μg ml_-1 for 8 and 16 h) on the ultrastructure of sporidia of Ustilago avenae were studied using transmission and scanning electron microscopes. Thin-section and freeze-fracture techniques demonstrated fine structural alterations to the cytology of the fungus. Compared with untreated controls, in which the cells were generally unicellular and uniform in shape and ultrastructure, fungicide-treated cells exhibited considerable thickening of peripheral cell walls and septa, an increased number of mitochondria that were enlarged and irregular in shape, greater amounts of endoplasmic reticulum, and extensive vacuolisation and accumulation of lipid bodies. The vacuoles were filled with membrane-like aggregates and vesicles, which could be extruded by exocytosis. The plasmalemma ultrastructure of fungicide-treated sporidia, observed by scanning electron microscopy after freeze-fracture was significantly different from that of control cells. The antifungal agents caused hemispherical pits, protrusions, invaginations, and aggregation of intramembrane particles on the membrane fracture faces. Specific structures, on the plasmalemma of plasmatic and extraplasmatic fracture faces, support the assumption of exocytosis. The ultrastructural changes were more pronounced after triadimefon and nuarimol treatment than after imazalil nitrate treatment. An extended incubation time (16 h) with triadimefon and nuarimol resulted in complete destruction of the cellular system of most sporidia.     

Behaviour of commercial systemic fungicides in conventional (non-systemic) tests

Biological screens specifically designed to detect systemic fungicides, besides being slow, labour-intensive, wasteful of chemicals and very expensive, may be expected to bias the results in favour of types of activity already encountered. Evidence is presented to show that simple, non-systemic tests are efficient in selecting active chemicals with systemic properties.     

Antisporulant action of fungicides against Podosphaera leucotricha on apple seedlings

A number of commercial fungicides and dinitrophenolic compounds were tested for their ability to suppress further sporulation of established infections of the apple mildew fungus Podosphaera leucotricha on seedlings in the greenhouse. Although they did not kill the fungus, two organophosphorus and two benzimidazole-type compounds gave powerful suppression for at least 18 days. Other fungicides known to control apple mildew in the field had a less marked effect. Sporulation occurred on subsequent untreated growth to the same extent as on control seedlings, except where either of two substituted dinitrophenols had been used.     

Mode of action,crop selectivity,and soil relations of the sulfonylurea herbicides

The sulfonylurea herbicides are characterized by broad-spectrum weed control at very low use rates (c. 2–75 g ha^?1), good crop selectivity, and very low acute and chronic animal toxicity. This class of herbicides acts through inhibition of acetolactate synthase (EC 4.1.3.18; also known as acetohydroxyacid synthase), thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. Crop-selective sulfonylurea herbicides have been commercialized for use in wheat, barley, rice, corn, soybeans and oilseed rape, with additional crop-selective compounds in cotton, potatoes, and sugarbeet having been noted. Crop selectivity results from rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range from 1–5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives > 20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative O-demethylation and direct conjugation with (homo)glutathione. Sulfonylurea herbicides degrade in soil through a combination of bridge hydrolysis and microbial degradation. Hydrolysis is significantly faster under acidic (pH 5) than alkaline (pH 8) conditions, allowing the use of soil pH as a predictor of soil residual activity. Chemical and microbial processes combine to give typical field dissipation half-lives of 1–6 weeks, depending on the soil type, location and compound. Very short residual sulfonylurea herbicides with enhanced susceptibility to hydrolysis (DPX-L5300) and microbial degradation (thifensulfuron-methyl) have been developed.     

Rates of reaction of singlet oxygen with some systemic pyrimidine fungicides and related compounds

Rate constants have been measured for the reaction in chloroform solution of singlet oxygen with the fungicides ethirimol (5-butyl-2-ethylamino-6-methylpyrimidin-4-ol) and dimethirimol (5-butyl-2-dimethylamino-6-methylpyrimidin-4-ol), and with the compounds 2-dimethylamino-5,6-dimethylpyrimidin-4-ol, 2-dimethylamino-6-methylpyrimidin-4-ol, 4-benzyloxy-2-dimethylamino-5,6-dimethylpyrimidine and 2-diethylamino-6-methylpyrimidin-4-ol. The values obtained show that singlet oxygen reacts readily with these compounds; the differences between the rate constant values have been rationalised in terms of the different structural features of the compounds studied. The possibility that singlet oxygen may react with agricultural chemicals under natural conditions is considered.