Absence Of Fungicide Research Articles

Plant density and the control of chocolate spot in spring and winter sown field beans (Vicia faba)

Trials with spring field beans of different growth habits and a single winter cultivar examined yield, yield components and straw strength responses, with and without fungicides to control chocolate spot, to plant densities greater than those recommended to farmers. Indeterminate spring cultivars showed only small responses in yield and components of yield to increased density, irrespective of fungicide treatment. Determinate cultivars responded to density only in the absence of fungicide ; the winter cultivar gave a linear response to increased density where fungicide was used but a diminishing response in the absence of fungicide. The different responses were attributed to differences in growth habit. Straw strength decreased as density increased, but did not appear to play a part in differential yield responses.

Studies on resistance to dichlofluanid and other fungicides in Botrytis cinerea

Amongst 489 mass isolates of Botrytis cinerea obtained from protected crops in Humberside during 1989, 67.3% were resistant to benomyl and 45.6% to vinclozolin. Two isolates resistant to dichlofluanid were obtained with EC50 values for mycelial growth on agar of 21.45 and 19.78 μg dichlofluanid/ml, respectively, compared with values of 1.55‐2.56 μg/ml for sensitive isolates. EC50 values for germ tube emergence and length were much lower than for mycelial growth. In population studies carried out on tomatoes in a polythene tunnel, resistance to dichlofluanid and to vinclozolin declined over a 12‐week period in the absence of fungicides. Field isolates of B. cinerea resistant to dichlofluanid were also resistant to vinclozolin. but strains resistant to dichlofluanid and sensitive to vinclozolin were obtained during population studies. Examination of a number of features of dichlofluanid‐resistant strains revealed a degree of lack of fitness as compared with sensitive strains. There is no indication that resistance to dichlofluanid poses a great threat to the control of grey mould in Humberside at present.

Effects of leaf susceptibility and fungicide seed treatment on leaf spot of Pima cotton caused by Altenaria macrospora

Effects of leaf susceptibility and seed treatment with the systemic fungicides tebuconazole (Raxil) or difenoconazole (Dragon) on Altenaria macrospora in Pima cotton were examined under glasshouse conditions and in the field. In the absence of fungicide, the susceptibility of cotyledons to Alternaria‐induced shedding increased with seedling age. Effects of seed treatment with fungicide were evident even on the third true leaves, and persisted up to 39 days after planting. Disease suppression was determined both by the fungicide and also by the susceptibility of the host tissue: seed treatment suppressed Alternaria‐induced shedding of cotyledons for 18 days, but not 29 days after planting In a field trial, disease progress was delayed by 20‐24 days in plots planted with treated seeds. The effect persisted for up to 86 days after planting. Alternaria‐induced defoliation at later stages of the season was not affected by the seed treatment. No significant differences in disease control were detected between the two fungicides.

Stand and yield of cucumber seeded with gel and fungicides in various tillage systems

A cucumber ( Cucumis sativis L.) crop was established in various tillage systems from gel-sown germinated seed. Fungicides (flutolanil plus metalaxyl) were mixed with a gel or applied as a drench after seeding to control Rhizoctonia and Pythium seedling diseases. The benefit of mixing fungicides with a gel was similar to that of drenching the seeded area with fungicides. There was no advantage in using germinated seed for cucumber production in conservation tillage. Germinated seed was more susceptible to fungal diseases in the absence of fungicides. Crop yield was greater in conventionally prepared soil than in conversation tillage.

YIELD OF CUCUMBER SEEDED WITH GEL AND FUNGICIDES IN CONSERVATION TILLAGE

Cucumber crop was established in conservation tillage from gel-sown germinated seed. Fungicides (flutolanil + metalaxyl) were mixed with gel or applied as a drench after seeding to control Rhizoctonia and Pythium seedling diseases. The benefit of mixing fungicides with gel was similar to drenching the seeded area with fungicides. There was no added advantage of using germinated seed for cucumber production in conservation tillage. In fact, germinated seed was more susceptible to fungal diseases in the absence of fungicides. Crop yield was greater in conventionally-prepared soil than in conservation tillage.

Studies on the inheritance of resistance to metalaxyl in Bremia lactucae and on the stability and fitness of field isolates

One hundred and eighty‐one sexual progeny isolates from three F1, one F2, five BC1 and one BC2 generations of crosses involving two metalaxyl‐resistant and two metalaxyl‐sensitive isolates of Bremia lactucae were assessed for response to metalaxyl. In most cases, isolates were also tested for virulence phenotype and sexual compatibility type. Isolates could be classified into three phenotypes (sensitive, resistant and intermediate) on the basis of their response to fungicide. Sensitive progeny isolates were similar to the sensitive parents and did not sporulate on lettuce seedlings grown in the presence of > 1.0 μg/ml metalaxyl. Resistant progeny isolates, in common with the resistant parents, sporulated as readily on lettuce seedlings grown in the presence of 100 μg/ml metalaxyl as on untreated seedlings. Progeny isolates which expressed an intermediate phenotype not observed in field samples were also recovered. This phenotype was more variable but isolates so classified were able to sporulate on seedlings grown in the presence of 100 μg/ml metalaxyl but exhibited a latent period (time from inoculation to appearance of sporulation) which was on average twice as long as that observed on seedlings grown in the absence of fungicide.Segregation patterns suggested that response to metalaxyl is controlled at a single locus exhibiting incomplete dominance but that the expression of response in the heterozygous condition may be affected by modifier genes. There was evidence that the metalaxyl‐resistant parental field isolates were heterokaryotic (possibly carrying nuclei homozygous and heterozygous at the locus controlling metalaxyl response). After 10‐30 asexual generations in the absence of fungicide, the response to metalaxyl of resistant field isolates began to approach that of an intermediate type. This phenomenon did not occur with a resistant sexual progeny isolate (presumed homokaryotic) and is additional evidence for the heterokaryosis of field isolates. The relative fitness of a range of isolates was examined by observing changes in the proportions of spores over three asexual generations after they were initially mixed in a 1:1 ratio with mutant isolates carrying a microscopically visible genetic marker (large lipid droplets). Most field isolates were more fit than the mutant isolates used, regardless of whether they were resistant or sensitive to metalaxyl, and there was no evidence that resistant isolates carried any fitness deficit in comparison with the sensitive isolates examined. In contrast, sexual progeny isolates (particularly from BC1 generations) were often less fit than the mutant isolates used, but there was no evidence that this was associated with response to fungicide. These findings are discussed in relation to the mode of action of metalaxyl and disease control strategy.

The influence of four fungicide formulations on the degradation rate and selective extractability of carbofuran in the soil

Abstract ‘Soil was incubated for 18 weeks with carbofuran alone (control) or in combination with Manate (maneb), Apron (metalaxil + mancozeb), Ridomil Special (metalaxil + maneb) and Pomarsol (TMTD) which were added to the soil at high (250 ppm) and lower (25 ppm) concentration. In the presence of fungicides the half‐life time value for carbofuran in the soil was 1.2 to 3 times higher than in the control depending on the nature of the fungicide and its concentration. The degradation rate, as reflected from the degradation ratio R measured at weekly time intervals, was greatly affected by the presence of the fungicides: at the lower concentration the R values were much lower only during the first weeks of incubation , whereas at the higher fungicide concentration this inhibitory effect on the degradation rate appeared to be very persistent. The extraction of the soil samples at the end of the incubation period has been performed using either water or a benzene‐acetone mixture as solvents. The differential extractability, which is linked to the insecticide bioavailability in the soil environment, has been shown to be influenced by the presence of the fungicides: at high application rate the carbofuran was relatively better extracted with water than at lower application rate or in the absence of fungicides.

Studies of fungicide resistance in Pyrenopeziza brassicae, cause of light leaf spot disease of oilseed rape and other brassicas

Resistance in the light leaf spot fungus, Pyrenopeziza brassicae, to benzimidazole or dicarboximide fungicides was not detected in field isolates. Resistant mutants were, however, obtained in vitro. Benzimidazole-resistant strains were generated by mutagenesis with ultraviolet light, and dicarboximide-resistant sectors developed when the fungus was grown on media containing iprodione or vinclozolin, without prior exposure to a mutagen. The mutant isolates remained resistant after several months of subculture in the absence of fungicides. The rate of growth in culture of isolates resistant to benzimidazoles was between 10 and 20% less than that of sensitive isolates, whereas isolates resistant to the dicarboximides grew and sporulated just as well as sensitive isolates. Genetic studies showed that the resistance to benzimidazoles was probably conferred by a single gene mutation, and that the same may also have been true of resistance to dicarboximides.

Relative parasitic fitness of isolates of Pyricularia oryzae Cav. with different sensitivities to fungicides

AbstractThe relative parasitic fitness of isolates of Pyricularia oryzae with different sensitivities to isoprothiolane (di‐isopropyl 1,3‐dithiolan‐2‐ylidenemalonate) and IBP (S‐benzyl O, O‐di‐isopropyl phosphorothioate) was studied in the absence of fungicides. Four field isolates (S‐1,S‐2,MR‐1 and MR‐2) and two in‐vitro mutants (Rvt‐1 and Rvt‐2) were used for disease epidemics. S‐l and S‐2 were wild types; MR‐1 and MR‐2 were sensitive to isoprothiolane and moderately resistant to IBP, and Rvt‐1 and Rvt‐2 were resistant to both fungicides. Twelve epidemics were made by inoculating rice seedlings with mixed conidial suspensions of two isolates or mutants. The value of relative parasitic fitness (W = 0‐1.0) was calculated for each isolate and epidemic. S‐l and S‐2 were stronger (W = 1.0) than MR‐2, Rvt‐1 and Rvt‐2; but weaker (W = 0.75, 0.73, respectively) than MR‐1. MR‐1 was strongest among all isolates and mutants used. MR‐2 was slightly weaker (W = 0.9‐1.0) than S‐l and S‐2, but stronger than Rvt‐1 and Rvt‐2. Rvt‐1 and Rvt‐2 had smaller values of W, ranging from 0.25‐0.58, in the epidemics with each field isolate. These results suggest that the proportions of in‐vitro mutants do not increase unless intensive selection pressure is given, and would be expected to decrease rapidly after the selection pressure is removed. Isolates moderately resistant to IBP, such as MR‐1 and MR‐2, however, had high values of W, suggesting that they would increase or would not decrease rapidly in the absence of selection pressure. These results may well explain why isolates highly resistant to isoprothiolane and IBP have seldom been found, and why a large number of isolates moderately resistant to IBP but sensitive to isoprothiolane have been observed in the field.

Some factors limiting the growth and yield of winter wheat and their variation in two seasons

SummaryMulti-factorial experiments on winter wheat cv. Hustler in autumn 1979 and 1980 sown on a clay loam soil following potatoes tested the effects of combinations of the following eight factors, each at two levels: sowing date; amount of nitrogen; division of nitrogen; timing of nitrogen; irrigation; autumn pesticide (aldicarb); summer aphicide (pirimicarb); and fungicide (carbendazim, tridemorph, maneb and captafol).The mean grain yields of all plots in 1980 and 1981 were respectively 9·6 and 8·3 t/ha; the best eight-plot means were 11·2 and 9·9 t/ha. Fungicides had the largest effect on grain yield, increasing it by 0·8 and 1·7 t/ha in 1980 and 1981, mainly by increasing grain size. Effects were greater with earlier sowing and the larger amount of nitrogen. Benefits from fungicide were well related to the control of leaf diseases, mainly Septoria spp., which became severe after anthesis. Fungicide temporarily decreased the number of saprophytic fungi on the developing ears. Aphids that appeared in autumn on plots sown in mid-September were controlled by autumn pesticide, which also prevented the spread of barley yellow dwarf virus that occurred only in 1981. Consequently, yield of early-sown plots in 1981 was increased by autumn pesticide, but only when the severe infection with leaf diseases was controlled by fungicide. Autumn pesticide decreased nematode populations. Aphid populations in summer were small and yield was unaffected by the decrease in numbers that followed application of an aphicide.Sowing on 20 or 15 September, as compared with 19 or 30 October, caused faster growth and development and greater uptake of N from the soil early in the season. Effects were smaller after April: earlier sowing increased total dry weight by 2·5–3·0 t/ha and, when leaf diseases and barley yellow dwarf virus were controlled, increased yield by 0·9-l·0t/ha. Increasing the amount of N applied by 70 kg/ha (from 105 or 80), increased yield only in 1980 and then only when fungicide was used. Extra N decreased yield in 1981 in the absence of fungicide. Extra N always increased N uptake and decreased grain size. Applying mostor all of the Non 4 or 19 March instead of 15 or 23 April resulted in less uptake of N from anthesis onwards and smaller yield, especially in 1980. N in three applications instead of one had negligible effect. Trickle irrigation decreased yield slightly, despite delaying leaf senescence and increasing straw weight. Attributes of wheat on best yielding plots differed little between years. Average values were: 534 ears/m2; 40·7 grains/ear; 40·4 mg/grain; 18·8 t/ha total dry matter and 214 kg N/ha uptake by grain plus straw.

Resistance of Botrytis cinerea to dicarboximide fungicides in protected crops

SUMMARYDicarboximide fungicides have been used for the control of grey mould in protected crops in Crete since 1977. During the 1980 growing season a decline of their efficacy was observed. In successive surveys carried out in May 1980, February 1981 and May 1981 in 28, 10 and 13 plastic houses repectively, a considerable proportion of resistant strains was found. From each of the plastic houses sampled mostly either only resistant or only sensitive strains were isolated. In three of the plastic houses with resistant strains there was an acute disease control problem. The ED50 of 15 resistant strains studied was in the area of 3·5 μg/ml vinclozolin as compared with 0·2 μg/ml for the wild type strains. The vinclozolin‐resistant strains were also resistant to procymidone, iprodione, and dicloran. In most of the cases strains resistant to vinclozolin were also resistant to benomyl and strains sensitive to vinclozolin were also sensitive to benomyl. In the absence of fungicide, resistant strains grew more slowly on PDA than sensitive ones, but spores germinated equally well. Vinclozolin (0·75 mg a.i./ml) did not protect eggplant seedlings against resistant strains but gave satisfactory control of sensitive ones.

Characterization of a dicarboximide-fungicide-resistant laboratory isolate ofMonilinia laxa

Monilinia laxa, the incitant of blossom blight in stone fruits in Israel, is sensitive to 5μg/ml of the dicarboximide fungicides vinclozolin and iprodione in the growth medium. When a large number of spores, from an isolate never exposed to these fungicides, was seeded on a medium containing 15 μg/ml iprodione, spontaneous resistant mutants appeared at 10-5 frequency. These mutants showed cross resistance to the dicarboximide fungicides vinclozolin, procymidone, l-(3,5-dichloro-phenyl)-3-propen(2)-pyrrolidin-2,5-dion (Co 4462), l-(3,5-dichlorophenyl)-3-methoxymethyl-pyrrolidin-2,5-dion (Co 6054), and to dicloran. Growth rate on fungicide-free medium was similar to that of the parental sensitive strain but sporulation was much reduced. Growth rate on media supplemented with dicarboximide fungicides decreased gradually with increasing fungicide concentrations. The resistance has been stable for more than one year in the absence of fungicides. Artificial inoculation of cherry, apricot and plum fruits previously treated with 500 μg/ml vinclozolin, iprodione, procymidone or Co 6054, with a resistant strain, resulted in brown rot; similar treatments provided full protection of the fruits against the sensitive strain.