Studies on the fungicidal control of stem lesions caused by Botrytis cinerea on glasshouse tomatoes

In glasshouse pot experiments, uptake of benomyl, thiophanate-methyl and carbendazim from equivalent soil applications (in the range 0–003– 0–035 %^a-i- ^atarateof ²&4 ml/plant) gave equal levels of control of Botrytis lesions developing from inoculations of freshly exposed leaf scars on tomato stems. Spray applications of benomyl to exposed leaf scars controlled infection at concentrations down to 0025 % a.i. The effect of lower concentrations of the stem spray could be markedly enhanced either by the addition of a mineral oil (2 % Actipron) or by a prior soil application of benomyl at a low rate which on its own had little effect on lesion development. Protectant spray applications of glycophene and vinclozolin gave levels of control quite comparable to that of benomyl at equivalent concentrations. Evidence was obtained that the lesions formed at the artificially-inoculated leaf scars at the top of the stems of young pot-grown tomato plants were larger than those lower down. In spite of this, the level of disease control with soil applications of fungicides containing or generating carbendazim (MBC) was greater at the top than at the bottom, probably because of the normal migration of the fungicides and their accumulation at the extremities of the plant. In an observation trial in a commercial crop of tomatoes, benomyl applied either as five soil drenches at approximately monthly intervals, or as two drenches followed by five sprays at three-weekly intervals, or as five sprays alone gave marked reductions in plant loss and number of Botrytis stem lesions in both cvs Eurocross BB and Cudlow Cross. Those stem lesions which did develop, however, were generally as large as those on untreated control plants. Five sprays of dichlofiuanid gave similar levels of disease control. All the treatments gave apparently higher yields (statistically untested) in Eurocross BB, but less consistent responses were recorded in Cudlow Cross.     

The effect of benomyl on the infection of tomatoes by Fusarium oxysporum f.sp. lycopersici and Botrytis cinerea

In glasshouse experiments at Auchincruive, drench applications of benomyl (100 or 142 mg per plant) to the soil surface around the stem bases of pot-grown tomato plants before inoculation with Fusarium oxysporum f.sp. lycopersici reduced the penetration of the fungus up the stems and/or decreased the development of vascular discoloration and associated severity of wilting. Similar drenches applied after establishment of the fungus in the stems either halted or considerably retarded the growth of the pathogen up the vessels. This again was reflected in reduced vascular discoloration and wilt symptoms. In experiments with benomyl over 2 years at a commercial holding in Argyll, the application of soil drenches (at the rates above) shortly after planting out and again 5 weeks later, coupled with a programme of stem and foliar sprays (at 0–05 % a.i.) during the summer, reduced the development of stem lesions caused chiefly by Botrytis cinerea and increased the general survival of plants more than did drench or spray treatments alone. There were indications that ‘ghost spotting’ of the fruit, particularly where spray applications were made, was also slightly reduced, but the magnitude of the effect was not consistent.     

The effect of seed treatment with benzimidazole-based fungicides on infection of the foliage of overwintered salad onions by Botrytis cinerea

Seed treatments of carbendazim (Bavistin 50% W.p.) and thiophanate-methyl (Mildothane 50% W.p.) applied to overwintered salad onions at 250 g a.i./kg seed protected the foliage of plants from infection by Botrytis cinerea during the seasons 1973–1976. Crop establishment and yield were also improved. Seed treatment with calomel was not effective. Chemical analysis of treated non-viable seeds, retrieved from the soil, indicated that 73% carbendazim and 46% thiophanate-methyl remained attached to the seeds after 9 months in the soil. Analyses of onion leaves revealed that each fungicide was represented by similar quantities of carbendazim, 5 μg/g fresh weight in October 1975 reducing to 1 μg/g fresh weight in May 1976. Bioassay tests showed that the fungicide was acropetally distributed and was present in all leaves early in the season (October) but was absent from some new leaves formed in the following spring. Carbendazim-insensitive isolates of B. cinerea occurred after three seasons' use of this chemical. Sensitive isolates failed to grow on agar containing 1 μg/ml benomyl but all insensitive isolates (31 total) grew normally at this concentration and some were capable of growth on agar containing 1000 μg/ml benomyl. The emergence of foliar isolates of the fungus insensitive to the benzimidazole-based compounds used in the treatment of seeds indicated that these fungicides did not provide a permanent solution to the disease problem.     

Evaluation of fungicides for control of South American leaf blight of Hevea brasiliensis

Of 43 fungicides tested in vitro, 19 showed strong, seven moderate and 17 weak inhibition of germination of conidia and ascospores of Microcyclus ulei. The formation of lesions on Hevea brasiliensis leaf discs was also suppressed by the first category of fungicides as well as by the five adjuvants tested. Ascospores were not released when perithecia were treated with urea, thiabendazole or alcoholic mercury chloride at 10.00, 0.10, 1.00 g/1 respectively; other fungicides had no such inhibitory effect. In field trials, thiophanate methyl (0.07% a.i.) and benomyl (0.025% a.i.) were most effective in controlling leaf infection, followed by chlorothalonil (0.15% a.i.) and mancozeb (0.32% a.i.). Benomyl suppressed conidial sporulation, whereas one application of thiophanate methyl (0.14% a.i.) to perithecia inhibited ascospore release; half of this concentration applied to conidial lesions or pycnidia caused the perithecia formed subsequently to abort. Thiophanate methyl thus shows promise for SALB control and elimination and benomyl may be valuable as a supplement in later rounds of spraying to control conidial sporulation. After 6 days of showery rain (2 mm for 17 min per day), water collected from sprayed leaves still gave complete inhibition of spore germination. However, inhibition was markedly reduced after 6 days of heavy rain (over 8 mm for 24 min per day).     

Experiments on soil drenching with fungicides against take-all in wheat

In short term pot experiments benomyl, iprodione and KWG 0599 applied as soil drenches in several types of soil significantly suppressed take-all symptoms from inoculum placed just below wheat seeds planted 1×5 cm deep, and in sand but not other soils when seeds were 5 cm deep. Benomyl was, however, effective against inoculum below seed planted 5 cm deep in a loam-sand mixture when the drench contained an alcohol ethoxylate surfactant. Computer simulations of fungicide distributions in the soils correlated well with disease control observations. In long term outdoor pot experiments two drenches with benomyl (without surfactant) controlled disease significantly for at least 3 months against inoculum placed 15 cm deep. The significance of these results for the practical control of take-all by fungicides is discussed.     

Duration of effects of benomyl on growth and wilt (Verticillium dahliae) development in the strawberry, cv. Cambridge Vigour, in relation to time of inoculation

Single doses of benomyl, 0–4 g/plant in 1972 or 0–04 g in 1973, were applied in 100 ml water to the surface of the soil in 12-5 cm pots containing Cambridge Vigour strawberry plants, either before inoculation with Verticillium dahliae or at various times up to 56 days afterwards. Pre-inoculation treatments were terminated by washing the potting medium from the roots at the time of inoculation and their effects on pathogenesis and growth were relatively small. Post-inoculation treatments with benomyl prevented or suspended pathogenesis for at least as long as fungistatic activity could be detected in petiole segments by bioassay; this was for more than 150 days after the larger dose, 50–60 days after the smaller dose applied 7–28 days after inoculation and 30 days when applied 56 days after inoculation. There were no consistent differences in the uptake and persistence of benomyl in inoculated and non-inoculated plants. Early post-inoculation treatment caused some retardation of plant growth, but benomyl-treated inoculated plants were generally comparable in size with similarly treated non-inoculated ones, and much larger than untreated inoculated plants. Increasing the dose of benomyl applied to the soil apparently had little influence on the intensity of its effects but greatly increased their duration, probably because of the low solubility of the systemic chemical.     

Some effects of benomyl and carbendazim on Aphis fabae and Acyrthosiphon pisum on field bean (Vicia faba)

Field bean plants were treated with benomyl in a glasshouse, then nymphs of Aphis fabae and Acyrthosiphon pisum were caged on the second pinnate leaf. Soil drenches at concentrations of 150 and 75 μg benomyl/ml or above increased mortality of A. fabae and A. pisum respectively; 250 μg a.i./ml increased mortality and decreased progeny production of alate A. fabae. The effect on mortality persisted for at least 16 days after treatment. Foliar sprays increased mortality at concentrations of 75 μg a.i./ml and above (A. fabae). Field populations of A. pisum were reduced when bean plants were drenched or sprayed at a concentration of 250 μg a.i./ml and A. fabae populations were reduced by drenches but not by foliar sprays. A commercial formulation of carbendazim (Bavistin) increased aphid mortality whereas the formulation medium did not. Under the experimental conditions, benomyl affected the distribution of both species on young bean plants but did not induce a repellent effect; aphids preferred untreated leaves. Mortality and preference tests, and a field experiment, indicated that A. pisum was affected more than A. fabae.     

Control of smoulder (Botrytis narcissicold) in narcissus with fungicides

Smoulder, caused by Botrytis narcissicola, is the most widespread foliar disease of narcissus (Narcissus cultivars) in the UK and causes significant losses in bulb and flower yield. A range of current and novel fungicide treatments was examined to determine if control of the disease could be improved and to measure the effect of treatments on bulb yield. In assays on pot-grown plants, carbendazim+flusilazole, epoxiconazole, iprodione + thiophanate-methyl, tebuconazole and vinclozolin reduced lesion size by more than 90%, compared with untreated plants, when applied 1 day before inoculation with the fungus; vinclozolin and tebuconazole were also effective when applied 2 days after inoculation. In field experiments in Cambridgeshire and Lincolnshire, significant reductions were observed in secondary smoulder symptoms (leaf lesions and stem rot) following fungicide sprays. Spray programmes of the novel fungicides azoxystrobin, cyprodinil, kresoxim-methyl, tebuconazole and pyrimethanil were as effective, or more so, than the standard treatments (chlorothalonil, vinclozolin) used by growers. Mixtures of carbendazim + tebuconazole and azoxystrobin + tebuconazole also gave effective control of smoulder. Programmes of four to six sprays, using two or three fungicides with different modes of action, applied alternately, reduced smoulder by 35–69% and increased bulb yields by 7–59%. Although treatment around and after flowering resulted in the greatest control of secondary smoulder, treatment before flowering was required for the highest bulb yields. The choice of fungicides for use in spray programmes to provide effective and reliable control of smoulder, and the possibility of control using fewer sprays, are discussed.     

Benzoic acid induces tolerance to biotic stress caused by Phytophthora cinnamomi in Banksia attenuata

Banksia attenuata plants were treated with soil drenches or foliar sprays of benzoic acid (BZA) to determine induced resistance to Phytophthora cinnamomi. Stems of B. attenuata were inoculated with the pathogen 1 week after treatment with BZA. Resistance was estimated by measuring P. cinnamomi lesions on stems. Treatment with 0.10 mM, 0.25 mM or 0.50 mM BZA caused a reduction in lesion size with 0.50 mM BZA applied as a soil drench being the most effective treatment at suppressing the development of lesions. This is the first report of BZA induced host resistance in any plant species to any pathogen.     

Control of Verticillium wilt transmission in strawberry runners with systemic benzimidazole-type fungicides

Pre-planting root dips and soil drenches with benomyl, carbendazim (MBC), thiabendazole and thiophanate-methyl were used to reduce disease severity in mother plants. Transmission of the pathogen Verticillium dahliae through stolons and into young runners was prevented by two applications of benomyl or thiophanate-methyl at o-i% active ingredient applied at stolon appearance and 1 month later. Sufficient concentrations of fungistatic residues were detected in all plant tissues to suppress the internal spread of the pathogen.     

The control of apple canker (Nectria galligena) in a young orchard with established infections

In a factorial experiment on canker control the efficiency of phenylmercurie nitrate (PMN) applied at leaf fall, before bud burst or both was compared with the application of dodine, dithianon, triforine, thiabendazole, benomyl or carbendazim in May and June. Of the total number of cankers which developed in unsprayed trees 76–78% resulted from infections in April to August of each year of the experiment. Infection was reduced significantly by all of the fungicides applied in summer. Of these carbendazim was outstanding, controlling both summer and autumn infections. Sporulation throughout the summer was suppressed by carbendazim and to a lesser extent benomyl, but whereas the suppressant effect of carbendazim persisted until long after leaf fall that of benomyl was evident only until August. Dodine, dithianon, triforine and thiabendazole had no significant effect on spore production. Dithianon and dodine showed highest toxicity to the germination of Nectria galligena spores of all fungicides used in summer. All of the fungicides controlled apple scab (Venturia inaequalis) although thiabendazole was the least effective. Autumn applications of PMN reduced canker incidence but their value alone was not as great as carbendazim, dithianon, benomyl or dodine applied in summer. PMN applied in spring reduced the number of cankers in trees receiving no other fungicides but tended to increase the incidence of infections, particularly in autumn, when used in conjunction with fungicides applied in May and June. PMN applied before bud burst reduced sporulation of N. galligena for a few weeks, after which production resumed and in late summer and autumn actually exceeded the controls. None of the fungicides had any direct effect on leaf fall. Infection of the crotch and basal leaf scars was more common in summer than in autumn whereas infection of leaf scars above the basal region was more common in autumn than in summer. The rootstocks of many of the trees became infected via callus tissue associated with adventitious root development and this was controlled by the carbendazim treatment.     

Effect of some fungicides on the yield of grass swards

When benomyl, drazoxolon, thiabendazole or zineb were applied as soil drenches to established grassland drazoxolon reduced the incidence of rhizoplane fungi. Benomyl and thiabendazole significantly increased yield at one harvest but only benomyl had a significant effect on total yield. When benomyl was applied at monthly intervals from March to July significant increases in total yield were obtained with the June applications which also significantly increased yield at the first harvest in the following year. Repeated applications from March to July also had a significant residual effect on total yield. Yield increases were not significantly affected by increasing the benomyl level from 2.5 to 5.0 g/m².     

Comparison of fungicides for the control of Rhizoctonia solani causing datnping-off of mung bean (Phaseohis aureus)

Of 41 fungicides tested in the laboratory, copper carbonate, copper sulphate, mercuric chloride, Agrosan GN, quintozene, kasugamycin, carboxin, pyracar-bolid, carbendazim, chloroneb, benomyl, Ohric, RH 893 (2-n-octyl-4-isothiazole-3-one) and Terrazole were most inhibitory to the mycelial growth of Rhizoctonia solani on Czapek's agar plates and had EC₅₀ values of less than 1 μg a.i./ml, while copper oxychloride, Udonkor, zineb, ziram, F 319 (3-hydroxy-5-methyl isoxazole) and anilazine were much less toxic, ziram being least inhibitory with an EC₅₀ of 214 μg a.i./ml. Of 17 fungicides tested in the greenhouse as seed treatments, thiabendazole, carbendazim, benomyl, thiophanate-methyl, dichlozoline and Ohric gave 80–90% control of damping-off of mung bean seedlings. A single soil drench with thiophanate-methyl and two drenches with benomyl gave about 90% disease control, More seedlings with R. solani infection survived when thiophanate-methyl was used as a post-inoculation soil drench than when benomyl or chloroneb were used.     

Chemical Control of Basal Stem Rot Disease of Tomato (Lycopersicon esculentum Mill.) in Northern Nigeria

Six fungicides (Aatopam-N, Aldrex T, Calixin M, PCNB, captan and captafol) were evaluated at 200 mg a.i.l^?1 for their effectiveness in reducing basal stem rot disease incited by Sclerotium rolfsii on tomato in pre- and post-inoculation soil drenches in the glasshouse. The results showed that only PCNB effectively reduced disease severity when applied to soil 10 days before inoculation. Of the two application methods only pre-inoculation soil drenching with the fungicides was efficacious in reducing disease severity. In field trials conducted in Samaru with Aldrex T, captan and PCNB, only PCNB was effective in combating the severity of the disease. It reduced the disease by about 88% in one trial.     

The effect of Benlate and cytokinins on the content of tobacco mosaic virus in tomato leaf disks and cucumber mosaic virus in cucumber cotyledon disks and seedlings

Tomato leaf disks were inoculated with tobacco mosaic virus (TMV) and floated for 7 days on solutions of kinetin and benzyladenine in the range 20-0-002 mg/1. Virus content was reduced at the higher and increased at the lower concentrations. Benlate and benomyl showed a peak of cytokinin activity in the Amaranthus betacyanin bioassay equivalent to c. 0–002 fig/l kinetin. At concentrations above 25 and 100 mg a.i./l for Benlate and benomyl respectively, both compounds increased the TMV content of tomato leaf disks. Cucumber mosaic virus (CMV) content in cucumber cotyledon disks was increased by Benlate and benomyl treatment (50–100 mg/1). Applied as a soil drench (50–500 mg a.i./l) when the plants were inoculated, Benlate increased the CMV content of infected seedlings. The number of starch-iodide lesions (a measure of susceptibility) was unaltered in cotyledons treated with Benlate 7 days before or immediately after inoculation. Infectivity of crude infective cucumber sap was unaffected by benomyl incorporation, whereas Benlate reduced infectivity at higher concentrations (1000–5000 mg/1). Under the experimental conditions described, Benlate, benomyl, benzyladenine and kinetin had no effect on the chlorophyll content of tomato leaf disks, and intact seedlings.     

The effects of fungicide-coated seeds on the establishment of VA mycorrhizal infection

Commercial fungicide-coated seed of five plant species was tested to determine whether the fungicides used affected seedling infection by VA mycorrhizal fungi. All the plants became mycorrhizal, with no differences between those emerging from fungicide-coated or non-fungicide-coated seed. However, when the same fungicides were applied as soil drenches, mycorrhizal infection was greatly inhibited, and in some instances was absent throughout the duration of the experiment.     

Efficacy of benzimidazole and related fungicides against Rhizoctonia solani and R. bataticola

Five formulations of four benzimidazole derived fungicides, carbendazim, benomyl, thiophanate methyl and methyl 4-[2-(2-dimethylamino acetamide) phenyl]-3-thioallophanate were compared for their toxicity towards two pathogenic isolates of Rhizoctonia solani and three of R. bataticola. The isolates of two fungi showed significant differences in mycelial growth inhibition by the five fungicides. Benomyl and carbendazim were most inhibitory to all isolates of both fungi while the sesame isolate of R. bataticola was least sensitive to all fungicides. Disease control (90%) was obtained with low concentrations of benomyl against root rot of cowpea caused by R. solani, and with thiophanate methyl against root rot of sesame and sunflower, and leaf blight of mung bean caused by R. bataticola. The spread of stalk-end rot of sunflower heads was best checked with a spray of thiophanate methyl. The results suggest that benzimidazole fungicides having similar toxophores act differently for disease control in different host-parasite combinations.     

Tolerance in Isolates of Colletotrichum coffeanum to Prochloraz-Mn in Kenya

Coffee berry disease, caused by Colletotrichum coffeanum is a serious disease of Coffea arabica in Kenya. Control of this disease is achieved mainly through the use of fungicides which currently include chlorothalonil and prochloraz-Mn (Octave). Fungicide resistance in C. coffeanum to benzimidazoles has been well documented. Isolates of C. coffeanum highly sensitive to prochloraz-Mn have been shown to acquire tolerance to high levels of prochloraz-Mn in culture. These isolates were able to grow and sporulate in malt extract agar (MEA) amended with 250 μg ml^-1 prochloraz-Mn. The prochloraz-Mn tolerant isolates of C. coffeanum showed low level of tolerance to benomyl. The benomyl tolerant isolates of C. coffeanum equally showed low level of tolerance to prochloraz-Mn. The prochloraz-Mn and benomyl tolerant and sensitive isolates were found to be highly pathogenic and induced sporulating lesions on seedlings and berries of coffee cv. SL 28 whichis, very susceptible to C. coffeanum. Mixed inoculation tests using prochloraz-Mn and benomyl tolerant isolates and prochloraz-Mn tolerant and sensitive isolates of C. coffeanum showed that the prochloraz-Mn tolerant isolate was competitive.     

Reaction of incompatible isolate of Pyricularia grisea on rice leaves stripped of epicuticular wax reflects blast conduciveness of soils

Upland rice cultivars were evaluated in the greenhouse for susceptibility to the rice blast disease caused by Pyricularia grisea Sacc., on two upland soils from the Philippines previously considered to be “blast conducive” and “blast non-conducive”. Under monocyclic inoculation tests plants grown in conducive soil showed significantly greater lesion development than plants of the same cultivar grown in non-conducive soil: cultivars considered to be susceptible to the isolates used showed increased number of susceptible-type lesions; resistant cultivars showed increased number of hypersensitive resistant-type lesions. A similar effect was observed under polycyclic tests where several generations of the pathogen were allowed to develop on the test plants. Dilution of conducive soil with non-conducive soil resulted in a corresponding reduction of disease severity, although this was most pronounced on resistant cultivars. Removal of leaf epicuticular waxes (LEW) using organic solvents increased the number of resistant-type lesions on resistant cultivars grown in both soils following inoculation. Susceptible plants were not suitable for quantifying the relative blast conduciveness of a soil because of the extreme environmental sensitivity of the bioassay and the tendency of lesions to coalesce. Comparing numbers of resistant-type lesions on leaves of plants stripped of LEW and inoculated with an incompatible P. grisea isolate among plants grown in different soils proved to be a satisfactory means of distinguishing the relative blast conduciveness of soils under controlled conditions. This method was field tested in eastern India and results corroborated farmer assessment of which soils were blast conducive. Using incompatible isolate-cultivar combinations and LEW-free leaves is proposed as a simple bioassay for assessing blast conduciveness of soils and should prove useful in regional characterization of rice blast risk.     

Induced Resistance Against Plasmopara helianthi in Sunflower Plants by DL-β-Amino-n-butyric acid

Sunflower plants treated with the nonprotein amitio add, DL-β-attiino- n -butyric acid (BABA) were protected against infection with Plasmopara helianthi. Soil drenches at the highest rates (150-250 mg/kg soil), applied one day before the inoculation induced high levels of protection (80-83%) against the disease and more than 90% control was observed when BABA was applied at 300 mg/kg soil. However, at this concentration phytotoxic symptoms were observed. This compound also provided a curative activity when applied one day post-inoculation. BABA had no antifungal activity in vitro against P. helianthi. The effect of BABA on zoosporangia germination was evaluated by treating pre-germinated seeds with the compound solution and the zoosporangia suspension for 3 h. Then, seeds were sown and the percentages of infected plants were determined. The other two aminobutyric acid isomers (a and g) were ineffective against downy mildew. The mechanisms by which DL-β-amino- n -butyric acid protect sunflowers against downy mildew awaits more detailed elucidation.