Development of resistance to thiabendazole in Helminthosporium solani (silver scurf) as a result of potato seed tuber treatment

Applying thiabendazole to potato seed tubers affected with silver scurf caused by Helminthosporium solani sensitive to thiabendazole decreased the severity of disease on progeny tubers at harvest, but about 50% of the isolates from these were resistant to the fungicide. The disease was not decreased when samples of the progeny tubers were treated with thiabendazole and planted in the following year, and the incidence of resistant isolates increased. Resistant isolates continued to be present when tubers were planted in the next 2 years without fungicide treatment. Treatment with a mixture of thiabendazole and imazalil also decreased the disease and fewer isolates were resistant than when treated with thiabendazole alone, although the proportion increased after treatment with the mixture in the following year. When seed tubers were infected with thiabendazole-resistant H. solani , silver scurf on progeny tubers was not affected by thiabendazole applied to the seed tubers but was decreased by the mixture of thiabendazole and imazalil. Imazalil was equally effective against H. solani sensitive or resistant to thiabendazole.
Some isolates of H. solani had grey aerial mycelium and of 516 of these isolates obtained in 4 years 29% were resistant to the fungicide. Other isolates produced small, black colonies and their frequency increased with thiabendazole treatment of seed tubers. Of 244 of these isolates, 62% were resistant.     

Characterization of Fusarium spp. responsible for causing dry rot of potato in Great Britain

A 3-year survey was undertaken to establish the relative frequency of different Fusarium spp. present as inoculum on potato tubers collected from four regions of Great Britain. A total of 219 samples (comprising 10 950 tubers) were collected from the 2000, 2001 and 2002 crops and processed to recover dry rot-producing isolates. In total, 228 isolates of Fusarium spp. were recovered. Most (94·7%) of these isolates were attributed to one of four Fusarium species: F. coeruleum, F. avenaceum, F. culmorum and F. sambucinum (formerly F. sulphureum) . The incidence of the combined Fusarium spp. increased the further south the crops had been grown. Fusarium coeruleum was the most commonly isolated species in each survey year, comprising 37 to 52% of the total Fusarium species. Selected isolates of each species were evaluated for their ability to produce rots in potato tubers. Fusarium sambucinum was a more aggressive pathogen than the other Fusarium species in eight out of 10 cultivars. Fusarium avenaceum and F. culmorum were relatively weaker pathogens. However, these species were aggressive on some cultivars, notably Hermes. The selected isolates were also assessed for their sensitivity to the fungicides thiabendazole and imazalil. Using in vitro tests, 65% of F. sambucinum isolates were resistant to thiabendazole and 7% of F. avenaceum isolates were resistant to imazalil. Tubers treated with imazalil yielded a higher proportion of isolates of F. avenaceum than those that were untreated. Similarly, a higher proportion of F. sambucinum isolates were recovered from tubers treated with thiabendazole than from those that were not treated.     

The occurrence of thiabendazole-resistant isolates of Polyscytalum pustulans and Helminthosporium solani on seed potato tubers in relation to fungicide treatment and storage

Tubers infected with thiabendazole-sensitive isolates of Polyscytalum pustulans and Helminthosporium solani were treated annually for 4 years with benomyl, thiabendazole or a formulated mixture of thiabendazole and imazalil, and grown at three farms in Scotland. The proportion of thiabendazole-resistant isolates of H. solani increased, and isolates producing black colonies became more common with successive annual applications of thiabendazole or benomyl. Silver scurf was not reduced after three annual applications. When these fungicides were applied once to untreated seed the incidence of resistant isolates of H. solani was much less in 1988 than in 1991 when tubers had been grown on farms for 3 years from untreated seed.
The proportion of resistant isolates of P. pustulans increased with the number of successive applications of thiabendazole or benomyl but at differing rates on each farm. At one farm, skin spot was not reduced by three annual applications of these fungicides whereas at the other farms it was reduced by 90–100% by four annual applications.
A smaller proportion of resistant isolates of P. pustulans and H. solani was obtained after applying the mixture of thiabendazole and imazalil than after benomyl or thiabendazole alone. Their occurrence was not related to the number of fungicide applications. The mixture also reduced both diseases by more than 75% over the 4 years.
At one farm where resistant isolates of H. solani were present, tubers were infected when stored on trays but not when stored in bags.     

Resistance to thiabendazole in Fusarium species isolated from potato tubers affected by dry rot

Fusarium species obtained from stored potato tubers affected with dry rot were grown on agar containing thiabendazole. All 40 isolates of F. coeruleum and 60 isolates of F. avenaceum tested were sensitive to the fungicide, but 68% of the 85 isolates of F. sulphureum and one isolate of F. culmorum were classified as resistant. When isolates were made from dry rots on tubers that had been treated with thiabendazole during loading into store, all 81 isolates of F. sulphureum were resistant, whereas all the isolates of F. coeruleum (25), F. avenaceum (4) and Phoma foveata (10) were sensitive. Resistance was not found in five isolates of Cylindrocarpon destructans. All the Fusarium spp. were sensitive to imazalil and were pathogenic when inoculated into potato tubers. Resistant and sensitive isolates of F. sulphureum caused rots of similar size.     

Resistance to thiabendazole in isolates of Polyscytalum pustulans (skin spot) and Fusarium solani var. coeruleum (dry rot) in Scotland

Between 1988 and 1991, isolates of Polyscytalum pustulans were obtained from potato tubers grown in Scotland and tested for sensitivity to thiabendazole (TBZ) on agar. Sensitive isolates did not grow at 1 mg TBZ/1, whereas growth of resistant isolates was unaffected at 1 mg and reduced by 10–30% at 10 mg/1. Resistant isolates were present on all classes of seed potatoes, and were also recovered from the air of stores during grading. Isolates from a stock of seed tubers from Northern Ireland were resistant. All 44 isolates of Fusarium solani var. coeruleum collected in 1991 were sensitive to TBZ.     

Resistance to thiabendazole in isolates of Helminthosporium solani, the cause of silver scurf disease of potatoes

Isolates of Helminthosporium solani were obtained from potato tubers and cultured on agar containing thiabendazole. Some grew on agar containing 100 mg/l and these were classified as resistant, whereas sensitive isolates produced no mycelial growth on agar containing 5 mg/l. Isolates were also found that were intermediate in their sensitivity to the fungicide. All isolates from Scottish virus tested stem cutting stocks (VTSC) on two farms were resistant whereas those from another farm were sensitive to the fungicide. The proportion of resistant isolates from eight English once-grown seed stocks ranged from 0 to 96%. There has been no previous report of fungicide resistance in H. solani and the possible reasons for these results and their implications are discussed.     

Increased Incidence of Erwinia Soft-rot on Calla Lilies in the Presence of Phosphorous

Erwinia soft-rot is an important disease of many ornamental potted crops and is one of the most limiting factors in greenhouse calla lily (Zantedeschia spp.) production. Experiments were conducted to test the effect of phosphorous added to soil-less mixes or to nutrient solutions used for irrigation on soft-rot caused by Erwinia carotovora subsp. carotovora (Ecc). Soft-rot incidence increased to 51% when soil-less mix was amended with superphosphate in comparison to regular soil-less mix (no superphosphate added) (31%). In contrast, addition of phosphorous in the nutrient solution met the phosphorous needs of the plant without enhancing soft-rot. Plant height, fresh mass, and number of flowers per plant were greater in calla lilies irrigated with nutrient solution containing phosphorous than no phosphorous treatments. Similar results were obtained in tests conducted in a commercial greenhouse with larger sample size. No statistical differences were found between tubers sprayed with water (control) or with a 0.5 mM solution of KH₂PO₄ in laboratory experiments to determine the effect of phosphorous on tuber root development. In other experiments, tubers were sprayed with either water, a bacterial cell suspension 1 × 10² cfu ml^–1, a solution of 0.5 mM KH₂PO₄, or a suspension of bacteria in KH₂PO₄. The results from these tests showed a significant increase of soft-rot development in tubers treated with the suspension of Ecc prepared in a solution of KH₂PO₄ relative to other treatments. Further laboratory tests indicated that enzymatic activity (polygalacturonase and pectate lyase) of Ecc increased when grown in the presence of phosphorous. These experiments suggest that increased soft-rot in the presence of phosphorous is due to increased virulence of Ecc.     

Effect of additives in aqueous formulation on the foliar uptake of dimethomorph by cucumbers

BACKGROUND: The efficacy enhancement of dimethomorph formulation by several adjuvants is thought to be through increased foliar uptake. In order to identify the most effective adjuvants, the adjuvancy of 36 additives was examined in aqueous formulations in relation to the absorption of dimethomorph by cucumber leaves. RESULTS: Polyethylene glycol monohexadecyl ethers with ethylene oxide (EO) contents of between 7 and 20, polyethylene glycol monooctadecyl ethers with EO contents of between 10 and 20 and polyethylene glycol monooctadecenyl ethers with EO contents of between 6 and 20 were effective adjuvants for promoting dimethomorph uptake from both aqueous acetone solutions and aqueous wettable powder (WP) suspensions into cucumber leaves. Polyethylene glycol monododecyl ethers with EO contents of between 7 and 9 were effective in promoting dimethomorph uptake from aqueous WP suspensions but less effective relative to the other adjuvants tested with aqueous acetone solutions. Foliar uptake of dimethomorph was also facilitated by the addition of methyl hexadecanoate, methyl octadecenoate and methyl octadecadienoate. CONCLUSIONS: Although the foliar uptake of dimethomorph from both aqueous WP suspensions and aqueous acetone solutions was greatest in the presence of fatty alcohol ethoxylates generally having a C₁₆ or C₁₈ lipophile, uptake from aqueous surfactant–acetone solutions was, on average, 7.6‐fold greater than that from aqueous WP suspensions containing surfactant. Copyright © 2009 Society of Chemical Industry     

Assessment of fungicides for the prevention of storage rot of yam tubers

Twelve fungicides were screened against the soft rot pathogens of yam tubers. Benomyl and thiabendazole were the only fungicides which completely prevented infection either by spore suspensions or mycelial discs of the pathogens. Captan, iprodione and mancozeb prevented infection by spore suspensions, but did not give complete protection against infection from the mycelial disc of all the pathogens.     

Occurrence of the A2 mating type and oospores ofPhytophthora Infestans in potato crops in Israel

Seventeen metalaxyl-sensitive and 21 metalaxyl-resistant isolates ofPhytophthora infestans collected from blighted potato fields during the years 1983–1988 were tested for mating type on rye seed agar medium. All isolates except one (MS3, collected in 1986 at Sufa, in the western Negev) were found to belong to the A2 mating type. A2 isolates produced oospores within 2 weeks when cultured together with isolate 163 (A₁ from the U.S.A.) or with the A₁ isolate MS3 from Israel. When cultured singly, A₂ isolates produced some oospores within 4–8 weeks. Blighted potato tubers harvested from potato crops artificially inoculated with a mixture of A1+A2 sporangia were found to contain some oospores. No oospores were detected in blighted tubers harvested from A₂ + A₂ inoculated crops. It was concluded that the A2 mating type ofP. infestans has occurred in Israel since 1983 or even earlier. The rare occurrence of the A1 mating type was unexpected and indicated that sexual reproduction of the fungus in the country might be limited.     

Goitrogenic effects of ethylenethiourea on rat thyroid

Two 90-day feeding trials were conducted in which rats were treated with 75 or 100 ppm ethylenethiourea (ETU) in the diet. Parameters of thyroid function; serum T₄, T₃, and TSH; T₃ uptake; ¹³¹I uptake into thyroid; and thyroid to body weight ratios were measured at Days 46 and 91. In addition, a thyroid hydrolysis study (100 ppm trial) was conducted in an effort to trace the fate of incorporated ¹³¹I. Treated groups from the lower feeding level and treated females from the higher feeding level were functionally euthyroid whereas treated males (100 ppm) were somewhat hypothyroid despite hyperstimulation. ETU inhibits MIT utilization and the coupling of DIT residues to form T₄. The capacity of serum to bind T₃ was reduced; however, no other evidence for extrathyroidal effects of ETU were observed. Thus ETU mimics the mode of action of methimazole.     

Investigations on potato mop-top furovirus in Sweden1

Potato mop-top furovirus (PMTV) was isolated for the first time in Sweden in 1985. During 1986 and 1987, soil samples from potato fields, mainly in the south-west of Sweden, were tested for PMTV. The virus was detected in 49 out of 305 samples tested. PMTV was also found to be present in soil collected from potato riddles during potato grading in early spring. The soil samples were tested by using Nicotiana clevelandii and N. debneyi as bait plants in a glasshouse. Seven potato cultivars were grown in a PMTV-infested field. PMTV symptoms in the tubers, i.e. external and/or internal brown rings and arcs (spraing), were observed in cvs Evergood, Provita, Saturna and Ukama but not in cvs Bellona, Bintje or King Edward. Very few, or no, PMTV symptoms were found in tubers of susceptible cultivars when examined 2-3 weeks after lifting. In later examinations spraing appeared and increased. The increase was greater in tubers stored at 9°C than in those at 4°C, and in cut than in uncut tubers.     

Analysis of Colletotrichum musae populations from Brazil reveals the presence of isolates with high competitive ability and reduced sensitivity to postharvest fungicides

In this study, the sensitivity of 218 isolates of Colletotrichum musae to imazalil and thiabendazole was evaluated, as well the fitness and competitive ability of less sensitive isolates. There was a positive correlation between the sensitivity to the two fungicides, but the isolates were more sensitive to imazalil. The estimated effective concentration of the fungicide able to inhibit mycelial growth by 50% (EC₅₀) was used to select four isolates with the lowest and the highest values for both fungicides, which were considered as sensitive (S) and less sensitive (LS), respectively. The level of sensitivity was maintained after 10 successive transfers on fungicide-free medium. Both fungicides were effective in controlling the disease caused by S isolates of C. musae in detached banana fruit when recommended doses were used. However, only imazalil was able to control the disease caused by LS isolates. For both fungicides, analysis of fitness-related variables (mycelial growth, sporulation, germination, and virulence) showed no difference between the groups of S and LS isolates, but a large variation was observed within the group. The LS isolates to thiabendazole that showed a mutation (F200Y) in the β-tubulin gene did not have fitness penalties. Our results allow a better understanding of the sensitivity and fitness of isolates of C. musae from Brazil, and demonstrate the importance of periodic monitoring to determine the frequency of LS isolates in populations, aiming at more effective management of anthracnose in banana orchards in Brazil.     

Accumulation patterns and fungicidal effect of carbendazim and thiabendazole in peanut plants after soil injection

Carbendazim and thiabendazole applied to the rhizosphere of peanut plants by injection into the soil at doses of 0.1, 0.5 and 2.5 g per 1 m row, accumulated in the leaves during the first week after treatment, the concentrations being related to the doses applied. Two weeks after treatment the concentrations of both fungicides dropped sharply. During the rest of the season, whilst the concentration of carbendazim in the high-dose treatment was 0.1 mg/kg fresh weight, the concentration of thiabendazole increased from 0.1 to 2.0 mg/kg and remained at this level until harvest. At the end of the growing season treated plants were less infected with Cercospora leaf spot (Cercospora per sonata) than the untreated control, thiabendazole affording better protection than carbendazim. Plants treated with the highest dose of thiabendazole were the only ones that did not wilt, even though infected. Laboratory studies of the adsorption of these fungicides on soil revealed that carbendazim is less adsorbed and also less persistent than thiabendazole.     

The extraction and assay of thiabendazole in strongly adsorbing soils

Thiabendazole is strongly adsorbed on soils, and is not readily extracted by conventional solvents. two-phase solvent system consisting of a mixture of aqueous ammonium chloride solution (pH 10), ethanol and chloroform was developed which extracted thiabendazole from a range of soils including high-organic fen peat soils and mushroom compost. Subsequent partition of the extract between organic solvents and acid or alkaline solutions provided sufficient clean-up for assay by ultraviolet or fluorescence spectroscopy, of concentrations of less than 1 mg thiabendazole kg^?1 of soil. Investigation of the adsorption of thiabendazole from aqueous solution by soils indicated the importance of both soil organic and mineral matter and the adsorption of both neutral and protonated molecules.     

Effect of temperature on the transmission of Potato mop-top virus from seed tuber and by its vector, Spongospora subterranea

Potato mop-top virus (PMTV) causes disease in both the growing plant and tubers (spraing) of potato and is transmitted by the plasmodiophorid Spongospora subterranea , the cause of powdery scab. The effect of temperature during plant growth on the transmission of PMTV from infected seed tubers and from infested growing media was investigated in a series of glasshouse experiments. Symptoms developed on foliage of plants derived from infected seed tubers but none developed when PMTV was transmitted by S. subterranea in soil. The incidence of foliar symptoms was greatest on plants grown at 12°C, less at 16°C, few at 20°C and absent at 24°C. The transmission of PMTV from infected seed tubers was not significantly affected by temperatures between 12 and 24°C, but when the virus was transmitted by S. subterranea , minimal tuber infection occurred at 24°C and no differences were recorded at temperatures between 12 and 20°C. The incidence of powdery scab on tubers was greatest at 12 and 16°C and very low at 20 and 24°C. However, the incidence and severity of root galling caused by S. subterranea , was greatest at 20 and very low at 24°C. The incidence of powdery scab was greater on tubers of plants derived from infected seed tubers grown in a fluctuating temperature regime of 12 h at 20°C followed by 24 h at 12°C than on those grown at a constant 20°C, whereas the incidence of tuber infection by PMTV and spraing was similar for both regimes. This demonstrates that infection of roots can occur at a higher temperature than that for powdery scab on tubers and that this root infection can enable the transmission of PMTV into the potato plant.     

Mechanism of cellular absorption of imidazolinones in soybean (Glycine max) leaf discs

The mechanism of uptake of imazapyr, imazethapyr, and imazaquin into soybean leaf discs was determined. Uptake into the leaf discs was linear with respect to external concentration from 10^?8 to 10^?3 M. Metabolic inhibitors and uncouplers (DNP, FCCP, CCCP, NaN₃, NaCN, DCMU) decreased uptake 58 to 85 %. Uptake was also sensitive to temperature, with maximum uptake occurring at 23°C. The Q₁₀ for uptake between 13 and 23°C was 1-7. The uptake of all 3 imidazolinones was sensitive to the pH of the uptake solution. Maximum uptake occurred at pH 4. However, there were differences in the rate of uptake among the three herbicides. Imazaquin was absorbed the most rapidly followed by imazethapyr and then imazapyr. These differences in uptake reflected the differences in the lipophilicity of the chemicals at pH 4 and 7. The apparent K_ow of imazaquin, imazethapyr and imazapyr at pH 4 was 7-7, 1-4, and 0-1, respectively, and at pH 7 was 0-04, 0-02, and 0-004, respectively. Based on these results the mechanism of uptake is probably best explained by ion trapping.     

Gas chromatography–mass spectrometry analyses of volatile organic compounds from potato tubers inoculated with Phytophthora infestans or Fusarium coeruleum

Volatile organic compounds (VOCs) collected from potato tubers inoculated with Phytophthora infestans (late blight), Fusarium coeruleum (dry rot) or sterilized distilled water (as a control) were analysed using gas chromatography–mass spectrometry (GC–MS) and gas chromatography–flame ionization detection (GC–FID). A total of 52 volatiles were identified by GC–MS in the headspaces above P. infestans- and F. coeruleum- inoculated tubers after incubation for 42 days in the dark at 10°C. Of these VOCs, the six most abundant were common to both pathogens. These were benzothiazole (highest abundance), 2-ethyl-1-hexanol (second highest abundance), and at approximately equal third abundance, hexanal, 2-methylpropanoic acid-2,2-dimethyl-1-(2-hydroxy-1-methylethyl)-propyl ester, 2-methylpropanoic acid-3-hydroxy-2,4,4-trimethyl-pentyl ester and phenol. In addition, styrene also occurred at approximately equal third abundance in the headspace of F. coeruleum- inoculated tubers, but at lower abundance in the headspace of P. infestans- inoculated tubers. Some VOCs were specific to each pathogen. Butanal, 3-methylbutanal, undecane and verbenone were found at low levels only in the headspace of tubers inoculated with P. infestans , while 2-pentylfuran and copaene were found only in the headspace of tubers inoculated with F. coeruleum . Additionally GC–FID analysis identified ethanol and 2-propanol in the liquid exudate from both P. infestans - and F. coeruleum -inoculated tubers after incubation for 35 days, and in the headspace after incubation for 42 days. These data provide key information for developing a sensor-based early warning system for the detection of postharvest diseases in stored potato tubers.     

Variations among field isolates of Botrytis cinerea in their sensitivity to antifungal compounds

Seventeen field isolates of Botrytis cinerea were compared by determining their radial growth on synthetic media containing various amounts of 21 antifungal compounds. Twelve of these compounds were fungicides that are recommended for the control of Botrytis infections. There were marked differences between the isolates in their sensitivity to the compounds. Individual isolates displayed high levels of resistance to some of the fungicides, including benomyl, carbendazim, iprodione, thiabendazole, thiophanate-methyl, vinclozolin and zineb. The most potent growth inhibitors were benomyl and carbendazim (ED₉₅ values for most isolates <0.1 μg fungicide ml^?1 media), dichlofluanid, iprodione, nystatin, thiabendazole, thiophanatemethyl and vinclozolin (ED₉₅ values for most isolates < 1.0 μg ml^?1), and captan, chlorothalonil, dicloran and thiram (ED₉₅ values for most isolates < 6.0 μg ml^?1). Zineb was much less potent than the other recommended anti-Botrytis fungicides; it was no more effective than carboxin, dinocap and mancozeb (ED₉₅ values for most isolates > 25 μg ml^?1).     

Uptake and translocation of fungicides in wheat after seed treatment,as measured by disease response to Fusarium culmorum

The toxicities of five systemic fungicides [benomyl, carbendazim, methyl 4-(2- aminophenyl)-3-thioallophanate (NF48), thiabendazole and thiophanate-methyl] and of two non-systemic fungicides (guazatine and phenylmercury acetate) against Fusarium culmorum were compared on agar plates; their performance as seed treatments was measured by inoculating the shoot bases of wheat seedlings with F. culmorum in pot experiments. The two most effective compounds, benomyl and thiabendazole, and the less effective thiophanate-methyl, were evaluated in further seed treatment experiments in which leaf sheaths and roots of slightly older plants were infected. The three fungicides protected the first leaf sheath for more than 5 weeks, but thiophanate-methyl was least effective. Against root disease, they were effective when infection was mild, but only thiabendazole significantly controlled severe infection. Bioautography confirmed that fungicide levels in shoots were greater after benomyl and thiabendazole, than after carbendazim and thiophanate-methyl treatments, and that concentrations of fungicide after benomyl, carbendazim and thiophanate-methyl treatments generally declined between 5-13 weeks after treatment. Thiabendazole produced a second fungitoxic component detectable in plants after 34 days.