Plant sensitivity to atrazine and chlorsulfuron residues in a soil-free system

The sensitivity of 22 major crops, pastures and weeds from the north-east grain region of Australia to atrazine and chlorsulfuron residues was determined in a glasshouse using a soil-free bioassay system. A logistic equation was fitted to the seedling fresh weights as a function of the logarithm of herbicide concentration by non-linear regression and used to calculate the doses for 10%, 30% and 50% inhibition of seedling growth (ID₁₀, ID₃₀ and ID₅₀). The ID₅₀ for atrazine ranged from 0.03 to 0.04 mg a.i. L^–1 for Salvia reflexa Hornem. and barley to 1.47 mg a.i. L^–1 for sorghum. The ID₅₀ for chlorsulfuron ranged from 0.19 to 0.21 μg a.i. L^–1 for lucerne and snail medic to 102 μg a.i. L^–1 for wheat. Based on ID₅₀ values measured, the predicted responses of each species to a range of concentrations of atrazine and chlorsulfuron were classified into four categories ranging from no damage to severe damage. These sensitivity data will assist in planning cropping sequences in soils previously treated with atrazine or chlorsulfuron.     

A rapid reliable test for screening aryloxyphenoxy- propionic acid resistance within Alopecurus myosuroides and Lolium spp. populations

A reliable seedling bioassay was developed and tested for the rapid screening for resistance to aryloxyphenoxypropionic (APP) herbicides in Alopecurus myosuroides and Lolium spp. populations. It is based upon the difference in coleoptile length of resistant and susceptible A. myosuroides and Lolium seedlings, respectively, exposed to fenoxaprop-P acid and diclofop acid solution for 6 days in a plastic box. A 6 mg L⁻¹ fenoxaprop-P acid solution was selected as the best concentration for a reliable screening of resistant biotypes within A. myosuroides populations. At this concentration, coleoptile lengths of susceptible and resistant seedlings were shorter and longer than 10 mm respectively. Similarly, resistant seedlings within Lolium populations were easily detected at 10 mg L⁻¹ diclofop acid. At this concentration, coleoptile lengths of susceptible and resistant seedlings were shorter and longer than 20 mm respectively. For both populations, the coleoptile length distributions appear to discriminate between two kinds of APP-resistant biotypes (highly and slightly resistant).     

A qualitative quick-test for detection of herbicide resistance to tribenuron-methyl in Papaver rhoeas

A qualitative seed-based method useful for the detection of resistance to the herbicide tribenuron-methyl in Papaver rhoeas L. is described. Seeds were germinated on 35 mL of a 1.3% agar medium containing 2 g KNO₃ L^–1 in 8.5 cm Petri dishes in a growth chamber under 20 μmol s^–1 m^–2 of fluorescent light. When 0.24 μM tribenuron-methyl or more was added, growth in susceptible plants stopped after the cotyledon stage and they turned chlorotic. The resistant plants continued developing new leaves. The same effect was achieved when 0.2 g gibberellin (GA₃) L^–1 and 7.68 μM tribenuron-methyl or 0.5 g GA₃ L^–1 and 61.44 μM tribenuron-methyl were added. Germination percentage rose with gibberellin in the presence or absence of the herbicide. Plants developed rapidly, with only about 14 d needed to finish the test but sometimes root growth was reduced because of the addition of gibberellin. In the absence of gibberellin but in the presence of the herbicide, plants grew more slowly and developed smaller leaves with a 17-d evaluation period requirement. The test was validated with pot experiments in a greenhouse and also with field trials. The best combination was found to be 0.2 g GA₃ L^–1 and 7.68 μM tribenuron-methyl, assuring homogenous germination and testing of dormant seeds but avoiding root inhibition associated with too much gibberellin.     

Resistance to Sphaerotheca pannosa in roses induced by 2,6-dichloroisonicotinic acid

The susceptible rose cv. Madelon and the partially resistant cv. Sonia both responded with reduced development of rose powdery mildew when they were treated with the synthetic inducer 2,6-dichloroisonicotinic acid (INA). The EC₅₀ for number of colonies cm⁻² was approximately 0.4 mg L⁻¹ in both cultivars when treated 4 days prior to inoculation. However, conspicuous differences were observed with respect to number of spores per cm². For sporulation, the EC₅₀ was 0.37 mg L⁻¹ in cv. Madelon and only 0.08 mg L⁻¹ in cv. Sonia. A comparison with the pathosystems cucumber/ Sphaerotheca fuliginea and red cabbage/ Peronospora parasitica is made and the importance of the observed phenomenon for the selection of parents in a breeding programme for (partial) resistance is discussed.     

White bean (Phaseolus vulgaris) tolerance to preplant-incorporated herbicides

There is a limited number of registered herbicides in white beans. Field trials were conducted at two Ontario, Canada, locations (Exeter and Ridgetown) in 2001 and 2002 to evaluate tolerance of two white bean cultivars, AC Compass and OAC Thunder, to preplant-incorporated applications of S -metolachlor plus imazethapyr (1600 + 75 and 3200 + 150 g ai ha⁻¹, respectively), flumetsulam plus S -metolachlor ( premixed at 1443 and 2886 g ai ha⁻¹) and cloransulam-methyl (35 and 70 g ai ha⁻¹). There were no differences between the two cultivars in their responses to the herbicide treatments. S -metolachlor plus imazethapyr caused as much as 5% visual crop injury and decreased plant height up to 20%, shoot dry weight up to 39% and yield as much as 21%. Flumetsulam plus S -metolachlor caused as much as 7% visual crop injury and reduced plant height by up to 25%, shoot dry weight by up to 46% and yield as much as 24%. Cloransulam-methyl caused as much as 10% visual crop injury and decreased plant height up to 35%, shoot dry weight up to 55% and yield as much as 44%. There were no differences in seed moisture content among any of the herbicide treatments. This research suggests that the margin of safety of white bean is inadequate to support the preplant-incorporated registration of S -metolachlor plus imazethapyr, flumetsulam plus S -metolachlor and cloransulam-methyl in Ontario.     

Sensitivity of fungi from cereal roots to fluquinconazole and their suppressiveness towards take-all on plants with or without fluquinconazole seed treatment in a controlled environment

The take-all fungus, Gaeumannomyces graminis var. tritici , was highly sensitive to fluquinconazole ( in-vitro EC₅₀ 0·016–0·018 mg L⁻¹), a fungicide developed for use as a seed treatment to control take-all, and to prochloraz (EC₅₀ 0·006 mg L⁻¹). Fungi of other genera that were commonly isolated from cereal roots were sensitive in varying degrees to prochloraz but were relatively insensitive (e.g. Fusarium culmorum , EC₅₀ > 20 mg L⁻¹) or slightly sensitive (e.g. Epicoccum purpurascens , EC₅₀ 0·514 mg L⁻¹) to fluquinconazole. Gaeumannomyces graminis var. graminis and G. cylindrosporus , weak parasites that can protect roots against take-all, and an unnamed Phialophora sp., all closely related to the take-all fungus, were highly or moderately sensitive to fluquinconazole. Alternaria infectoria and E. purpurascens were most consistently effective in suppressing development of take-all on pot-grown wheat plants dual-inoculated with G. graminis var. tritici and the nonpathogen. Take-all was decreased more on dual-inoculated wheat plants grown from seed treated with fluquinconazole or fluquinconazole plus prochloraz than when only an antagonistic fungus ( A. infectoria , E. purpurascens , Fusarium culmorum or Idriella bolleyi ) or a seed treatment was applied. These fungi were less effective in combination with seed treatments on barley. Gaeumannomyces graminis var. graminis and G. cylindrosporus , tested on wheat, suppressed take-all only in the absence of fungicides. It is suggested that the performance of seed treatment containing fluquinconazole against take-all may in some circumstances be enhanced by its partial specificity for the take-all fungus.     

Sensitivity to cymoxanil in populations of Plasmopara viticola in northern Italy

In 1993, control failures were reported on grapevine in northern Italy under severe downy mildew pressure after postinfection application of cymoxanil in mixtures with copper or mancozeb. A monitoring survey was started immediately in Piedmont (north-western Italy) in order to determine the sensitivity of populations of Plasmopara viticola to cymoxanil from those vineyards where the fungicide was not controlling the disease satisfactorily. In 1994 and 1995, monitoring surveys were extended to north-eastern Italy, where cymoxanil mixtures were not performing as well as in the past. Sampled populations were tested on detached leaf discs and on whole potted plants under controlled conditions. In 1993, 12 populations, sampled in Piedmont, showed MIC values (minimum inhibitory concentration) varying from 10 to more than 100 mg L⁻¹ cymoxanil. With a baseline reference population having a MIC value of 3 mg L⁻¹, resistance factors ranged from 3 to more than 30. In 1994, 17 populations out of 27 sampled in Trentino (north-eastern Italy) showed MIC values of 100 mg L⁻¹ or higher and in 1995, 32 populations out of 38 showed the same behaviour. In similar experiments, the MIC values of populations from nontreated plots were between 3 and 10 mg L⁻¹. In whole potted plant tests, populations with MIC values higher than 200 mg L⁻¹ in a leaf disc test were not controlled by 500 mg L⁻¹ of cymoxanil. The results of our study suggest that resistance to cymoxanil in P. viticola may contribute to a lack of disease control in Italian vineyards.     

Antifungal vapour-phase activity of allyl-isothiocyanate against Penicillium expansum on pears

The fungitoxicity of allyl-isothiocyanate (AITC) vapour against Penicillium expansum , the causal agent of blue mould on pears (cvs Conference and Kaiser), was evaluated. The best control of blue mould was obtained by exposing fruits for 24 h in a 5 mg L⁻¹ AITC-enriched atmosphere, the extent of control depending on the inoculum density. Lesion diameter was inversely related to AITC concentration. In treated fruits the percentage of infected wounds increased with conidial concentration, with fewer than 20% affected at 1 × 10³ conidia mL⁻¹ to almost 80% at 1 × 10⁶ conidia mL⁻¹. In comparison, >98% of wounds were infected in untreated fruits irrespective of conidial concentration. AITC treatments were effective up to 24 h after inoculation for Conference and 48 h for Kaiser. AITC treatments also controlled a thiabendazole-resistant strain of P. expansum , reducing the incidence of blue mould by 90% in both cultivars. The use of AITC produced from pure sinigrin or from Brassica juncea defatted meal may be an economically viable alternative to synthetic fungicides against P. expansum .     

Phytotoxic activity of root absorbed glyphosate in corn seedlings (Zea mays L.)

Growth chamber experiments were conducted in order to study the absorption, translocation and activity of glyphosate when applied to roots with aqueous solution avoiding any glyphosate–substrate interaction. Corn seedlings at the first leaf stage were set up in individual graduated cylinders containing different solutions of ¹⁴C-glyphosate (0–30 mg ae kg⁻¹). After 26 h of root exposure, plants were transferred to fresh nutrient solution and grown for the next 5 days. After harvest, plants were separated into seed, root, mesocotyle, coleoptile, cotyledon, first leaf and all new leaves (apex), and quantified ¹⁴C radioactivity contained in each part. Glyphosate uptake was only 11% of the theoretical mass flow into the plant. The amount of glyphosate translocated from roots was positively correlated with plant uptake ( P  < 0.01). Total plant fresh weight presented a logistic response to glyphosate amounts, including a growth stimulant effect (hormesis), when plants absorbed less than 0.6 µg. The treated plants presented a normal pattern of glyphosate allocation, with the apex the principal sink, accumulating more than 38% of mobilized glyphosate. When corn plants absorbed more than 0.6 µg they showed a decrease in growth. The relatively high glyphosate quantities allocated in the new leaves showed the relevance of the symplastic pathway in the translocation process for root absorbed glyphosate.     

Influence of light-enhanced glycoalkaloids on resistance of potato tubers to Fusarium sulphureum and Fusarium solani var. coeruleum

Increases in resistance to Fusarium sulphureum and Fusartium solani var. coeruleum were determined in seed tubers of cultivars Desiree, Pentland Hawk and Kerr's Pink with light-enhanced glycoalkaloids. Glycoalkaloid concentration in the tubers ranged from 78.0 to 885.9 mg kg⁻¹ fresh weight. Exposure of tubers to light prior to inoculation decreased fungal invasion, the greater reductions generally reflecting longer light exposure periods. In most cases, no direct relationship between resistance and glycoalkaloid concentrations was recorded for F . sulphureum. Surface infection indices and reduced depth of rot caused by F. solani var. coeruleum were closely correlated with tuber glycoalkaloid content. No significant effects on spore length and width were recorded. Exposure to light of seed tubers that are not destined for human or animal consumption could provide a pretreatment to reduce fungal invasion by F. sulphureum and F. solani var. coeruleum .     

Prohexadione-calcium induces sunflower (Helianthus annuus) resistance against the root parasitic weed Orobanche cumana

Prohexadione-calcium (PHDC) is a plant growth retardant that inhibits gibberellin biosynthesis and transiently alters the spectrum of flavonoids and their phenolic precursors. In this study, root chamber and pot experiments were conducted to evaluate the effects of PHDC on Orobanche cumana seed germination and induction of sunflower resistance to this root parasitic weed. Root chamber experiments indicated that PHDC did not interfere with the progress of Orobanche seed germination, but retarded O. cumana tubercle formation and development. In pot experiments, control of O. cumana by PHDC depended on its dose, method of application and sunflower cultivar. In sunflower cv. HA89, PHDC at 10 mg LS⁻¹ (litre soil) as two drenches or at 1 m m as two foliar sprays significantly reduced the total number and dry matter of O. cumana without influencing flower, leaf and root development. In sunflower cv. Albena, PHDC at 20 and 30 mg LS⁻¹, but not at 10 mg LS⁻¹, as two drenches significantly reduced O. cumana infection. Sunflower cv. HA89 was less susceptible to O. cumana and more inducible by PHDC than sunflower cv. Albena. The biochemical analysis of root extracts revealed a significantly higher accumulation of free phenolics in sunflower cv. HA89 after PHDC soil drench, but not in Albena. No significant lignification was observed after PHDC treatments. The results suggest that PHDC reduces O. cumana infection in sunflower by inducing host resistance, not by directly interfering with the parasite. Free phenolics play an important role in sunflower response to the parasite.     

Association of induction of protease and chitinase in chickpea roots with resistance to Fusarium oxysporum f.sp. ciceri

To improve the breeding of chickpea varieties with resistance to Fusarium wilt, an attempt was made to analyse the biochemical basis of disease resistance by measuring levels of chitinase, β-1,3-glucanase, protease and proteinase inhibitor activities in dry and soaked seeds and in root and shoot tissues of wilt-resistant and wilt-susceptible cultivars. Marginal variation was observed in the levels of the candidate proteins in dry or soaked seeds. Chitinase activity was higher in roots than in shoots or cotyledons. No proteinase inhibitor activity was detected in root and shoot tissue of any of the cultivars. When the levels of these proteins were analysed in resistant (Vijay) and susceptible (JG-62) cultivars during development of wilt by growing plants in soil infested with F. oxysporum f.sp. ciceri , race 1, both cultivars showed induction of chitinase activity in the roots. However, the induced activity in JG-62 (3.82 U g⁻¹) was equivalent to the constitutive level in Vijay (3.90 U g⁻¹) and much lower than that induced in Vijay (5.18 U g⁻¹). Induction of protease activity was observed only in root extracts of Vijay when challenged by the pathogen. The root extract of Vijay showed in vitro antifungal activity in a plate assay. Simultaneous induction of proteolytic and chitinolytic activities specifically in the resistant cultivar was correlated with antifungal properties of root extracts effective in conferring resistance.     

Control of the invasive liana, Hiptage benghalensis

The liana, hiptage ( Hiptage benghalensis ), is currently invading the wet tropics of northern Queensland and remnant bushland in south-eastern Queensland, Australia. Trials using seven herbicides and three application methods (foliar, basal bark, and cut stump) were undertaken at a site in north Queensland (158 700 hiptage plants ha⁻¹). The foliar-applied herbicides were only effective in controlling the hiptage seedlings. Of the foliar herbicides trialed, dicamba, fluroxypyr, and triclopyr/picloram controlled >75% of the treated seedlings. On the larger plants, the cut stump applications were more effective than the basal bark treatments. Kills of >95% were obtained when the plants were cut close to ground level (5 cm) and treated with herbicides that were mixed with diesel (fluroxypyr and triclopyr/picloram), with water (glyphosate), or were applied neat (picloram). The costings for the cut stump treatment of a hiptage infestation (85 000 plants ha⁻¹), excluding labor, would be $A14 324 ha⁻¹ using picloram and $A5294 ha⁻¹ and $A2676 ha⁻¹, respectively, using glyphosate and fluroxypyr. Foliar application using dicamba for seedling control would cost $A1830 ha⁻¹. The costs range from 2–17 cents per plant depending on the treatment. A lack of hiptage seeds below the soil surface, a high germinability (>98%) of the viable seeds, a low viability (0%) of 2 year old, laboratory-stored fruit, and a seedling density of 0.1 seedlings m⁻² 12 months after a control program indicate that hiptage might have a short-term seed bank. Protracted recolonization from the seed bank would therefore be unlikely after established seed-producing plants have been controlled.     

Characterization of aryl acylamidase activity from propanil-resistant barnyardgrass (Echinochloa crus-galli [L.] Beauv.)

The enzyme, aryl acylamidase, was characterized in propanil-susceptible and propanil-resistant barnyardgrass with respect to kinetic parameters, the effects of inhibitors, and the levels of activity in dark- and light-grown tissues. The enzyme reaction in the resistant tissue preparation proceeded linearly with time over a 5 h time course, while activity in the susceptible tissue preparation was 2- to 4-fold lower and the activity tended to decrease after 2 h. The apparent K_m values were 62.1 mmol L⁻¹ and 3.1 mmol L⁻¹ for the enzyme activity in the susceptible and resistant tissue preparations, respectively. Two herbicides (anilofos and piperophos), previously shown to synergize propanil injury against the resistant biotype, were found to be potent inhibitors of the in vitro aryl acylamidase activity.     

Effect of method of inoculation, inoculum density and seedling age at inoculation on the expression of resistance of cocoa (Theobroma cacao L.) to Verticillium dahliae Kleb.

Soil drench and stem puncture inoculation were compared as methods for selecting cocoa cultivars with resistance to Verticillium dahliae. Disease progress was more rapid and induced symptoms were more severe following stem puncture and, under glasshouse conditions, differences between cultivars were detected 15 days after inoculation. Moreover, using stem puncture, inoculum densities of 10⁴ conidia/ml were sufficient to differentiate resistant and susceptible cultivars, whereas with the soil drench method, inoculum densities of 10⁷ conidia/ml were necessary. Although a substantially higher proportion of plants were affected by stem puncture inoculation, the resistance of cultivar Pound-7 remained effective at high inoculum densities of 10⁸ conidia/ml. With either method, older seedlings were more susceptible to V. dahliae than younger ones. However, with stem puncture, 15-day-old seedlings were sufficiently susceptible for a valid disease assessment. In contrast, with soil inoculation, 60-day-old plants were required. In a nursery trial with 15-day-old seedlings, seven cocoa genotypes previously selected as resistant, moderately resistant or susceptible to Verticillium dahliae , on the basis of root inoculation, were ranked in the same order when stem punctured. Stem puncture inoculation of young seedlings is cost-effective in terms of time and space, and is therefore recommended for screening of cocoa for wilt resistance, especially in large-scale breeding programmes.     

Sensitivity of Sclerotinia homoeocarpa to demethylation-inhibiting fungicides in Ontario, Canada, after a decade of use

In late 2003, nine populations of Sclerotinia homoeocarpa in Ontario Canada (seven of which had been previously sampled in early 1994, prior to the registration of sterol demethylation-inhibiting (DMI) fungicides for turf disease control in Canada) were sampled and tested for sensitivity to propiconazole. Four of the nine populations had not been treated with DMI fungicides during the intervening years, and isolates from these locations were sensitive to propiconazole (geometric mean EC₅₀ values of 0·005–0·012 µ g mL⁻¹, compared with 0·005–0·008 µ g mL⁻¹ for the original 1994 populations). Among the five populations from 2003 that had been exposed to DMI fungicides, mean EC₅₀ values were significantly greater, ranging from 0·020 to 0·048 µ g mL⁻¹. A significant correlation of determination was found between estimated number of fungicide applications and log EC₅₀ ( R ² = 0·832, P  = 0·0001), and the equation predicted that 42·3 applications of propiconazole would be needed to bring a sensitive population (EC₅₀ < 0·01  µ g mL⁻¹) to a resistant level (EC₅₀ > 0·10  µ g mL⁻¹). Fungicide sensitivity vs. duration of fungicide efficacy was also tested, and it was found that isolates with decreased sensitivity were able to more quickly overcome the inhibitory effects of fungicide application, reducing the duration of control from 3 weeks to 2 weeks.     

A green fluorescent protein-based screening method for identification of resistance in anthurium to systemic infection by Xanthomonas axonopodis pv. dieffenbachiae

Resistance of cultivars of Anthurium andraeanum to systemic infection by Xanthomonas axonopodis pv. dieffenbachiae , the causal organism of bacterial blight disease of anthurium, was investigated using a bioengineered bacterial strain containing p519ngfp plasmid. Successful infection establishment in anthurium was found to be cultivar and inoculum density dependent, but independent of plant age. Injection of cut petioles (stage-2 leaf) with 100  µ L inoculum (10⁹ CFU mL⁻¹) resulted in 100% infection establishment in susceptible cultivars on a repeatable basis, and differentiated between various levels of observed field resistance. Time to death (weeks) and proportion of dead plants best differentiated between levels of resistance and cultivars were placed in four groups based on these criteria. The susceptible group (32 cultivars) rapidly declined within 6–12 weeks of inoculation (WAI) and resulted in 100% plant death; the moderately resistant group (10 cultivars) declined within 12 WAI, but resulted in less than 100% plant death; the resistant category had less than 100% plant death with a slow decline taking over 20 weeks; and the highly resistant category (15 cultivars) showed 0% infection. The correlation coefficient between green fluorescent protein (GFP)-fluorescence and eventual death of plants was 0·90, indicating that the final death of individual plants can be reasonably well predicted based on GFP-fluorescence data at 5 WAI. Hence GFP data at 5 WAI can be used for early detection of latently infected plants and may assist screening for resistance in segregating populations of anthurium.     

Resistance in water yam (Dioscorea alata) cultivars in the French West Indies to anthracnose disease based on tissue culture-derived whole-plant assay

Reactions of 60 water yam ( Dioscorea alata ) cultivars to three isolates of the yam anthracnose fungal pathogen ( Colletotrichum gloeosporioides ) were evaluated using tissue culture-derived whole-plant assay. Three disease parameters: single score on a scale of 0–6 at the seventh day after inoculation (SD7); area under the disease progress curve (AUDPC); and disease progress rate ( R _d) were compared, and cultivars were classified into disease-response groups using a rank-sum method based on AUDPC scores for the two most virulent isolates. A wide range of variation in resistance of the D. alata cultivars, and significant effects of pathogen isolate and isolate–cultivar interactions, were observed for all disease parameters. The three disease parameters were positively correlated; however, four cultivars showed great dispersions from the regression lines for comparisons of SD7 with the multiple assessments based AUDPC and R _d. The 60 cultivars were separated into resistant ( n  = 12), moderately resistant ( n  = 19), moderately susceptible ( n  = 18) and susceptible ( n  = 11) groups. The potential of the tissue culture-derived whole-plant assay to resistance breeding programmes and further understanding of the yam anthracnose pathosystem is discussed.     

Sulphur accumulation after Verticillium dahliae infection of two pepper cultivars differing in degree of resistance

Elemental sulphur levels, sulphur localization in stems, and levels of sulphate, glutathione and cysteine were studied in pepper ( Capsicum annuum ) cvs Yolo Wonder (higher resistance) and Luesia (lower resistance) after inoculation with Verticillium dahliae , the cause of vascular wilt. Accumulation of elemental sulphur (S⁰) was first detected 10 days after inoculation in Yolo Wonder (mean S⁰ level 7·3  µ g g⁻¹ DW), and 15 days after inoculation in Luesia (mean S⁰ level 3·3  µ g g⁻¹ DW). The maximum level was reached 21 days after inoculation in Yolo Wonder (14·1  µ g g⁻¹ DW). In control plants, elemental sulphur was not detected. SEM-EDX (scanning electron microscopy-energy dispersive X-ray microanalysis) indicated that the sulphur was not restricted to a specific location, but was dispersed throughout the vascular tissue. Sulphate levels showed a decline at the end of the experiment in inoculated plants, possibly related to the increase in sulphur levels seen in the two cultivars. The differences in sulphate levels between the two cultivars may be due to faster sulphate breakdown in cv. Yolo Wonder.     

Occurrence of Corynespora cassiicola isolates resistant to boscalid on cucumber in Ibaraki Prefecture, Japan

A total of 651 isolates of cucumber corynespora leaf spot fungus ( Corynespora cassiicola ) collected from cucumber in Japan, either with (438 isolates) or without (213 isolates) a prior history of boscalid use, were tested for their sensitivity to boscalid by using a mycelial growth inhibition method on YBA agar medium. Additionally, seven isolates of C. cassiicola obtained from tomato, soybean, eggplant (aubergine) and cowpea in different locations in Japan were tested before boscalid registration. Minimum inhibitory concentration (MIC) and 50% effective concentration (EC₅₀) values for 220 isolates from crops without a prior history of boscalid use ranged from 0·5 to 7·5 μg mL⁻¹ and from 0·04 to 0·59 μg mL⁻¹, respectively. Two hundred and fourteen out of 438 isolates collected from ten cucumber greenhouses in Ibaraki Prefecture, Japan, which received boscalid spray applications showed boscalid resistance, with MIC values higher than 30 μg mL⁻¹. Moreover, resistant isolates were divided into two groups: a moderately resistant (MR) group consisting of 189 isolates with EC₅₀ values ranging from 1·1 to 6·3 μg mL⁻¹, and a very highly resistant (VHR) group consisting of 25 isolates with EC₅₀ values higher than 24·8 μg mL⁻¹. MR isolates were detected from all ten greenhouses, but VHR isolates were detected from only three. As a result of fungus inoculation tests which used potted cucumber plants, control failures of boscalid were observed against resistant isolates. Efficacy of boscalid was remarkably low against VHR isolates in particular. This is the first known report on boscalid resistance in Japan.