A total of 618 isolates of corynespora leaf spot fungus (Corynespora cassiicola) collected from 24 commercial cucumber greenhouses in 12 cities in Ibaraki Prefecture, Japan, were tested for their sensitivity to boscalid. Boscalid-resistant isolates were detected in 17 out of 19 greenhouses with a history of use of this fungicide and detection frequencies of the resistant isolates exceeded 47% in nine greenhouses. Frequencies of very highly resistant (VHR) isolates with 50% effective concentration (EC 50 ) values of boscalid exceeding 30 lg mL )1 were higher than those of moderately resistant (MR) isolates with EC 50 ranging from 2AE0 to 5AE9 lg mL )1 in 11 greenhouses. Additionally, highly resistant (HR) isolates with EC 50 from 8AE9 to 10AE7 lg mL )1 were first detected. Furthermore, molecular characterization of genes encoding succinate dehydrogenase (SDH) subunits (SdhA, SdhB, SdhC and SdhD) was carried out to elucidate the amino acid substitution responsible for the resistance to boscalid. All 23 VHR isolates had the same mutation from CAC to TAC in the SdhB gene leading to the substitution of histidine with tyrosine at amino acid position 278 (B-H278Y). At the same position, the substitution to arginine conferred by a mutation to CGC (B-H278R) was detected in all four HR isolates. Some MR isolates showed a substitution from serine to proline at position 73 in SdhC (C-S73P), from serine to proline or from glycine to valine at position 89 (D-S89P) and 109 (D-G109V), respectively, in SdhD. There was no common mutation in SDH genes of all MR isolates.
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 (EC 50 ) values for 220 isolates from crops without a prior history of boscalid use ranged from 0AE5 to 7AE5 lg mL )1 and from 0AE04 to 0AE59 lg mL, 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 lg mL )1. Moreover, resistant isolates were divided into two groups: a moderately resistant (MR) group consisting of 189 isolates with EC 50 values ranging from 1AE1 to 6AE3 lg mL, and a very highly resistant (VHR) group consisting of 25 isolates with EC 50 values higher than 24AE8 lg 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.
The functional responses of each instar larva and adult of the coccinellid Harmonia axyridis to adults of the green peach aphid Myzus persicae were estimated under laboratory conditions. Linear parameter estimates from a logistic model of the proportion of M. persicae consumed by H. axyridis were negative at all development stages. Although a realistic estimate for the handling time of first instar larvae could not be produced, functional response curves of fitting the data with a random predator equation exhibited a Type II curve in most development stages. Fourth instar larva had the highest attack rate and shortest prey handling time. The implications of these results are discussed with more effective stages of H. axyridis in the context of biological control.
Females of the migrant skipper, Parnara guttata guttata, that are reared under lower temperatures lay smaller eggs. The adaptive significance of egg size plasticity in response to temperature is unknown in this species. We suggest, based on the following experimental results, that P. g. guttata uses temperature as an indirect cue to predict the host condition (leaf toughness) of the next generation. First, larvae were reared under the typical conditions of temperature and photoperiod experienced during the immature stages in the first, second, and overwintering (third) generations (LD 16:8 at 25°C, LD 14:10 at 25°C and LD 14:10 at 20°C). Females reared under LD14:10 at 20°C produced more, smaller eggs than those reared under LD14:10 and LD16:8 at 25°C. Secondly, survival rates of first instar larvae derived from females reared under the three photoperiod/temperature treatments were measured on young soft rice leaves (“soft”), or tough, old rice leaves (“tough”). Survival rates of hatchlings reared on soft and tough leaves did not differ when females were reared under LD16:8 and LD14:10 at 25°C. However, hatchling survival was significantly higher on soft than on tough leaves when females were reared under LD14:10 at 20°C. Thirdly, we found that egg size plasticity in response to temperature in P. g. guttata may be a threshold response. Temperatures below 20°C experienced during the immature stages may be effective for production of smaller and more eggs in the overwintering generation of P. g. guttata.
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