High genetic diversity of Fusarium circinatum associated with the first outbreak of pitch canker on Pinus patula in South Africa

The disease known as pitch canker results from infection of Pinus species by the fungus Fusarium circinatum. This fungus also causes a serious root disease of Pinus seedlings and cuttings in forestry nurseries. Pinus radiata and P. patula are especially susceptible to the pathogen, but there are no records of pitch canker on P. patula in established plantations. To date, only planting material of this tree species in nurseries or in plantations at the time of establishment have been infected by F. circinatum. Symptoms of pitch canker have recently emerged in an established P. patula plantation in South Africa and this study sought to determine whether the symptoms were caused by F. circinatum. Isolates from cankers were identified as F. circinatum using morphology and DNA-based diagnostic markers. Microsatellite markers were then used to determine the genetic diversity of a collection of 52 isolates. The entire population included 17 genotypes representing 30 alleles, with a greater number of genotypes collected from younger (three- to six-year-old) than older (12- to 19-year-old) trees. Both mating types of F. circinatum were present, but no evidence of sexual recombination was inferred from population genetic analyses. This is the first record globally of pitch canker on P. patula trees in managed plantations. It is of significant concern to South Africa, where P. patula is the most important Pinus species utilised for plantation forestry.     

Optimizing aphid biocontrol with the predator Aphidoletes aphidimyza,based on biology and ecology

Aphidoletes aphidimyza is one of the most important predators used in the augmentative biological control of aphids, key pests of many crops worldwide. Adult females are very efficient in locating aphid infestations over a relatively long range, up to 45 m, and deposit eggs near or within aphid colonies. The predatory larvae are aphid generalists preying on several agriculturally important aphid species. The successful use of this biocontrol agent in agricultural systems depends on several biotic and abiotic factors. Among biotic factors, aphid species, plant structure, interspecific competition and intraguild predation may significantly impact the predator´s population dynamics. Key abiotic conditions include day lengths (above a critical threshold to prevent diapause), availability of mating sites in the crop, temperature (above 15 °C to enable egg laying), air relative humidity (above 70%) and availability of pupation sites. Although several successes have been reported in open field crops with naturally occurring or released populations, commercial releases are primarily used in protected crops. Optimized emergence boxes combining provisioning of food sources for the adults, integration with the technological advances that occurred in the greenhouse environment lately, insights into the nutritional ecology in open field crops and exploration of the genetic variability are proposed as future directions to improve adoption and efficacy of A. aphidimyza in crop protection. © 2018 Society of Chemical Industry     

Applicability of ground‐penetrating radar as a tool for nondestructive soil‐depth mapping on Pleistocene periglacial slope deposits

Soil depth plays a decisive role in determining soil properties in mountainous regions for ecological site assessment. To evaluate the use of ground‐penetrating radar (GPR) for fast and high‐resolution mapping within mountainous regions, we examined the possibilities and limitations of GPR to determine soil depth over bedrock and to delineate individual substrate layers formed during the Pleistocene in a periglacial environment (Pleistocene periglacial slope deposits, PPSD). Selected catenae in representative subregions of the study area (Dill catchment, SE Rhenish Massif, Germany) have been successfully mapped using GPR. A practicable method was developed using a 400 MHz antenna to reach a mean penetration depth of 1.5 m and to map different substrates and layers of PPSD based on calibrations of the GPR at soil pits along 12 catenae. Colluvium, the three types of PPSD layers, as well as the in situ bedrock could be distinguished in most sections of the GPR surveys. Characteristic GPR facies caused by intrinsic material properties of the different substrates, such as stone content and soil moisture content, could be distinguished in different geomorphologic and lithological settings. A layer‐based velocity distribution was determined for characteristic substrate layers at soil pits enabling us to considerably enhance the accuracy of soil‐depth prediction. Compared to traditionally surveyed soil profiles, our results demonstrate an accuracy of layer thickness surveying within a standard deviation of approx. 0.1 m. It is demonstrated that the combination of GPR with conventional soil‐pit mapping is an efficient and valid method to produce high‐resolution data of substrate distribution.     

Effects of Azoxystrobin on Mycotoxin Production in a Carbendazim-Resistant Strain ofFusarium sporotrichioides

Carbendazim-resistant (RS) and control (CS) strains ofFusarium sporotrichioides Sherb., previously developed in our laboratory, were exposed to graded concentrations of azoxystrob in in broth media under shake-culture conditions for 2, 3, 4 and 8 days. Azoxystrobin concentrations were 0, 1, 10 and 100 mg 1^-1 broth and cultures were incubated at a constant 25°C. Mycelial growth was significantly affected by strain (P<0.01), azoxystrobin concentration (P<0.001) and incubation time (P<0.001). Combined results for the four incubation times showed that CS yielded higher mycelial mass than RS (P<0.01) only in the absence of azoxystrobin. At fungicide additions of 1, 10 and 100 mg P^-1 mycelial growth was reduced (P<0.001) with minimal strain differences (P>0.05) at all three doses of azoxystrobin. Significant (P<0.05 or better) strain-fungicide interactions were recorded in trichothecene production following exposure to azoxystrobin. At 4 and 8 days of incubation, the 10 mg 1^-1 addition of azoxystrobin stimulated T-2 toxin synthesis (P<0.05) only in RS cultures. In contrast, T-2 toxin enhancement in CS cultures occurred only on day 8 but at a lower level of azoxystrobin (1 mg1^-1). Thus, the stimulation of T-2 toxin synthesis depended upon strain and azoxystrobin level. Production of diocetoxyscirpenol (DAS) was affected by a more complex set of interactions. Overall means showed that, in comparison with initial values (on day 2 or 3), DAS output maximized significantly(P<0.05) on day 4 in RS cultures and on day 8 in CS. Marked strain effects were observed on exposure to the 10 mg 1^-1 level of azoxystrobin. At this level, DAS production was enhanced in RS only after 4 (P<0.01 ) and 8 (P<0.05) days of incubation, while in contrast, CS reduced DAS production. As with T-2 toxin, DAS production in CS was stimulated (P<0.05 or better) only at low exposure levels of azoxystrobin. In the case of neosolaniol (NEO), however, the main effect of strain was significant (P<0.05), with CS producing consistently more of the mycotoxin than RS on day 4 of the experiment. At this point, the NEO:T-2 toxin ratio was also higher in CS (0.63) than in RS (0.12), a feature reported by us previously. In conclusion, the present investigation has shown for the first time that the development of resistance to one fungicide can affect trichothecene production inF. sporotrichioides on exposure to a second fungicide. These results have been incorporated into a new classification scheme for fungicide efficacy which is also presented in this paper. http://www.phytoparasitica.org posting Oct. 7,2001.     

Distribution of fenvalerate and permethrin residues on cattle hair following variable application rates of impregnated ear tags

Fenvalerate and permethrin residues on hair of groups of cattle that received either two tags per adult animal or one tag for every other adult animal were determined using gas chromatography over a three-month period in 1987 and 1988. Cattle with two tags had consistently higher residues than cattle with one tag. This difference was statistically significant (P < 0.05) in the first month for residues on the head and in the first two weeks for residues from the body in 1987. Residues on cattle with one tag and without a tag in the same herd were similar (P > 0.05) on each sampling day on all regions. Residues on the hair from the head of cattle with two tags were greater than on the body and rump (P < 0.05) during the first 28 days. Residues found on hair on days 14 and 84 following tag application declined by 80–86% on the head, 73–78% on the body, and 36–86% on the rump. Isomeric compositions of fenvalerate (range 51–61% SR, RS: 39–49% SS, RR) and permethrin (range 61–67% trans: 33–39% cis) were consistent during the study. Rainfall reduced residues on hair.     

Involvement of esterases in diazinon resistance and biphasic effects of piperonyl butoxide on diazinon toxicity to Haematobia irritans irritans (Diptera: Muscidae)

Resistance to insecticides remains a major problem for the successful control of the horn fly, Haematobia irritans irritans (L.), one of the most important pests of cattle in many countries including the United States. The organophosphate (OP) insecticide diazinon has been used to control pyrethroid-resistant populations of the horn fly. There are only a few reported cases of horn fly resistance to diazinon in the United States and Mexico. Piperonyl butoxide (PBO) has been used successfully as a synergist of pyrethroid insecticides to control horn flies. PBO-synergized diazinon products are also available for horn fly control in the United States, although PBO is known to inhibit the bio-activation of certain OP insecticides including diazinon. A study was conducted to evaluate the effect of PBO on diazinon toxicity to horn flies using a filter paper bioassay technique. These bioassays in both the susceptible and diazinon-resistant horn fly strains revealed a biphasic effect of PBO on diazinon toxicity to horn flies. PBO inhibited diazinon toxicity when the PBO concentration used was high (5%), and no effect was observed when PBO concentration was intermediate (2%). However, at low concentrations (1% and lower), PBO significantly synergized diazinon toxicity. We demonstrated that enhanced esterase activity was associated with survivability of horn flies exposed to diazinon alone. PBO has been shown to inhibit esterase activity in other insect species. However, results of biochemical assays with esterases from this study suggest that PBO did not have significant effect on the overall esterase activity in the horn fly. The observed synergistic effect of PBO at lower concentrations on diazinon toxicity to horn flies could not be explained by reduced esterase activity due to PBO inhibition. It is likely that PBO synergized diazinon toxicity at lower concentrations by facilitating penetration of diazinon through the cuticle and/or inhibiting the oxidative detoxification of diazinon, and reduced diazinon toxicity at high PBO concentration by inhibiting the bio-activation of diazinon.     

Responses to ultraviolet-B radiation by purely symbiotic and NO3-fed nodulated tree and shrub legumes indigenous to southern Africa

Purely symbiotic and NO3-fed nodulated seedlings of Virgilia oroboides (Bergius) T.M. Salter, Cyclopia maculata (L.) Vent and Podalyria calyptrata Willd. were exposed to biologically effective ultraviolet-B radiation (UV-B) to assess the effects of above- and below-ambient UV-B on growth, symbiotic function and metabolite concentrations. Seedlings were grown outdoors either on tables under ambient or 34 or 66% above-ambient UV-B conditions (UV-B100 control, UV-B134 and UV-B166, respectively), or in chambers providing below-ambient (22% of ambient) UV-B (UV-B22) along with a UV-A control and a photosynthetically active radiation (PAR) control. Exposure of seedlings to UV-B166 radiation reduced (P < or = 0.05) leaf and stem dry mass by 34 and 39%, respectively, in C. maculata, and reduced leaf nitrogen concentration (%N) by 12% in V. oroboides. Nodule %N in C. maculata and stem %N in P. calyptrata also decreased (P < or = 0.05) in response to UV-B22 radiation compared with the UV-A control, but not compared with the PAR control. Concentrations of flavonoids, soluble sugars and starch were unaltered by the UV-B treatments. Application of 1 mM NO3 to UV-B166-treated seedlings increased whole-plant dry mass of V. oroboides and P. calyptrata by 47 and 52%, respectively. Dry mass of organs, nodule %N and total N concentration of these species also increased with NO3 application. However, NO3 supply decreased (P < or = 0.05) nodule dry mass, stem %N and leaf %N as well as root and leaf anthocyanin concentrations in C. maculata. In terms of UV-B x N interactions, dry mass of stems, roots, nodules and total biomass of NO3-fed C. maculata seedlings were reduced, and nodule %N, total N and leaf anthocyanins were depressed by the UV-B134 and UV-B166 treatments relative to UV-B100-treated seedlings. Although we found that above-ambient UV-B had no effects on growth and symbiotic function of V. oroboides and P. calyptrata seedlings, feeding NO3 to these species increased (P < or = 0.05) seedling growth. In contrast, purely symbiotic C. maculata seedlings were sensitive to the UV-B166 radiation treatment, and adding NO3 further increased their sensitivity to both the UV-B134 and UV-B166 treatments.     

Correction to: Agroforestry can enhance foraging and nesting resources for pollinators with focus on solitary bees at the landscape scale

Agroforestry Systems - A correction to this paper has been published: https://doi.org/10.1007/s10457-021-00641-7