Occurrence of Canker and Wood Rot Pathogens on Stone Fruit Propagation Material and Nursery Trees in the Western Cape of South Africa

Dieback and canker of young stone fruit trees can cause suboptimal growth and even death under severe conditions. One source of inoculum of canker pathogens could be through nursery trees harboring latent infections that would not be visible to inspections done according to the deciduous fruit scheme. The objectives of this study were to identify the canker and wood rot fungal pathogens present in nursery stone fruit trees as well as propagation material and to evaluate their pathogenicity. Isolations were made from scion and rootstock propagation material and from certified nursery stone fruit trees. The plant material sampled did not have any external symptoms. The certified nursery trees when cross-sectioned displayed brown discoloration from the pruning wound, bud union and often from the crown. Fungal species isolated were identified by sequencing of the relevant barcoding genes and phylogenetic analyses thereof. Canker and wood rot associated fungi were identified. Buds used for budding had low levels of infection, with 1.2% of dormant buds infected and 0.4% of green buds infected. The dormant rootstock shoots had canker pathogen incidence of 6.2% before it was planted in the nursery fields and increased as the ungrafted, rooted rootstock plants had 11.1% infection with canker and wood rot pathogens. Out of 1080 nursery trees, the canker and wood rot associated fungi infected 21.8% of trees. The canker causing pathogens that were isolated the most were Cadophora luteo-olivacea and Diplodia seriata. A low incidence of wood rot fungi was found with only 1.5% of nursery trees infected. In total 26 new reports of fungal species on stone fruit in South Africa were made. Of these, 22 have not been found on stone fruit world-wide. The pathogenicity trials’ results confirmed the pathogenic status of these newly reported species. All of the isolates tested formed lesions significantly longer than the control, 4 months after wound inoculation of 2-year-old shoots of two plum orchards. Lasiodiplodia theobromae was the most virulent species on both plum cultivars. The results of this research showed that nursery stone fruit trees and propagation material can harbor latent infections. Different management practices need to be evaluated to prevent these infections to ensure healthier stone fruit nursery trees.

Survey of apple collar and rootstock blight in Serbia

Apple production in Serbia is characterized by the introduction of new technologies and the establishment of intensive plantations based on modern systems. In intensive apple production systems, sudden and rapid decay of young apple trees exhibiting collar and rootstock blight symptoms in the first few years after planting is becoming a prevalent issue. However, knip or nursery trees with lateral branches (feathers) on rootstock M.9 i.e. oneto five-year old are most seriously affected. Based on a longitudinal survey of young apple plantations in Serbia and the results of pathogen detection and identification using conventional and molecular methods, the plant pathogenic bacterium Erwinia amylovora was found as the causative agent. By increasing the incidence of collar and rootstock blight in young apple trees, E. amylovora is gaining importance as the most destructive disease in apple production. In order to control E. amylovora effectively and apply appropriate control measures, it is necessary to survey the development of lesser-known symptoms caused by this pathogen, which are becoming prevalent in Serbian agroecological conditions.

Permethrin Residual Activity Against Ambrosia Beetle (Coleoptera: Curculionidae: Scolytinae) Attacks Following Field Aging and Simulated Rainfall Weathering

Adult ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) bore into ornamental nursery trees resulting in trunk vascular tissue damage, which can potentially kill trees. Ambrosia beetle exposure to surface-applied insecticides is minimal after internal trunk galleries are formed, so effective management requires insecticide treatments to be applied near the time of infestation or to have residual activity on the bark. Tree trunk sections (bolts) were used to determine the effect of field aging or irrigation (i.e., simulated rainfall weathering) on permethrin residual activity against ambrosia beetles. In all experiments, 30-cm-long bolts from Liriodendron tulipifera L. (Magnoliales: Magnoliaceae) were hollowed and filled with 70% ethanol at field deployment to induce ambrosia beetle attacks over a 2-wk period. To evaluate insecticide residual activity, permethrin was sprayed onto tree bolts at 0, 8, 17, or 24 d before ethanol addition, and then bolts were deployed along a wooded border in fall 2017 and spring 2018. Tree bolts with permethrin residues ≤17 d old had significantly fewer ambrosia beetle attacks than bolts with 24-d-old residues or the non-permethrin-treated control bolts. To evaluate simulated rainfall weathering, permethrin was applied to tree bolts 8 or 22 d before ethanol (spring 2018) or 10 or 24 d before ethanol (fall 2018) with half of the bolts receiving regular irrigation events. Irrigation had no significant effect on permethrin residual activity against ambrosia beetles during either test. This study determined ambrosia beetle control was affected by permethrin residue age more than simulated rainfall weathering, and a reapplication interval of ≤17 d maximized beetle control.

Canker and Wood Rot Pathogens Present in Young Apple Trees and Propagation Material in the Western Cape of South Africa

Canker and wood rot pathogens cause dieback and, in severe cases, the death of young apple trees. Recently, a higher occurrence of cankers was observed on 1-year-old apple trees in the Western Cape Province of South Africa. This study aimed to assess the phytosanitary status of nursery trees and propagation material as possible inoculum sources for canker pathogens. Thirteen 1-year-old apple orchards showing canker or dieback symptoms were sampled. Certified nursery apple trees were collected from four nurseries as well as scion and rootstock mother plant material. Isolations were made from the discoloration observed in the vascular tissue of the plant parts and from asymptomatic material. Possible canker and wood rot species were identified with PCR and sequence comparisons of the relevant gene regions and phylogenetic analyses. Similar canker and wood rot species were isolated from 1-year-old diseased apple trees, nursery apple trees, and the propagation material. Forty-five fungal species associated with canker or wood rot symptoms were identified. The top five most abundant fungal species found causing disease on commercial 1-year-old trees were also found in high numbers causing latent infection in certified apple nursery trees. These species were Didymosphaeria rubi-ulmifolii sensu lato, Diplodia seriata, Schizophyllum commune, Didymella pomorum, and Coniochaeta fasciculata, with D. rubi-ulmifolii sensu lato being the dominant species in both sampling materials. In all, 65% of certified nursery apple trees, 5% of scion shoots used for budding, and 21% of rooted rootstock cuttings from layer blocks had latent infections of canker and wood rot pathogens. Pathogenicity trials were conducted with isolates of 39 species, inoculated onto 2-year-old branches of 14-year-old Golden Delicious trees. All species caused lesions that were significantly longer than the control. This study confirmed the presence of canker and wood rot pathogens in apple propagation material as well as certified nursery apple trees, which will aid the improvement of management practices in nurseries.

Utilizing a Thermal Time Model to Estimate Safe Times to Transplant Tilia Trees

City trees planted in parks and along streets are typically grown to large size in nurseries before being transplanted to their final growing sites. According to tendering rules within the European Union (EU), any business may compete for public contracts in any EU country, and this applies to purchases of valuable lots of nursery trees. There is however a risk of poor transplanting success if the trees are imported from very distant locations with a different pace of spring development. The aim of this study was to implement a Thermal Time model to predict the spring development of Tilia trees to find out in which geographical area the spring development is sufficiently similar to conditions in southern Finland, so that the success of transplantation of the trees is not unduly risked. We used phenological observations collected at the International Phenological Gardens (IPGs) over the whole of Europe, together with ERA-Interim weather data to estimate the model parameters, and then used the same date to predict the onset of leaf unfolding ofTilia during the years 1980 to 2015. Producing maps of phenological development of Tilia, we concluded that there are no large risks of frost damage if tree import area is limited to northern parts of Baltics or to the west coast of Scandinavia.

Stock Plant and Tree Production: Irrigation and Fertilization

Citrus tree production in nurseries can be divided into three stages: production of rootstock liners, transplant of rootstocks into larger containers, and bud grafting. The objectives of this new 6-page publication of the UF/IFAS Horticultural Sciences Department are to provide general information on irrigation and fertilization for production of citrus nursery trees in seedbeds, nurseries and budwood multiplication blocks. This article, written by Rhuanito Soranz Ferrarezi, is chapter 8b of the forthcoming Citrus Nursery Production Guide. https://edis.ifas.ufl.edu/hs1333

Heat treatments of dormant scion wood killed the European canker pathogen in planta, while chemical treatments did not

Neonectria ditissima, the causal agent of European canker, can be present in symptomless scion wood. Sanitation treatments could minimise this risk to nursery trees. In this trial, six heat treatments and five chemical treatments were tested for their effectiveness in removing this pathogen from dormant ‘Royal Gala’ wood. In July 2018, 120 symptomless inoculated shoots (three inoculations/shoot) were harvested and stored at 1oC for 3 months. Bundles of five inoculated shoots (45 cm) were placed in the centre of 24 bundles, each consisting of 25 wood pieces. Heat-treated bundles were submerged in water (45oC for 45 min or 50oC for 15 min), or wrapped in moist cloth, vacuum sealed inside plastic then submerged for 3–6 h at the same temperatures. Chemical-treated bundles were submerged for 16 h at room temperature. Treatments were compared with untreated wood. After surface sterilising, isolation of N. ditissima from inoculated wounds was attempted on apple-sap amended water agar. All wounds from the untreated wood and from the chemical-treated wood yielded the pathogen. However, N. ditissima was not isolated from wounds that had been heat treated. Therefore, heat treatments that do not affect scion wood viability may prove an effective tool to remove European canker from nursery material.