Silver leaf has been assessed in experimental and commercial fruit orchards in Norway and Chile. Observations so far indicated increased incidence during season, but also that symptoms can be reversed and that it might differs between years. Cultivar differences are pronounced. Development of fruiting bodies were investigated in different locations, but the incidence was low even in heavily infected orchards. It is observed cultivar difference in how fast silver colored leaves appear and that also other fungi are present in trees with symptoms. Identification of such fungi showed a number of fungi familiar with stone fruit trees and also broad leaf trees in general. Their potential to infect healty plum trees are currently assessed by inoculation, and vegetation around and in plum orchards are assessed for their presence.
Development time was documented in inoculation experiments, together with cultivar differences. Seedling populations of two cultivars were inoculated in autumn 2019 and they had silver leaf in spring 2020. Similar development time were observed on trees inoculated autumn 2019. In 2020 graftings were inoculated and they developed symptoms quickly, indicating that it is possible to spot infected plants in nurseries.
Different methods for detecting the pathogen prior to visible symptom development were adapted in Chile and Norway. The latter were used to document how far from an inoculation point the fungi could be found. Isolates of the fungi were collected in Chile and Norway. In Norway five isolates were tested for growth rate at different temperatures. It was highest growth at 25°C and none or limited growth at 5 or 30°C. It was similar temperature response for the five isolates, but the differed in growth rate. Two of them were used in inoculation experiments in 2020, both at 15°C in a growth chamber and outside. The development was faster in the growth chamber. In Chile isolates were tested for growth at 25, 30 and 35°C. Also there the growth was optimal at 25°C. It was no growth at 35°C.
Negative effect of silver leaf on fruit quality and yield was documented in Chile. Similar experiments in Norway gave no clear results in two seasons.
No silvered foliage was found on surrounding trees in Norway, but the pathogen was isolated from different trees surrounding plum orchards in Chile. Neither fruiting bodies of the pathogen were found on surrounding trees in Norway.
Bacterial canker is investigated in commercial orchards in Norway. From isolations, only Pseudomonas syringae is found from plum and sweet cherry. Quantification of the bacteria on leaves has been challenging but gave more promising results on wood samples. Plum trees can have both the silver leaf fungi and bacteria in the main stem.
In Poland it is performed experiments with plum, sweet and sour cherry. Spraying experiments in season 2019 gave few results because the season was dry in Poland and with low incidence of bacterial canker. Seasons 2020 and 2021 had more symptoms and in spraying experiment both copper and biological treatments reduced the attach without phytotoxicity. In Norway phytotoxicity was observed in different experiments across the fruit districts. In 2021 the treatemtns causing less toxicity were repeated.
Leaf spots in plum. A quantitative molecular screening indicated Pseudomonas syringae as a possible cause in 2018, and possibly in 2019. Inoculation experiments were performed on plum with both the bacteria and a fungal specie that could be causal agent of leaf spots in 2019 but gave few or no leaf spots. Molecular analysis showed clearly higher levels of bacteria on the inoculated trees. Inoculation experiments in 2020 resulted in leaf spots of the fungi and it is now documented that leaf spots on plum can be caused of both P. syringae and the rust fungi Pucciniastrum areolatum, but that Pseudomonas might be the dominating cause in commercial plum orchards.
The knowledge is disseminated through field experiments in orchards, in demonstration plots and by experience groups and knowledge network across national borders with fruit growers, advisors and researchers.
Fruktgården AS har gjennom de siste tre årene satset stort på frukt og bær, gjennom planting av blant annet plommer og epler. Plommesatsingen har ikke gitt de ønskede avlingsresultatene og hovedgrunnen er for dårlig trehelse. Fruktgården ønsker å bedre denne situasjonen og søker sammen med plomme og søtkirsebærdyrkere i andre dyrkingsområder, rådgivere og forskere prosjektet Bedre Trehelse. Det vil finansiere forskningsarbeid i Norge, Finland, Chile og Polen. I dette skal det bygges kunnskap om sølvglans i plomme, bakteriekreft i søtkirsebær og plomme og bladflekker i plomme. Både sølvglans og bakteriekreft er sykdommer som det har vært problematisk å bekjempe i dyrking av steinfrukt, selv med omfattende forskningsinnsats over mange år. Vår tilnærming er kunnskapsbygging på tvers av landegrenser, ny kunnskap om sorter og kobling mot dyrkingsteknikk og lokale klimaforhold. Ny grunnleggende kunnskap om sykdommene sammen med utprøving av nye tiltak og god agronomi vil gi et løft i sykdomshåndtering i steinfruktfelt. Kunnskapen skal formidles gjennom feltforsøk i våre frukthager, i demonstrasjonsfelt og gjennom erfaringsgrupper og kunnskapsnettverk på tvers av landegrenser. Kunnskapsnettverket og erfaringsutvekslingen skal foregå mellom fruktdyrkere, rådgivere og forskere, med et felles mål: å bedre trehelsen gjennom et betydelig kunnskapsløft. Vi vil utnytte våre lokalklimatiske fordeler og dyrke steinfrukt til norsk marked på friske trær med høyt avlingspotensiale. Bedre Trehelse vil gi oss og store deler av steinfruktdyrkingen i mange land et nødvendig løft i kampen mot disse alvorlige sykdommene. Vi forventer en økt verdiskaping fra norsk steinfruktdyrking med minst 100 millioner årlig.