Evaluation of susceptibility to fruit tree canker caused by Neonectria ditissima

Abstract

Fruit tree canker caused by Neonectria ditissima is a major bark canker disease on apple especially in temperate regions with high humidity and precipitation. The fungus can infect plant material through natural or artificial wounds. It can lead to loss of yield with possible high impact on economy for the apple producer. It has been reported that there is more fruit tree canker attack on apple in recent years. To evaluate susceptibility of genotypes to fruit tree canker, one year old potted tree and detached shoots from top of main stem of one year old trees were tested in the greenhouse and growth chamber. All experiments were inoculated with macroconidia suspension after artificial wounding. The inoculum used in these trials was kept in deep freezer and activated on rootstock before inoculation on wounded areas. In general, infection percent and area under the disease progress curve (AUDPC) are commonly calculated to examine resistance of apple genotypes to fruit tree canker. This study showed that infection percent or AUDPC values alone were not enough to conclude about resistance level to fruit tree canker in apple genotypes. It may be beneficial to determine the resistance level by using several parameters, as evaluation of several parameters together seem to ensure more reliable results. In this study, a range of parameters were applied for recording symptom development. External lesion size was measured at fixed points during experiment period, also giving number of days until the first symptoms were visible. Several extra parameters were recorded at the last observation date, such as internal lesion size, wilting, girdling, swelling, flakes, bubbles, ring pattern, color and sporulation. In general, the developing of lesion surface size increased rapidly 4 weeks after first symptoms was visible in potted tree trial and in detached shoot trial symptoms increased rapidly already one week after first symptoms were visible. We investigated susceptibility of apple cultivars/genotypes by analysis of datasets including or not including inoculation points with no infection symptoms. It was a clear pattern that when including only inoculation points with infection in data analysis, susceptibility of cultivars with low infection percentage can be overestimated. The number of days until the first symptoms were visible after inoculation seem to be a good parameter to investigate susceptibility to fruit tree canker, together with other parameters such as AUDPC level and lesion size at the end of experiment. Wilting and girdling are important parameters to include when examining resistance of apple genotypes, both parameters presented a good association with other parameters such as AUDPC level and external lesion size at last iii recording date. Swelling is a parameter that may discriminate significant difference among apple cultivars. ‘Aroma’ is known to have low to medium susceptibility to N. ditissima, and on swelling character ‘Aroma Fagravoll’ differed from the susceptible cultivars ‘Elise’ and ‘Discovery’. PCA plot showed a relationship between the characters swelling and bubbles. These two characters might be related to resistance, while flakes seem to be related to susceptibility. The tested apple genotypes differ in level of susceptibility to fruit tree canker. MA042 10041 and ‘Bramley’s Seedling’ showed stronger resistance against the canker than ‘Aroma’ in this study. The study shows that recording of several symptoms can give a clearer picture of susceptibility to canker in apple genotypes. Phenotyping with several characters is necessary to get reliable results on resistance in breeding material for use as parents and for introduction of new resistant cultivars. Such phenotyping is also necessary when searching for resistance genes or markers for resistance genes.

Frukttrekreft forårsaket av soppen Neonectria ditissima er en viktig plantesykdom på eple både på treet og fruktene, spesielt i omåder med mildt og fuktig klima. Soppen kan infisere plantemateriale gjennom naturlige sår som for eksempel frostskade, sårdannelse etter bladfall (bladarr) eller beskjæring. Angrep av frukttrekreft kan medføre økonomisk tap for epleprodusenter. Det er rapportert at det er mer frukttrekreft nå enn det har vært før. For å evaluere motstandsevne mot angrep av frukttrekreft hos nummersorter og sorter ble det gjennomført forsøk på ettårige tre (pisker) og avskårne toppskudd av ettårige tre i veksthus og vekstrom. Plantemateriale i alle forsøk ble inokulert med makrokonidia etter såring, der inokulum ble lagret i fryseboks og aktivert på grunnstammer før det ble brukt i forsøkene. Vanligvis blir infeksjonsprosent og «area under the disease progress curve» (AUDPC) beregnet for å kartlegge motstandsevne mot angrep av fruktrekreft hos eplesorter. I denne studien fant vi at infeksjonsprosent og AUDPC-verdier alene ikke var nok til å konkludere om motstandsevne. Evaluering av flere parameter sammen ser ut til å sikre mer pålitelige resultater. I denne studien ble et vidt spekter av parametre brukt til å registrere ulike symptomer i forsøkene. Lengde av ytre skade ble registrert med faste intervall undervegs i løpet av forsøksperioden, og dette ga samtidig antall dager til første symptomer ble synlig etter inokulering. Sluttregistrering omfattet registrering av både ytre symptomer og indre symptomer, som for eksempel visning, ringing (sår vokser rundt tre), svelling, flassing, bobling, ringmønster, farge og sporulering.