Methods of inoculation and evaluation of Erwinia psidii in eucalypt

Abstract

The dieback and wilting caused by Erwinia psidii are emerging eucalypt diseases that have been observed since 2014 in the south and central-south regions of Brazil. Field observations have shown variability in disease severity resistance among Eucalyptus spp. clones and species. It is hypothesized that this variability is due to genetic resistance. To confirm this hypothesis, inoculations in genetically distinct eucalypt plants are necessary. However, lack of an inoculation method and disease assessment makes difficult to select resistant genotypes for use in commercial plantations or genetic breeding programmes. Three inoculation methods were tested on eight clones of Eucalyptus spp. Among them, inoculum deposition with bacteria-impregnated toothpick on the axillary buds was the simplest and most effective, capable to reproduce the disease symptoms observed under conditions of natural infection. We also developed a rating scale for disease assessment. Among eight clones tested, only Clone 1 (Eucalyptus saligna) and Clone 2 (Eucalyptus urophylla) were resistant.

Host range ofErwinia psidiiand genetic resistance ofEucalyptusandCorymbiaspecies to this pathogen

“… Rating scale for evaluating disease severity on eucalypt clones inoculated in the axillary buds with Erwinia psidii LPF534. 0 = plants without symptoms, 1 = necrosis of the midrib and/or secondary veins, 2 = death of some leaves without loss of apical dominance, 3 = presence of a minicanker and/or canker, 4 = dieback with loss of apical dominance, and 5 = dieback of the apical third portion of the main stem (Ferraz et al, ). …”

“…Inoculated plants were kept in a greenhouse at 28 ± 3°C and evaluated weekly for disease severity up to 35 days after the first inoculation. A rating scale from Ferraz et al () with six classes of disease symptoms was used to quantify disease severity, in which 0 = plants without symptoms, 1 = necrosis of the midrib and/or secondary veins, 2 = death of some leaves without loss of apical dominance, 3 = presence of a minicanker and/or canker, 4 = dieback with loss of apical dominance, and 5 = dieback of the apical third portion of the main stem. The frequency of each disease class with respect to the total number of plants from each species or provenance was determined.…”

“…Prior to inoculation, five branches (from the top to bottom of the plants) were labelled with adhesive tape delimiting the first three axillary buds, which were inoculated using sterilized toothpicks impregnated with a mass of cells from a bacterial colony of isolate LPF534 grown on 523 medium at 28°C for 24 hr in the dark (Ferraz et al, 2016). Ten plants of each species were inoculated.…”

“…Inoculation was carried out as mentioned above (Ferraz et al, 2016). At 21 days after the initial inoculation, asymptomatic plants were reinoculated in order to ensure successful inoculation of all experimental plants.…”

“…Inoculated plants were kept in a greenhouse at 28 ± 3°C and evaluated weekly for disease severity up to 35 days after the first inoculation. A rating scale from Ferraz et al (2016) with six classes of disease symptoms was used to quantify disease…”