Infection process of Phoma koolunga on stem and leaf tissue of resistant and susceptible field pea (Pisum sativum)

Abstract

This study investigates the infection process of Phoma koolunga on field pea (Pisum sativum) stems and leaves using different susceptible and resistant pea genotypes for each tissue, viz. 05P778‐BSR‐701 (resistant) and 06P830‐(F5)‐BSR‐5 (susceptible) for stems and ATC 866 (resistant) and ATC 5347 (susceptible) for leaves. On both resistant and susceptible genotypes, light and scanning electron microscopy showed P. koolunga conidia infect stem and leaf tissues directly via appressoria or stomatal penetration, but with more infections involving formation of appressoria on stems than on leaves. On leaves of the resistant genotype, at 72 h post‐inoculation, P. koolunga penetrated more frequently via stomata (5.2 conidia per 36 893 μm2) than by formation of appressoria (1.8 conidia); yet no such difference was observed on stems of the resistant genotype. In contrast, at the same time point, the number of conidia infecting the susceptible genotype by formation of appressoria on either stems (135 conidia) or leaves (11.3 conidia) was significantly greater than via stomata (8 and 7.3 conidia, stems and leaves, respectively). Mean germ tube length of germinating P. koolunga conidia on both stems (29.8 μm) and leaves (32.9 μm) of the resistant genotype was less than on the susceptible genotype (40.5 and 63.7 μm, stem and leaves, respectively). In addition, there were differences in the number of germ tubes emerging from conidia on resistant and susceptible genotypes. These are the first insights into the nature of leaf and stem resistance mechanisms operating in field pea against P. koolunga.

Expression of defence‐related genes in stems and leaves of resistant and susceptible field pea (Pisum sativum) during infection by Phoma koolunga

“…Tran et al . () reported that P. koolunga infected both stems and leaves of field pea via stomata or direct penetration following the formation of appressoria, noting that the number of infection sites involving formation of appressoria on stems were more than those on leaves. Results of the current study partly explain those earlier findings.…”

“…All pots were arranged in a complete randomized design. For inoculation on stems, a new method of seedling preparation described by Tran et al (2017) was used that ensures consistent infection on stems by preventing inoculum runoff, which commonly occurs when inoculations are made onto vertical stems. In brief, for the latter, seeds were sown into 4 9 8 cm clear plastic bags filled to approximately two-thirds of their capacity with pasteurized potting mix.…”

“…Phoma koolunga single-spore isolate UWA H2012-81, obtained during 2012 field pea disease surveys in Western Australia (Tran et al, 2014a), was used. This was the same isolate used in the previous studies to identify field pea genotypes with stem and leaf resistance (Tran et al, 2015) and for observations on the infection process of P. koolunga (Tran et al, 2017). Coon's agar media (Ali et al, 1978) was used for subculturing the isolate.…”

“…Stems were inoculated using a procedure described by Tran et al (2017). Prior to inoculation, seedlings sown into plastic bags were carefully removed from pots and the whole plastic bag area covered with aluminium foil to prevent light from interacting with plant roots.…”

“…There have been no studies as those described above involving P. koolunga. Resistance against P. koolunga is known to be associated with one or more factors influencing the number of conidia forming appressoria and impedance of germ tube development on leaves and stems (Tran et al, 2017). Hence, using these latter findings as a foundation, studies were undertaken to determine the differential expression of 13 defence-related genes during infection of P. koolunga on stems and leaves of susceptible versus resistant field pea using quantitative reverse transcription-PCR (qRT-PCR).…”