- Bacterial canker, caused by Pseudomonas syringae pv. syringae is
recognized as one of the greatest limiting factors in cherry production
in Oregon. Disease incidence may be decreased when susceptible
cultivars are high-grafted onto tolerant/resistant rootstocks. This
research was begun to develop a rapid screening method which could be
used to test cherry rootstocks for tolerance to Pseudomonas syringae pv.
syringae. In 1988, one-year-old wood of 'Napoleon', 'Corum' and F/12-1
was collected at monthly intervals from November until January.
'Napoleon' and 'Corum' are found to be susceptible, and F/12-1 to be
tolerant to Pseudomonas syringae pv. syringae. Twigs were inoculated
with water, one avirulent, and three virulent strains of bacteria at
10⁵, 10⁶, and 10⁷ cfu/ml. Browning, gummosis, and callus production
were evaluated at the inoculation site after incubation for 4 weeks.
Generally, browning and gummosis induced by concentrations of 10⁶ and
10⁷ cfu/ml across the virulent strains were not different. No gummosis
and browning were observed on twigs inoculated with water or the
avirulent strain. Callus production did not appear to be a usable
criterion of the disease in this assay. 'Napoleon' and 'Corum' had
significantly higher browning and gummosis ratings than F/12-1.
In 1989, one-year-old twigs from 'Napoleon', 'Corum', and a number
of cherry rootstocks were collected from October until December . The
rootstocks included: F/12-1, M x M 2, M x M 39, M x M 60, GI 148-1,
GI 148-8, GI 154-2, GI 154-5, GI 169-15, GI 172-9, and GI 173-9. Twigs
were inoculated with water alone as a check, one avirulent and three
virulent strains at 10⁷ cfu/ml. Incision browning, gummosis and callus
production were evaluated after incubation for 4 weeks. Based on
incision browning and gummosis all the rootstocks tested were more
tolerant than 'Napoleon' and 'Corum', and most did not differ from
F/12-1. However, rootstocks GI 172-9 and GI 169-15 showed more browning
than F/12-1 in a number of instances. On the last sampling date,
rootstocks M x M 2, GI 173-9, GI 148-8, and GI 154-2 showed less
browning than F/12-1.
The expression of virulence genes in P. syringae has been shown to
be induced by plant signals extracted from cherry leaves. Crude aqueous
extracts from 'Napoleon', 'Corum', and various cherry rootstock twigs
were adjusted to a concentration of 0.2, 1.0, and 2.0 mg-ml⁻¹, and
evaluated for the ability to induce virulence in Pseudomonas syringae
pv. syringae. A decrease in activity was generally observed at the two
highest concentrations of plant extracts. Reduced activity may have
been due to inhibition of the assay or saturation of active sites of the
enzyme produced by the bacteria. Compared to tolerant genotype F/12-1,
'Napoleon' and 'Corum' had the highest activity of all the genotypes
tested. Rootstocks M x M 60, GI 148-1, GI 148-9, GI 154-5, and
GI 169-15 had higher activity than F/12-1 in plant extracts of
0.2 mg-ml⁻¹. Activity induced in rootstocks MxM2, MxM39,
GI 154-2, GI 172-9, and GI 173-9 was not different from that of F/12-1.
The lowest activity observed was for 'Colt', however this activity was
not statistically different from that induced by F/12-1.