First Report of White Root Rot of Japanese Serissa Caused by Rosellinia necatrix in Taiwan

Japanese serissa (Serissa japonica (Thunb.) Thunb.) is a very popular
ornamental in Taiwan. During the summer of 2005, serissa plants in a
central Taiwan nursery had decaying roots, leaf yellowing, and were
wilting. Wilted plants had white fan-like mycelium under the bark. The
disease caused 70% loss of seedlings at the nursery. Associated
synnemata were rigid, erect, dark, setaceous, 0.8 to 2.1 mm long, 90 to
200 µm wide, and tapering to enlarged whitish gray heads composed of
geniculate conidiophores and conidia. Conidia were 3.1 to 5.8 × 2.6 to 3.4
µm, unicellular, hyaline, and subglobal with a truncated base. Root rots
were washed, disinfested for 1 min in 0.5% NaOCl, cut into 3 mm3 pieces,
plated on Merck (Darmstadt, Germany) potato dextrose agar (PDA)
amended with 100 ppm of ampicillin (Sigma, St. Louis, MO), and
incubated at 24°C in the dark until hyphae emerged. Single hyphal tips
were transferred to PDA, and two isolates were established as pure
cultures. Mycelia were cut, stained with 1% cotton blue in lactophenol,
and pear-shaped hyphal swellings adjacent to the septa were observed.
According to these hyphal, synnematal, and conidial characteristics, the
fungus was identified as Dematophora necatrix Hartig, the anamorph of
Rosellinia necatrix Prill. Inoculum for pathogenicity tests were produced
on oat-wheat medium composed of 20 ml of oat grain and 20 ml of wheat
grain, mixed, and soaked in water for 3 h. The grains were placed in 200-
ml flasks, autoclaved at 121°C for 30 min, inoculated with two isolates of
D. necatrix separately, and grown for 14 days. Six 3-month-old Japanese
serissa seedlings were grown in pots. The grain inoculum was added to
unsterilized field soil and the plants were transplanted into this mix.
Control plants were transplanted into a similar mix without the inoculum.
Two replicates were used for a total of 24 inoculated plants and 24 control
plants. All plants were kept in a growth chamber at 25 to 35°C with 20
min of irrigation per day, 12 h of irradiation, and relative humidity at
more than 95%. Inoculated plants developed root rots after 1 month, and
after 4 months, all plants were dead, while control plants remained
healthy. D. necatrix was reisolated, hyphal characteristics confirmed, and
synnemata were observed on collars of dead plants. The teleomorph was
not formed by our cultures, and the identification of Rosellinia necatrix
was confirmed by molecular studies. The nuclear ribosomal internal
transcribed spacer (ITS) amplified with two primers, ITS1 and ITS4, from
our representative isolate demonstrated 99.63, 99.81, and 99.27%
similarity to two R. necatrix isolates from Japan and one R. necatrix
isolate from Italy, respectively. This disease has been reported on many
species of plants (1), but to our knowledge, this is the first report of white
root rot of Japanese serissa seedlings caused by R. necatrix in Taiwan.