S. Moricca et al., Antagonism of the two-needle pine stem rust fungi Cronartium flaccidum andPeridermium pini by Cladosporium tenuissimum in vitro and in planta, PHYTOPATHOL, 91(5), 2001, pp. 457-468
Selected isolates of Cladosporium tenuissimum were tested for their ability
to inhibit in vitro aeciospore germination of the two-needle pine stem rus
ts Cronartium flaccidum and Peridermium pini and to suppress disease develo
pment in planta. The antagonistic fungus displayed a number of disease-supp
ressive mechanisms. Aeciospore germination on water agar slides was reduced
at 12, 18, and 24 h when a conidial suspension (1.5 x 10(7) conidia per ml
) of the Cladosporium tenuissimum isolates was added. When the aeciospores
were incubated in same-strength conidial suspensions for 1, 11, 21, and 31
days, viability was reduced at 20 and 4 degreesC. Light and scanning electr
on microscopy showed that rust spores were directly parasitized by Cladospo
rium tenuissimum and that the antagonist had evolved several strategies to
breach the spore wail and gain access to the underlying tissues. Penetratio
n occurred with or without appressoria. The hyperparasite exerted a mechani
cal force to destroy the spore structures (spinules, cell wall) by direct c
ontact, penetrated the aeciospores and subsequently proliferated within the
m. However, an enzymatic action could also be involved. This was shown by t
he dissolution of the host tell wall that comes in contact with the myceliu
m of the mycoparasite, by the lack of indentation in the host wall at the c
ontact site, and by the minimal swelling at the infecting hyphal tip. Cultu
re filtrates of the hyperparasite inhibited germination of rust propagules.
A compound purified from the filtrates was characterized by chemical and s
pectroscopic analysis as cladosporol, a known beta -1,3-glucan biosynthesis
inhibitor. Conidia of Cladosporium tenuissimum reduced rust development on
new infected pine seedlings over 2 years under greenhouse conditions. Beca
use the fungus is an aggressive mycoparasite, produces fungicidal metabolit
es, and can survive and multiply in forest ecosystems without rusts, it see
ms a promising agent for the biological control of pine stem rusts in Europ
e.