Classical and augmentative biological control against ... - IOBC-WPRS

Chapter 6
Persistence, physiological stresses, timing and coverage of others biological agents
Other references have been screened for biocontrol agents considering the analysis of:
persistence on the target,
resistance to physiological stresses,
timing and coverage.
Cladosporium cladosporioides. The antagonist has been effective in reducing sporulation of
Venturia inaequalis under orchard conditions. Furthermore, the results of the pre-screening indicate
that it is cold and drought tolerant and results of experiments on spore production in solid state
fermentation show that mass production is economically feasible. These results have been obtained
in a stepwise selection approach (Köhl, 2009, Köhl et al., 2009).
Ulocladium atrum and Gliocladium roseum. Köhl et al., 1998 described the effect of treatments
with conidial suspensions of Ulocladium atrum and Gliocladium roseum on leaf rot of cyclamen
caused by Botrytis cinerea was investigated under commercial greenhouse conditions. Spraying U.
atrum (1 × 106 conidia per ml) or G. roseum (2 × 106 conidia per ml and 1 × 107 conidia per ml) at
intervals of 2 to 3 weeks during the production period and spraying U. atrum (1 × 106 conidia per
ml) at intervals of 4 to 6 weeks resulted in a significant reduction of natural infections of petioles by
B. cinerea. U. atrum or G. roseum (1 × 107 conidia per ml) was as effective as the standard
fungicide program. B. cinerea colonized senesced leaves within the plant canopy and infected
adjacent petioles and leaves later. The antagonists colonized senesced leaves and reduced B. cinerea
development on these leaves. Thus, the inoculum potential on petioles adjacent to necrotic leaf
tissues was reduced. The fate of U. atrum conidia on surfaces of green cyclamen leaves during a 70-
day period after application was studied. The number of conidia per square centimetre of leaf
surface remained relatively constant during the entire experiment. Sixty percent of the conidia
sampled during the experiments retained the ability to germinate. When green leaves were removed
from the plants to induce senescence and subsequently were incubated in a moist chamber, U.
atrum colonized the dead leaves. Senesced leaves also were colonized by other naturally occurring
fungi including B. cinerea. On leaves treated with U. atrum from all sampling dates, sporulation of
B. cinerea was significantly less as compared with the untreated control. Our results indicate that
early applications of U. atrum before canopy closure may be sufficient to achieve commercially
satisfactory control of Botrytis leaf rot in cyclamen.
Kessel et al., 2005 developed a spatially explicit model describing saprophytic colonization of
dead cyclamen leaf tissue by the plant-pathogenic fungus Botrytis cinerea and the saprophytic
fungal antagonist Ulocladium atrum. Both fungi explore the leaf and utilize the resources it
provides. Leaf tissue is represented by a two-dimensional grid of square grid cells. Fungal
competition within grid cells is modelled using Lotka-Volterra equations. Spatial expansion into
neighbouring grid cells is assumed proportional to the mycelial density gradient between donor and
receptor cell. Established fungal biomass is immobile. Radial growth rates of B. cinerea and U.
atrum in dead cyclamen leaf tissue were measured to determine parameters describing the spatial
dynamics of the fungi. At temperatures from 5 to 25°C, B. cinerea colonies expanded twice as
rapidly as U. atrum colonies. In practical biological control, the slower colonization of space by U.
atrum thus needs to be compensated by a sufficiently dense and even distribution of conidia on the
leaf. Simulation results confirm the importance of spatial expansion to the outcome of the
competitive interaction between B. cinerea and U. atrum at leaf scale. A sensitivity analysis further
emphasized the importance of a uniform high density cover of vital U. atrum conidia on target
leaves.
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