Biological control agents in plant disease control - Disease Control in Crops - page 34

but numerous other PGPR species are known (Vessey, 2003; Zahir et al ., 2004).
Mechanisms involved may include associative N 2 fi xation, solubilisation of nutrients
such as P, promotion of mycorrhizal and rhizobia function, regulating ethylene produc-
tion in roots, release of phytohormones, and decreased heavy metal toxicity (Whipps,
2001; Vessey, 2003; Zahir et al ., 2004). Root and seed colonisation to various degrees are
the key features for growth promotion with the signifi cance of endophytic growth in this
process gradually being recognised (Rosenblueth & Martinez-Romero, 2006).
Of the fungal plant-growth promoters, Trichoderma spp. are probably the most
well known (Harman et al ., 2004; Harman, 2006). Trichoderma have the ability to
solubilise P and numerous other micronutrients, enhance effi ciency of N use, increase
root development and root hair formation (Altomare et al ., 1999; Harman, 2006), and
like most bacterial PGPs, exhibit ability to colonise roots, often becoming endophytic
(Yedidia et al ., 2000; Harman, 2006).

3.2.6 Hypovirulence

A highly specialised mode of action concerns the use of hypovirulent isolates of fungal
pathogens. Hypovirulent fungal isolates contain mycoviruses that intrinsically cause the
fungus to be less fi t. When hypovirulent isolates are introduced into plant tissues infected
with a virulent pathogen isolate, the viruses can be transmitted via hyphal anastamoses,
spreading the viral infection, and decreasing disease. The classic example of this process
is that of hypovirulent isolates of Cryphonectria parasitica , containing unencapsidated
double-stranded RNA viruses of the virus family Hypoviridae which have been used to
control Chestnut blight in Europe (Heiniger & Rigling, 1994). Hypovirus infection is per-
sistent and non-lytic, and is associated with inability to effectively penetrate the host plant,
reduced sexual sporulation, female infertility, and reduced pigmentation. Genetically
modifi ed strains of C. parasitica transformed with a severe hypovirus are being explored
for control of Chestnut blight in the United States (Dawe & Nuss, 2001).

3.3
Production, formulation and application of BCAs have been investigated extensively with
the aim of producing successful and cost-effective products (Burges, 1998; Hall & Menn,
1999). A major aim is to produce the greatest quantity of viable propagules with the
best quality for formulation as cheaply as possible, preferably using inexpensive growing
media such as industrial wastes. Production of bacteria and fungi can be done using
large-scale liquid fermentation which often involves manipulating the culture medium
to induce production of the desired propagules for formulation. Factors which are often
manipulated include temperature, pH and osmotic potential, as well as nutritional factors
such as carbon source and C:N ratio (Jackson, 1997). Recently, solid-state fermentation
has been used for the production of fungal biomass. For example, conidia produced by
solid-state fermentation are incorporated into the wettable granule formulation of the
commercial C. minitans product, Contans WG (de Vrije et al ., 2001).
Unless inocula of BCAs are used immediately following production, cells or biomass
are usually dried and formulated as products capable of storage, distribution and appli-
cation (Fravel et al ., 1998; Fravel, 2005). Drying can be done by a range of different

Production, formulation and application