FUSARIUM HEAD BLIGHT AND DON CONTAMINATION MANAGEMENT IN SOFT AND DURUM WHEAT CULTIVATION - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 134

been reported to be the most important types of inoculum and both of them are abundantly
produced on the infected residues (Khonga and Sutton, 1988; Fernando et al., 2000). F.
graminearum has an additional epidemiological advantage because it regularly and
abundantly forms perithecia of its sexual stage ( Gibberella zeae ), resulting in the production
of ascospores on the colonized residues (Xu and Berrie, 2005).
3.2. Inoculum Production, Dispersal and Deposition
The production of conidia and ascospores is critically influenced by temperature, the
water activity (a w ) of the host, rainfall, relative humidity and light (UV light required). Sutton
(1982) reported 29°C and 25-28°C as the optimum temperature for perithecia and ascospore
production, respectively. Moisture is required for formation of both macroconidia and
ascospores: Xu (2003) reported that when the soil moisture content is below 30%, ascospore
production is not possible. When it is greater than 80%, ascospore production is at its
maximum. Macroconidia production is also dependent on the temperature o a great extend:
Rossi et al. (2002b) reported that they can be produced in the 5-35°C range while the
optimum temperature for M. nivale is 26°C, 28°C for F. avenaceum and 32°C for F.
graminearum and F. culmorum . Macroconidia are formed in mucilage in sporodochia and
rain and wetting are necessary for their liberation.
The dispersal of both macroconidia and ascospores is associated with rainfall events and
high relative humidity (Paulitz, 1996; Horberg, 2002). In a study conducted in Italy, very few
macroconidia were sampled in the air before rainfall, but their number progressively
increased with the beginning of a rain event. In the presence of high humidity, conidia
continued to be sampled at high densities for some hours after rain had ceased and they
usually reached their peak under these conditions. Finally, the density of the airborne conidia
rapidly decreased when the relative humidity dropped (Rossi et al., 2002a).
Turgor pressure represents the driving force behind ascospore discharge. It has been
suggested that the discharge of ascospores results from the buildup of turgor pressure
generated by ion fluxes and the accumulation of mannitole, which is the main simple sugar
component in the fluid (the epiplasm) with which the ascospores are discharged from asci
from within the ascus (Trail et al., 2002). Ascospores are discharged to distances from <1 mm
to nearly 10 mm with an average between 4.6 mm to 3.9 mm. This distance is sufficient for
the ascospores to surpass the laminar boundary layer of air over which they become airborne
and can encounter turbulent air currents which help them ascend into the atmosphere. This
mainly happens during daylight hours, when the laminar boundary layer is considered to be
only millimetres thick, while during the night, it can extend to nearly 10 m above ground
level. Therefore, ascospores of G. zeae discharged during the daylight hours have the greatest
probability of becoming airborne in turbulent air currents (Schmale et al., 2005). Once
ascospores become airborne, they are subjected to air turbulence: during the day, energy from
the sun heats the earth's surface and there is an upward transfer of heat toward the cooler
atmosphere. Ascospores can be transported in the atmosphere to significant vertical and
horizontal distances over the surface of the earth by turbulent mixing masses of air. At night,
the surface of the earth cools rapidly in comparison to the atmosphere and there is a
downward transfer of heat, which means that at night, as turbulent mixing slows down, the
ascospores may settle out of the atmosphere over the surface of the earth (Maldonado-