Environmental Engineering Reference
In-Depth Information
vegetative planting material and soil may reduce or eliminate the popu-
lation of certain pathogens in the seedbed. Preventive measures are quite
effective in reducing occurrence of diseases caused by nematodes, but
elimination of numerous fungal pathogens by prevention alone is very
difficult. Many fungi are easily spread by wind, equipment or animals,
and turf usually harbours them permanently. An important aspect of
disease prevention is development of turf-grasses less susceptible to
pathogens. Numerous disease-resistant cultivars have been developed
for temperate climates, where diseases are widespread and often devas-
tating. Because warm-season species are naturally much more resistant,
considerably less effort has been put into development of disease-resist-
ant varieties. Among warm-season species, several cultivars resistant to
the virus causing St Augustine grass decline (SAD) have been developed.
The most effective preventive measure is to provide unfavourable
environmental conditions for the growth of pathogens. Some factors, like
temperature, are quite difficult to modify in the tropics, but others, such
as plant nutrition, soil moisture and duration of leaf wetness, can be
altered.
Temperature has a direct effect on the growth rates of fungi, but
unlike the situation in temperate climates, where temperature is very
important, high temperature is not a primary factor promoting develop-
ment of disease. In the tropics, humidity within the turf-grass canopy
seems to be more important. Moisture-saturated air allows germination
of most fungal spores and the growth of fungal mycelia. After sunset,
humidity inside the turf canopy usually increases and reaches the dew
point. The moisture then condenses in the form of little droplets, which
remain on the turf until sunrise. When the humidity is high and evapor-
ation of water from the leaf surfaces in the morning is slow, the favour-
able period for growth of fungal pathogen growth becomes longer. More
than 12 h of leaf wetness are needed for most pathogens to grow and
penetrate plant tissue. If actively growing fungal structures are dried
rapidly before the fungus has penetrated the plant surface, the patho-
gen's life cycle is broken, and disease does not develop. Therefore, any
measure that reduces the duration of leaf wetness decreases development
of disease, and any measure increasing leaf wetness promotes it. In the
morning, the period of leaf wetness can be reduced mechanically by
dragging of ropes or water hoses across the grass to knock the dew off
the blades, as well as by watering around the time of sunrise. Humidity
in the canopy can also be reduced by removal of obstructions that reduce
the movement of air. Spacing or pruning shrubs and trees in the land-
scape greatly improves air movement and speeds the evaporation of
dew. On the other hand, late-afternoon watering extends the period of
leaf wetness. Because the turf may not dry before evening dew is formed,
fungus can start its growth cycle several hours earlier, and mycelium
