Bradbrook - 2012 - Rice Farming complete with methods to increase ric

determine if fungicide application is warranted, check the percentage of tillers that
are infected from a sample number of plants. In susceptible varieties, if the
percentage of infection is 5 to 10%, fungicide should be applied. For moderately
resistant varieties the percentage of infected tillers should be 10 to 15%. The best
time to apply fungicide is during the booting stage (i.e. the panicle is 2 to 4 in
from the flag leaf sheath).
The spores of the causal organism overwinter in infected plant debris that is left on
the field after harvest and can spread through the air to other fields. When the fleld
is flooded again for the next cropping cycle, new infestation begins. Thus, it may
be necessary to spray the field twice to control further spread of the disease.
Sheath blight infestations cause sterility resulting in decreased harvestability and
quality of grains. Additional losses come from the cost of fungicide application
for the current and succeeding crop.
b. Rice blast
Causal organism: Pyricularia oryzae
The blast fungi infect rice plants from the vegetative stage to the harvest stage.
The fungi can infect all parts of the plant except the roots. Infection initially
appears on the leaves as diamond-, football- or spindle-shaped lesions with a lightcolored
center and dark brown edges. Lesions start small but enlarge as infestation
becomes more severe. During heading, lesions appear just below the head or on
individual panicles causing the parts to break off. Infected stems have blackened
nodes that lodge. As with sheath blight, the development of rice blast is favored by
the consequences of high nitrogen rates. Infestation is more severe in upland rice
culture. Sandy soils and fields lined with trees also favor blast development. Rice
blast infestation is considered the most serious disease of rice. Entire fields can be
completely destroyed in a short time. Losses stem from reductions in yield due to
plant deaths and the cost of applying fungicides.
Rice varieties with resistance to rice blast have been developed, but they do not
remain resistant for long because new strains of the fungi develop quickly.
Susceptible varieties should be scouted for signs of blast infection during the
vegetative stages. When blast is detected, preventive spraying is required. In
resistant varieties, fungicide is not applied to infected plants unless they are dying.
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