186 Agroecology and Sustainability
enables year round survival of the virus and the vector. Controlling the vector
population with insecticide does not always result in tungro control. Synchronous
planting effectively puts the disease at manageable levels. When and where plant-
ing synchrony is not possible, resistant varieties are recommended. In addition to
varieties with a certain degree of resistance to the vector, varieties highly resistant
to the virus itself became available recently. Farmers should also employ crop or
varietal rotation, and rogue intensively.
Fungicidal control of blast and sheath blight is increasing in many intensified
rice areas. It is extremely important that these fungicides be carefully screened not
only for efficacy as fungicides but also for their impact on natural enemies in the
rice ecosystem. One example is the release of iprobenfos as a fungicide for blast
control. Iprobenfos is an organophosphate that was originally developed for brown
planthopper control and is highly toxic to natural enemies. Its use in the rice eco-
system is likely to cause ecological destabilization and consequent outbreaks of
brown planthopper. Fungicides should also be carefully screened for their impact
on fish, both to avoid environmental damage in aquatic systems and to avoid dam-
age to rice–fish production.
In general, clean and high quality seed with resistance to locally known diseases
is the first step in rice IPM for diseases. An appropriate diversification strategy (vari-
etal mixture, varietal rotation, varietal deployment, crop rotation) should counter
the capacity of pathogens to adapt quickly to the resistance of the host. Management
of organic matter has to be geared not only towards achieving balanced fertility but
also in enhancing the population of beneficial microorganisms.
Box 9.3 Diversity defeats disease
Glutinous rice is highly valued in Yunnan, China, but like many varieties that have
been ‘defeated’ by rice blast, it cannot be grown profitably without multiple foliar
applications of fungicide. Rice farmers, guided by a team of experts from IRRI and
Yunnan Agricultural University, have successfully controlled rice blast simply by
interplanting one row of a susceptible glutinous variety every four or six rows of the
more resistant commercial variety. This simple increase in diversity led to a drastic
reduction of rice blast (94 per cent) and increase in yield (89 per cent) of the suscep-
tible variety. The mixed population also produced 0.5–0.9 tonnes more rice per ha
than their corresponding monocultures, indicating high ecological efficiency. By the
year 2001, this practice has spread in over 100,000ha of rice in Yunnan, and is being
tried by other provinces.Varietal diversity creates an entirely different condition that affects host pathogen
interaction. To begin with, a more disease-resistant crop, interplanted with a suscep-
tible crop, can act as a physical barrier to the spread of disease spores. Second,
with more than one crop variety, there would also be a more diverse array of patho-
gen population, possibly resulting in induced resistance and a complex interaction
that prevents the dominance by a single virulent strain of the pathogen. Finally, inter-
planting changes the microclimate, which may be less favourable to the pathogen.