thaibanana,
i'm in the tropics like you are.
here are some general tips:
1) fertilise. K & N are very important to bananas; resistance and yield improve with fertilisation. Don't apply fertiliser excessively..it's expensive!
2) micronutrient status is important... you don't have to use commercial preparations; if you know a little chemistry
3) stay AWAY from monoculture... run a highly polyclonal field ...include as many vars as possible. you're in Thailand!!!... there are really many vars there.
4) "inoculate" the field with leaves of ordinarily Sigatoka susceptible plant varieties (e.g Cavendish vars) from nearby anomalously "resistant" fields... in many such instances, a "beneficial" leaf surface microflora has developed to thwart Sigatoka.
I've seen this for myself.
These are
cheap methods that a small farmer can use... in addition to good field sanitation & field planning.
there are more...
shannon
shannon.di.corse@gmail.com