To prevent either bacteria, verticillium or fusarium wilt, please consider the following,

1. solarisation of nursery bed before planting to effectively reduce the bacteria or fungi population.
2. remove and burn plant debris after harvest to reduce inoculum pressure in the soil.
3. plant on raised beds or ridges to improve drainage.
4. reduce injury to crop roots as wounds are entry points of wilt pathogen.
5. avoid planting on land with history of wilt disease.6
6. practice long crop rotation for 4-6 years with cereals or legumes (maize, millet, sorghum or cowpea)
7. sterilize garden or farm tools after working in infected soils with fire.
8. extract seeds from healthy fruits.

Thank you Bernard.
Several factors could be the cause. Consider these points to help control the incidence of diseases and pest on your Dad’s vegetables.
– use certified and disease free seeds
– practice crop rotation and land rotation if possible
– uproot and burn affected plants
– scout regularly to detect diseases and pest around the farm.

For further discussions, kindly open a topic in the forum (http://myfarm.nobowa.com/support-forum). We would be glad to take you through the solutions.