Gabriele Stoll
Natural Crop Protection in the Tropics
Letting Information Come to Life
Methods of Field Protection
Plant Health and Disease Control
Plant diseases can often be as serious as insect pests in reducing the yield of crops. Diseases are caused by pathogens – these may be fungi, bacteria or viruses. The plants may be infected by pathogens that occur in the soil, adhere to the seed or planting material, or are carried in the wind or by insect vectors.
The natural control of diseases focuses first on producing a healthy plant which is capable of fending off many of the common plant diseases. The approach to accomplishing this is to first produce a healthy soil by managing organic matter and enhancing soil life including microorganisms, and optimizing nutrient availability by green manure, composts and other biofertilizers, including the use of nitrogen fixing crops. While a section on biofertilizers is included in this chapter, many of the details are outside the scope of this book. The interested reader is referred to Reijntes et al. An additional approach that is addressed in this section is the use of biofertilizers that aid in producing healthy plants.
The next step in disease control is to select disease free or resistant seeds or planting material. If that is not possible, seed or planting material can be treated. If disease is soil-borne, a variety of cultural methods and plant extracts can be used to treat the soil or the planting material. Air-borne diseases can be treated by the use of foliar, plant-based sprays and other products. Insect-borne viruses are best controlled by controlling the transmitting insects. This is particularly true of aphids, which carry a number of virus crop diseases. Indian scientists are presently studying the virus-controlling effects of aqueous plant extracts, which seem to be very promising.
The following section gives some information on producing healthy plants using biofertilizers and plant growth stimulants, controlling soil-borne diseases, and plants with potential disease-controlling properties.

Snails and Slugs
Plant Health and
Disease Control