Plant Disease Management : Points to Ponder upon

A crop is attacked by several diseases. For example, potato is attacked by early and late blight, black scurf, root-knot and several viral diseases. Each disease has specific control measures. It is not necessary that the control measure for one disease will control other diseases also. Therefore for controlling a number of major diseases the causal agents their mode of spread and activity should be taken into account. The approach should be so coordinated that the maximum number of diseases can be controlled partly or fully by a minimum number of operations, this multiple disease control method can reduce the cost of plant protection giving the grower considerable ease in applying them. For instance, all potato diseases can be checked by adopting the following measures.

1. For controlling early or late blight, seed treatment by chemicals can be practised before sowing the seeds. After the occurrence of disease, a spray of fungicide can check the damage caused by the pathogen.

2. For controlling black scurf disease, seed treatment with fungicide is recommended.

3. For controlling root-knot disease and potato wilt, deep ploughing of field before sowing is useful. In the soils where these pathogens are already present, the potato should not be grown for a long time. Crop rotation can help in removing these pathogens from the field.

4. Viral disease pathogens (i.e. virus) are carried by aphids. Therefore, removal of leaves and stem above the ground level, 30 days before harvesting minimizes the attack as aphids suck the sap from leaves.

Is disease control by individual farmer effective?  

For an effective control measure its adoption on a large contiguous area is necessary as pathogens are disseminated through air, water, insects or by shifting of soil. It is quite impossible to raise an effective and permanent barrier against these agencies transmitting plant diseases.

Suppose one farmer in the affected area follows certain control measures in this field to remove a particular disease, but his neighbouring farmers are not practising them in their crop. In such a situation there will not be much effect of control, because later on, pathogens will transmit again to the controlled field from its neighbouring affected fields. To avoid such a situation, the individual farmer may have to spend more money, time and energy in repeating the control measures. Therefore, if control measures such as chemical spraying on standing crop or crop rotation or deep ploughing are adopted on cooperative basis over adjoining areas under a crop its effectiveness or chances of success would be much more.

How to decide proper control in relation to the disease cycle?

For effective and economic control in relation to the disease cycle is necessary:

1. Source or cause of disease

2. Mode of penetration;

3. Transmission or spread of pathogen and

4. Effects of the environment on disease development.

For destroying the pathogen or suppressing its growth, knowledge of the various aspects of the disease cycle is helpful. The correct knowledge about the cause of the disease can prevent new problems and reduce expenses. For example, yellowing of rice or wheat leaves or stunted growth of their plant may not necessarily be due to the pathogen. It may be due to nutritional deficiency or improper drainage. Here we have to apply nutrients or provide proper drainage instead of applying any chemical.

Similarly, a disease may be sea-borne or spread through wind. Based on this information, the farmer can decide on seed treatment by a systemic chemical, which can kill the pathogen or some other measure like removal of diseases from the field. Similarly, loose smut  of wheat can also be controlled by the same measures. The knowledge of environmental relations makes it possible to determine the time of appearance and severity of the disease so that preventive measures can be adopted more accurately.

Always consider cost-benefit ratio

The plant disease reduces economic gain from the crop. The aim of disease control is to check this reduction. A farmer can adopt any control measure if it is beneficial in terms of money. Acceptance of any control measure will thus depend upon cost-benefit ratio. If a farmer sees more benefit in spraying chemical instead of control measures, then it is difficult to convince him about adopting cultural practices for controlling diseases.