Plant disease management strategies

M. Mithal Jiskani and Khadim H. Wagan
Department of Plant Pathology, Sindh Agriculture University, Tandojam
Oct 28 - Nov 03, 2002

The human beings and plant pathogens (biotic disease causing agents), both are competing for the same resources needed for food, clothing, shelter and other requirements for life survival. Therefore, the importance of plant protection in relation to crop production can be realized from the fact that the yields are not of the desired level in several crops. Among the various factors responsible for low yields, plant diseases are prominent and cause losses in world's crop production amounting to many million rupees, which comes next only to losses caused by insect pests.

Plant diseases damage the crop, reduce the yield, lower the quantity and quality, increase the cost of production and require cash outlays for material and equipments for control measures. Besides, chemicals used for disease control, are poisonous for human beings and are main cause of environmental pollution. The crops are subject to many diseases: biotic diseases are infectious, caused by pathogens that include fungi, bacteria, viruses, and nematodes; whereas a-biotic diseases are non infectious and are caused by environmental conditions such as soil imbalances (nutrients or pH), moisture imbalances, and chemical injury (herbicides and toxic chemicals). It is very difficult to discuss the importance of specific diseases on the different crops. Some diseases directly attack the fruit and make it unfit for consumption. Others indirectly reduce yields by killing plant parts prior to harvest or may cause defoliation, which reduces fruit size and quality, as well as exposing fruits to sun scald. The losses caused by the diseases are variable but their cumulative effect is considerable, and in certain cases fields have been found to show as much as 100 per cent damage.

The pathogen (fungus, bacterium, nematode or virus), once introduced in a field, can be spread by wind, water or rain, and equipment. The grower or field worker itself, the insect pests, birds, animals also help the disease-causing organism to spread more. Even the higher pathogens may become carrier for another. For example, viruses are transmitted through fungi and nematodes. The dry and wet as well as hot and cool climatic conditions sometimes help to reduce but may also increase development and spread of the disease. The disease inciting agents perpetuates (survive from season to season) through the seed given from diseased crop, weed plants, infested fields and all plant debris.

The diseases are most important factors limiting production of field, vegetable and fruit crops. The causes of diseases are very few but the diseases caused by them are so much and are difficult to count!? Some diseases are easy to identify but others are more difficult. Effective disease management is essential in the production of high quality production and the identification of diseases is first step in effective management. Control of the diseases may includes crop rotation; planting resistant varieties when available or sowing or planting of certified, disease-free seed or seedlings; timely sowing or planting in well-prepared, fertile soil; controlling weeds and insect pests; following a fungicide spray program when recommended and practicing clean plough down after harvest. It must be kept in mind that in case of any doubt, or incorrect identification of diseases may lead to the use of wrong management practices, wastage of time and wasted expenses that will result the failure of crop. For example, insecticides do not control diseases caused by bacteria, or viruses are also not controlled with most fungicides. Similarly, a particular fungicide may control only one fungal disease, but not another. Therefore, the growers should must learn to recognize the more common diseases by their symptoms and they have sufficient knowledge of disease development to select appropriate management practices for the particular disease situation.

Integrated pest management (IPM) involves the use of several different strategies and the judicious use of pesticides for management of diseases and other pests of the crops. The most beneficial, better and more economical control of all diseases and insect pests may be achieved through IPM as compared to any one, a single management practice, such as pesticide application. The weed and insect management are also important components of disease control in an IPM system, because infection and spread of some pathogens is associated with the presence of certain weeds (alternate hosts) and insects (vectors).

However, the disease management strategies that are effective components of an IPM system are given below, with a hope that the growers will adopt these for the benefit of themselves as well for nature, because the chemicals applied for the control of diseases and insect pests are equally poisonous for all livings and is major source of environmental pollution. Their effectiveness in controlling specific diseases could be learnt through regular study and observation.

CROP ROTATION: The fungi, bacteria, and nematodes cause soil and seed borne as well as foliar diseases. These pathogens may survive from season to season in the soil or on seed and other crop debris in soil and build up to damaging levels with repeated cropping, therefore 3-4 year rotation with non-host crops is recommended.

SITE SELECTION: Most crops are best grown on sandy loam, sand, or silt loam soils with a pH of 6 to 7. Growth on acid and/or poorly drained soils often results in increased incidence of Fusarium wilt, fruit rots and some other diseases. Therefore, maintaining records of the disease history of fields is beneficial for avoiding disease problems or implementing preventive measures. In such type situation, late plantings should not be situated near early plantings where a disease already exists.

SANITATION: Several destructive diseases of various crops can be spread from infested fields to clean fields in soil and crop debris carried on equipment and workers. Therefore the equipments and boots should be washed to remove all clinging soil and debris; when leaving infested fields to avoid contamination of clean fields. Clean fields can also be worked before entering infested fields.

VARIETY SELECTION: Resistance is the most effective and economical means of disease control. For some diseases, resistance is the only effective control. Therefore, available disease resistant varieties should be planted where possible.

SOIL FUMIGATION: Soil fumigation is expensive and potentially dangerous for inexperienced applicators, but increases yields, earliness, and controls soil borne diseases. However, row fumigation may be economically feasible mostly for vegetables.

PLANTING TIME: The planting time sometimes favours or may disfavours the multiplication of disease inciting organisms, therefore, change in planting time is recommended in various cases. For example, cultivation of early sowing and early maturing varieties are recommended for the control of rust in wheat.

DISEASE-FREE SEED AND SEEDLINGS: Some diseases may be seed borne or introduced into fields on infected seedlings (transplants). Efforts should be taken to obtain disease free seed and transplants. Seed dressing fungicides are found effective in minimizing the potential for seed borne diseases caused by fungi and to help assure establishment of an adequate stand. Only healthy or treated seed and seedlings should be used to initiate plantings.

WEED CONTROL: Mostly the weeds serve as alternate hosts or sources of infection for virus diseases. Therefore, effective weed control practices should be utilized in and around the crops.

INSECT CONTROL: The insects are major source to transmit virus and bacterial diseases. Therefore, insects must be controlled when warranted in and around the crops.

IRRIGATION: Excessive irrigation or frequent irrigations with small amounts of water as well as shortage of irrigation (drought conditions, due to long irrigation intervals) favours spread and development of many diseases. Therefore, the crops should be irrigated properly.

FERTILIZERS: Heavy doses of nitrogenous fertilizers may maximize but judicious use of potassic fertilizer help in minimising the susceptibility of plants. Therefore, proper use of fertilizers is recommended.

CHEMICAL CONTROL: Fungicides protect healthy plants from infection, but do not cure diseased plants. These are needed for effective management of some foliar diseases caused by fungi. Copper sprays are also useful in reducing foliar diseases caused by bacteria. The diseases are more difficult to control once established; hence timing of the first spray is critical. The first spray should be made before symptoms appear where diseases are anticipated or shortly after symptom appearance. Moreover, an adequate spray volume is needed to achieve thorough coverage of infected plant or tree. Thereafter, an approved schedule should be maintained. Sprays should also be applied before an anticipated rain event rather than after because this affords protection during periods favourable for infection. Most soil borne diseases cannot be controlled by foliar fungicide application, but drenching may be found useful in such condition.

SCOUTING: Scouting allows for early detection of all pests and diseases so that timely management practices can be implemented. Plantings should be scouted regularly (at least once per week), for assessment of the effectiveness of management programmes already implemented.

RESIDUE MANAGEMENT: Many of the pathogens survives in and on the plant debris or pruned parts of plants or trees, therefore theses should be incorporated into the soil by ploughing or disking after harvest to hasten decomposition.