Disease Forecasting PDF

# Forecasting of Plant Diseases Forecasting of plant diseases simply means predicting for the occurrence of disease in a specified area ahead of time so that suitable control measures can be undertaken well in advance to avoid heavy losses. It involves a well organised team work and expenditure of...

# Forecasting of Plant Diseases Forecasting of plant diseases simply means predicting for the occurrence of disease in a specified area ahead of time so that suitable control measures can be undertaken well in advance to avoid heavy losses. It involves a well organised team work and expenditure of time, energy and money. Once ascertained, the farmers are notified that the conditions are favourable for certain diseases and they are advised to apply suitable control measures. It is, therefore, prerequisite to have appropriate control measures of the disease for which the forecasting is done. There is no use of undertaking studies for forecasting of a disease which can not be controlled once it has started developing in the host. ## Informations Needed For Disease Forecasting Forecasting of disease is a part of applied epidemiology. Therefore, for accurate forecasting knowledge of epidemiology (i.e., development of the disease under the influence of factors associated with the host, the pathogen and the weather) is necessary. The factors of epidemic and its components should be known in advance before the forecasting is done. The various informations needed for accurate forecasting are as follows: ### (1) Factors associated with the host Among the various factors associated with the host, the important ones are: 1. **Prevalance of susceptible varieties in the given locality.** 2. **Response of host at different stages of its growth to the acitivity of pathogen.** For example, some diseases occur only during seedling stage of host while others attack the grown up plants. The plants having delicate and fragile foliage get more easily infected as compared to those with tough and healthy foliage. 3. **Density and distribution of host in the given area.** Usually a dense population of a susceptible variety invites the spread of an epidemic. On the other hand, if they are grown on limited scale and at scattered locations they are less prone to an epidemic. ### (2) Factors associated with the pathogen The factors associated with the pathogen, needed for disease forecasting, include almost all the stages of disease cycle viz., the amount of initial inoculum, dispersal of inoculum, spore germination, infection, incubation period, sporulation on the infected host, re-dispersal of spores or dissamination, perennating stages, inoculum potential and density in the soil, seed and air. The most important factor associated with pathogen is the amount of primary-inoculum present in the air, soil or planting material. The number of propagules present in the air can be determined by various air trap devices. Similarly the distribution and amount of inoculum in the soil and planting materials can also be examined by usual methods. However, it is difficult to estimate the amount of primary inoculum surviving at a remote place and brought to the locality by winds to initiate the disease. ### (3) Factors associated with the environment The most important environmental factors needed for disease forecasting are temperature and moisture. They effect both the host as well as the pathogen and play very important role in disease development. The weather conditions (i.e., temperature, humidity, light intensity, wind velocity, etc.) during the crop season and during the intercrop period (i.e., the period between the two crops) should be observed for a number of years in the field before the accurate disease forecasting is done. In fact the environmental factors are the final determinants of the disease development. ## Conditions For Disease Forecasting The plant diseases for which forecasting may be of practical value to the farmers are: 1. **Those destructive diseases which cause great economic losses** 2. **Which show variation in the manner of spreading, speed of spread and destructiveness.** 3. **The control measures of the disease are fully known and can be economically applied.** 4. **Informations regarding weather-disease relationship are fully known.** ## Methods of Disease Forecasting Forecasting of a disease requires certain field observations, collection of data on weather, variety and response of disease cycle on the pathogen, certain laboratory investigations and their correlations. Usually the forecasting of a disease is done by the following methods: ### (1) Forecasting based on primary inoculum Presence of primary inoculum, its density and viability is determined in the air, soil or planting material. Occurrence of viable spores (or propagules) in the air can be determined by various air trap devices. Similarly, presence of primary inoculum in the soil, in case of soil-borne diseases, can be determined by monoculture methods. The planting materials are also examined by various methods. Presence of primary inoculum of viral diseases of potato, loose smut of wheat, ergot of rye and bajra, etc. can be detected in random samples in the seed lots by different methods. On the basis of these estimations specific precautions are given to the farmers and specific control measures are suggested. ### (2) Forecasting based on weather conditions Weather conditions viz., temperature, humidity, rain fall, etc., during the crop season and during the intercrop periods are measured. The local weather conditions above the crop and at the soil surface are also recorded accurately forecast the disease. For example, forecasting the blast of rice (Pyricularia oryzae) can be made on the basis of minimum night temperature <ins>inge of 20 </ins>—26°C associated with 90% or above relative humidity for a <ins>riod of more than a week.</ins> ### (3) Forecasting based on correlative information In this method the weather data of several years are collected and related with intensity of the disease. These data are compared and then forecasting of the disease is done. ### (4) Use of computer for disease forecasting In some advanced countries the forecasting of disease is done by the use of computers. This system is much advantageous as it gives results much in advance, easily and much quickly. One such system is working in U.S.A. known as Blitecast. When an American farmer desires a forecast, he contacts the Blitecast along with recently recorded environmental data. The operator feeds these data into the computer programme which analyses within a fraction of second and returns the forecast along with the control measures. The control measures are recommended to the farmer so that he may protect his crop well in advance. The various environmental data needed for disease forecasting by computer are: 1. **Maximum and minimum temperature of the day** 2. **Number of hours of relative humidity more than 90%** 3. **Maximum and minimum temperature during the period when the relative humidity was more than 90%** 4. **The rain fall recorded at 24 hours basis and measured to the nearest of 0.1 cm.** ## Examples Of Disease Forecasting 1. **Late blight of potato:** The most popular disease for which forecasting has been tried since long is late blight of potato caused by Phytophthora infestans. In this disease, the suitable control measures are known and weather - disease relationship is also fully understood. According to Dutch rules, the appearance of this disease depends on (1) a night temperature below dew point for at least 4 hours ; (2) a minimum temperature of 10℃ or above; (3) the next day should be cloudy; (4) at least 0.1 mm rainfall during the next 24 hours. 2. **Tikka disease of ground nut :** Tikka leaf disease of ground nut caused by Cercospora personata ( = Cercosporidium personata or Mycosphaerella berkeleyvii) and Cercospora arachidicola is predicted in U.S.A. by diurnal periods of 10 hours or longer with relative humidity 95% or above and with temperature above 21°C. 3. **Blast of rice :** In India, the forecasting of the disease blast of rice casued by Pyricularia oryzae is done mainly by correlative information method. It is predicted on the basis of minimum night temperature 20-26°C in association with high relative humidity of 90% or above. 4. **Wheat rust :** Forecasting of rust epidemic is done by analysing the rain samples which gives precise data of inoculum present in the air. Moreover, several wind trajectories are also prepared to survey the air-borne inoculum and its deposition. It has been observed that primary inoculum in the plains of central and Northern India comes from South. 5. **Brown stripe downy mildew of maize :** This disease is caused by Sclerophthora rayssiae var. zeae. It is restricted to India. The forecasting of this disease is done on the basis of average rain fall 100-200 cm or more accompanied by low temperature (25°C or less). ## Questions 1. Write an account of forecasting of plant diseases. 2. Write short notes on methods of disease forecasting. 3. What are the environmental conditions needed for the development of the following diseases as epidemic in India : * Late blight of potato * Blast of rice * Wheat rust * Tikka disease of ground nut