What are the classifications of disease specimens?

Xu Zhigang

Sick plants or pathogens used for teaching demonstration, scientific research, academic exchange or exhibition. Plant disease specimens usually include diseased plants with typical symptoms, pure cultures of pathogens, slide specimens or photographs of pathogens. A complete specimen should be formally labeled, which is an index that can provide the most important information. The label on the diseased plant should record: host name (with Latin scientific name), disease name, pathogen name (with Latin scientific name), collection place, collector, appraiser, collection date, brief description of disease symptoms in the remarks column, geographical characteristics of the disease place, and other necessary explanations. For long-term preservation and use. There are many kinds of specimens, and the types of specimens are quite different because of the difference of plants and pathogens, the purpose and purpose of preservation, and the different methods of making. Common ones are wax leaf specimens, soaking specimens, slide specimens, agar film specimens, strain specimens, living specimens, photos, young lamps and videos.

Wax leaf specimen

Plants with typical symptoms are collected, dehydrated, dried, shaped, and then packed in glass boxes or fixed on cardboard and labeled. Wax leaf specimens are mostly limited to plant diseases on roots, stems, leaves, flowers and seedlings, and are rarely used to inhibit fruit diseases. Some diseases show different symptoms at different growth stages or different parts of plants, and should be suppressed after being collected at different stages or different parts. Pathogens of some diseases can invade different hosts and show different symptoms. For example, the symptoms of pear rust are different between pear and host Sabina vulgaris, so samples at different stages should be collected from each host. The drying process of wax leaf specimens has great influence on the quality of specimens. The traditional method is to put fresh specimens in dry absorbent paper and gradually dehydrate them by changing the paper several times. The dried specimen basically keeps its original color. You can also use the rapid drying method of hot sand or iron pressing, and then put the treated specimen in absorbent paper to absorb water 1 ~ 2 days. The advantage is that it can be flattened and dried quickly, but the disadvantage is that high temperature often leads to fading or discoloration, and green is not easy to preserve for a long time. The better effect of protecting green is impregnation with copper acetate or copper sulfate. Fresh specimens can remain green for a long time after soaking and drying. You can also seal the dried and pressed specimen in a special plastic film, which is more convenient for storage and can be observed from both sides.

Impregnated sample

For succulent plants, fruits or plants with swelling symptoms, in order to keep the characteristics of the disease or the original color as much as possible, it must be soaked with preservative solution or color protection solution. Copper acetate and sulfurous acid are the most commonly used impregnation solutions, which are different according to the sample type and preservation requirements. For example, formalin solution semen (FAA) can be used for simple preservation, copper acetate or copper sulfate solution can be used for green preservation, and hessler solution and WaFriedrich Hirth solution can be used for yellow preservation and orange preservation. Impregnated specimens are often kept in square or cylindrical specimen bottles filled with chemicals, and the bottle mouth should be sealed with paraffin to prevent the specimens from drying and deterioration after liquid evaporation.

Slide specimen

There are two kinds of temporary slides and permanent slides. Temporary slides are usually made by slicing pathogenic microorganisms or diseased tissues by hand, and water, creosote or Hill's solution are used as floating agents. The floating agent in the temporary slide is easy to evaporate and dry up, and it can't last long. The edge of the cover glass can be sealed with nail polish or resin to make a semi-permanent glass slide. Permanent slides are paraffin sections made of diseased materials or fruiting bodies of pathogens. The thickness of the material in the sample is uniform. After transparent staining, different tissues show different colors, with strong contrast and legibility, which can remain unchanged for decades, which is beneficial to pathogen identification or pathological study.

Strain sample

There are many kinds of pathogenic organisms, and the strains preserved in the laboratory are pure cultures after separation and purification. Most pathogenic bacteria and fungi can grow on artificial culture medium and be stored in refrigerator, and most of them are stored in vacuum ampoules after freeze-drying. Most fungi grow on agar plates in Petri dishes, forming colonies with specific shapes, then transfer to cellophane, evaporate and dry, leaving a layer of agar film with colonies, package it in clean paper bags, and keep it with wax leaf specimens.

living specimen

Many disease specimens should be kept alive as much as possible until they are completely identified or determined. Especially all kinds of downy mildew and bacteroides. Once the tissue dries up and dies, these obligate parasitic pathogens also die, so it is difficult to further isolate and identify them. Therefore, the collected materials should be kept alive as much as possible, and if necessary, they should be continuously transferred or inoculated to herbal host plants or even test-tube seedlings cultivated in laboratories or greenhouses. A few pathogenic fungi and bacteria, as well as all viruses, cannot grow on the culture medium and must be inoculated on the host to preserve their vitality. The seeds of parasitic seed plants can be sealed in dry seed bottles and stored at low temperature for many years.

Photos and videos

The ecological environment of the main disease plants and pathogens seen in the field during investigation or collection can be described in words, or photographed with a camera or video camera, which can save more complete, detailed and realistic images than naked eye observation.

transmission

transmission

Zhao meiqi

The process of pathogen vector spreading from diseased plants or places to healthy plants or places. It shows that the spatial distribution of diseases changes with time, that is, the dynamic change law of transmission distance and transmission speed during the epidemic process of diseases. The quantitative change law of disease transmission mainly depends on the species, biological characteristics, quantity, transmission mode and motivation of pathogenic vectors. The quantitative research on the law of disease transmission is mainly aimed at airborne diseases, while the research on other diseases is less.

Disease transmission is a series of complex biological and physical processes. Although it is based on the spread of pathogens (see vectors), it is not the same. Firstly, the formation and release of spores determine the quantity and quality of vectors. Its spread and landing determine the physical spread, infection and pathogenicity of spores, and finally realize the spread of diseases. Therefore, in the study of disease transmission distance, the concepts of physical transmission distance and disease transmission distance of communicators are put forward. Because the airflow propagation law of spores is almost the same as that of abiotic airborne particles, the theory and method of aerodynamics have been cited for a long time to describe the physical propagation distance of pathogen carriers. D.E.Aylor (1978) studied the relationship between spore release rate and external force on spores. Schrodt (H.Schr？ Dter, 1960) put forward the relationship between spore dispersion distance and updraft, horizontal wind speed and settling speed, and maximum spore height and updraft. Pasquale (F.Pasquill, 1962) transplanted a Gaussian plume model describing the law of airborne particles to study the physical transmission law of pathogen vectors. However, when a pathogen spreads far away through the air, whether it can germinate, infect or even cause epidemic consequences of diseases is restricted by a series of complex factors. Therefore, the physical transmission of pathogens is only the premise of disease transmission, and the transmission distance of diseases is actually the effective transmission distance of pathogens.

The spread distance of the disease is not only affected by the physical characteristics of the vector and the law of airflow movement, but also by the vector, host plants and their environment and other related biological factors. When a certain amount of spores spread from the center of the source, the spatial distribution of new diseases is generally the largest in the center of the source, and the farther away, the smaller the density, showing a certain gradient. This is the infection gradient or disease gradient. The gradient model established by Shigeru Kizawa and Mackenzie (D.R.Mac Kenzie, 1979) can be used to deduce the probability of disease spreading to a certain distance (see disease gradient). Different diseases have different transmission gradients and different transmission distances. The slower the gradient, the farther the propagation distance; On the contrary, the propagation distance is short. According to the gradient model, the disease density is close to zero only when the distance is infinite. But in fact, the spread distance of the disease is limited. Therefore, when calculating the propagation distance, we must first determine the "minimum condition" of the disease according to the type of the disease and the accuracy of the work requirements, and then we can deduce the propagation distance from the gradient model. In the actual epidemic process of the disease, the time of spore release can be artificially controlled, and the first transmission distance or the first generation transmission distance of the disease can be measured (see transmission distance).

In the primary transmission distance or the first transmission distance of the disease, the pathogen vector generated by the center of the bacteria source is affected by the wind in one direction in a short time, resulting in the fan-shaped distribution of the offspring diseases, which can sometimes be simplified to one-way linear transmission. This kind of transmission result is rare in nature. What's more, the wind direction and wind speed have changed many times in a period of time, resulting in a round, oval or even irregular distribution of new diseases after transmission. According to the measured data of disease transmission, various spatial dynamic models were developed, such as linear, circular and elliptical spatial dynamic models of wheat stripe rust epidemic in spring. We can also combine the spatio-temporal dynamics in the epidemic process of diseases to establish a spatio-temporal integrated model, which can predict the spread distance of diseases, infer the spread speed of diseases, and analyze the influence of host relative disease resistance and plant density on its field disease pattern. So as to serve for disease epidemic prediction and management decision-making.

Disease investigation

Investigation of plant diseases

Shang hongsheng

Collect basic data on the types, distribution, severity, harm and loss of diseases and related environmental factors at the scene of disease occurrence, so as to clarify the occurrence law of diseases and provide reliable basis for strengthening disease prevention and control. Disease investigation is an important basic work, which is not only the premise of carrying out experimental research, but also the basic work that must be carried out before making control strategies in production.

Survey types Disease surveys are divided into basic surveys (general surveys) and special surveys. The purpose of basic investigation is to understand the types, distribution and loss degree of various plants or specific plants in a certain geographical area, and use the obtained data to compile disease records, draw disease distribution maps and draw up prevention and control plans. The general survey data of plant quarantine diseases is an important basis for dividing epidemic areas and protected areas and determining or canceling quarantine objects. The objects and purposes of special investigation are different, and most of them are diseases with important economic significance, so as to deeply understand the key problems in the occurrence and prevention of diseases. ① Investigation on the regularity of disease occurrence. Mainly understand the relationship between the occurrence of diseases and environmental conditions, varieties, cultivation measures, or the characteristics of diseases in the key stages of epidemic (wintering and summering). Sometimes, through years of multi-point investigation, we can understand the temporal and spatial dynamics of disease development and accumulate systematic data for establishing digital models. ② Forecast and investigation. Focus on collecting bacterial count, illness and meteorological data, which can be used to establish prediction formulas or predict diseases according to existing methods. ③ Special investigation on disease control. It is to evaluate the control effect, benefit and existing problems of pesticides, varieties, natural enemies and comprehensive measures. ④ Investigation on disease resistance of crop varieties. Mainly understand the disease resistance performance and variation of field varieties.

Principle of investigation

The investigation of plant diseases should follow the following basic principles. (1) Having a clear investigation purpose and task; (2) There should be a careful investigation plan and appropriate investigation methods should be determined; (3) truthfully reflect the situation and prevent subjective one-sidedness; (4) Control the scale of investigation and try to save time, manpower and financial resources; ⑤ The survey data is complete, accurate, reliable, representative and comparable; ⑥ Closely combine with field experiments and indoor research, and connect with each other.

investigation method

According to the nature of the disease and the purpose of the investigation, choose the appropriate investigation method. There are two common survey methods: itinerant survey and fixed-point survey. The former is suitable for a large geographical area, and most of them are investigated according to the established route. Predict and investigate some diseases, and investigate them according to a certain route every year to accumulate comparable disease information. Fixed-point investigation is to select representative fields, fixed investigation points or fixed investigation plants, and conduct multiple investigations at certain time intervals to understand the law of disease growth and decline. In addition, during the prediction and variety resistance investigation, investigation nurseries (observation nurseries) were set up on plots suitable for disease. The former planted susceptible varieties to avoid the interference of variety resistance and obtain the real data of bacterial source and disease condition, while the latter planted a group of resistant varieties to observe the change of disease resistance.

Disease investigation is mainly based on the actual investigation of the disease site, supplemented by interviews and discussions and access to historical materials. In addition to the fixed-point system investigation, due to the limitation of time and manpower, the methods of field inspection and visual estimation are often used, and samples are carefully counted when necessary. The time interval and frequency of the survey vary according to the purpose of the survey. The general survey is conducted every 5 ~ 10 years, and the special survey can be conducted irregularly or regularly. Before the investigation, we should study and determine the investigation period, frequency and sampling method, select the recording standard of incidence, disease severity and infection type, print the investigation form, and prepare common instruments such as counters, magnifying glasses, telescopes, altimeters, tape recorders and cameras. For a few crops, a semi-automatic or automatic field disease data collector has been developed, which can record disease data and input them into a computer under the supervision of investigators. Remote sensing technology is also used for disease investigation and loss estimation.

Epidemic trends of diseases

Popular trends

Xiaoyueyan

Under certain environmental conditions, the number of diseases increases and decreases with time and space. The generalized epidemic dynamics of diseases include the changes of disease types (community structure) with time and space. This paper mainly studies the spatial distribution pattern, quantitative growth and decline rate and its changing law of diseases, which is the core issue of plant disease epidemiology and an important basis for disease prediction and control decision.

The epidemic dynamics of diseases can be divided into temporal dynamics (see temporal dynamics of disease epidemics) and spatial dynamics (see spatial dynamics of disease epidemics), which are two aspects of the same process. Taking time as the main dimension, this paper studies the prevalence rate (△ x/△ x/△ t) of disease number (x) changing with time (t), involving corresponding curve forms and various description formulas. Spatial dynamics takes spatial distance (d) as the main dimension, and studies the disease gradient (△X/△d), propagation distance, propagation speed and its change rate of disease density or quantity with spatial position. Disease gradient and propagation distance can be said to be the spatial pattern at a certain moment, and the propagation speed increases the time dimension, which becomes the rate of change of propagation distance in the time dimension. The differences between these concepts are only simplified for the convenience of analyzing the spread and prevalence of diseases. In the objective epidemic process of diseases, temporal and spatial dynamics go hand in hand and are inseparable. Without the spread of diseases, it is impossible to spread diseases; Without effective communication, it is difficult to achieve sustained population growth. However, most of the existing research and application results belong to the category of time dynamics. Although there are some achievements in the research of spatial dynamics, there are few practical ones, and even less comprehensive research on spatio-temporal dynamics (M.J.Jeger, 1983, P.Kampmeijer and J.C.Zadoks, 1977, Zhao Meiqi and others, 1985).

Because epidemiology is the science of diseases in the population (J.E.Van der Plank, 1963), the epidemic dynamics of diseases are also based on the research at the population level. In-depth analysis should be based on the infection process and infection cycle at the individual level, and some new quantitative concepts and parameters should be developed, such as infection probability, symptom rate, survival rate of overwintering and overwintering pathogens, spread rate of disease spots, spore production and spore landing. Prevalence rate, an important parameter of epidemic dynamics, is the synthesis of these parameters at the population level. The relationship between disease epidemic dynamics and infection process is shown in the figure. The scale of epidemiological dynamics research has developed to the community level in the macro direction, involving the succession or evolution of disease types, geographical distribution and so on. , with a larger time and space span.

The relationship between seasonal epidemic dynamics and infection process According to the viewpoint of system theory, plant diseases are an integral part of agricultural ecosystem or farmland ecosystem. The epidemic dynamics of diseases are determined by the whole system structure, which reflects some functions of the system. The most important thing in the study of disease epidemic dynamics is to clarify the internal structure and external influencing factors of the disease system and their functions. Analytical methods are as important as synthetic methods. No matter the study of time dynamics or space dynamics, it is necessary to establish the relationship between speed and related factors, involving which dominant factors to choose and what form to establish.

With the continuous development of epidemiology, the epidemic dynamics of diseases have entered the stage of quantitative research. It is based on systematic quantitative monitoring of pathogens, diseases and the environment (see disease monitoring). In 1963, J.E.Van dar Plank first described the epidemic dynamics of tomato diseases with logistic model (see logistic equation). In 1969, Wagner and J.G. Horsfall first reported the simulation model of tomato early blight -EpiDEM. Recent research focuses on the quantitative simulation of disease epidemic dynamics, and combines loss estimation, control effect model and crop growth model to form a disease management system. The prediction of fashion trends has developed from short-term and medium-term to long-term or even ultra-long-term

philology

Ceng Shimai, Yan Yang: Epidemiology of Plant Diseases, Agricultural Press, Beijing, 1986.

Zadoks, J.C. and R.D.Schein, Epidemiology and Plant Disease Management, new york, 1979.

Disease epidemic monitoring

Epidemiological surveillance

Ceng Shimai

A comprehensive, continuous, qualitative and quantitative observation record of the disease epidemic situation is called monitoring for short. The purpose is to grasp the dynamic changes of disease epidemic and its influencing factors, provide reliable basis for disease prediction and prevention decision-making in production, and provide research data of epidemic law and prediction method for scientific research. Disease prevention and control are regarded as systematic management. Under certain prevention and control objectives, prevention and control decision-making is the core of management, prediction is the basis of decision-making, and on-site monitoring is the basis of prediction and decision-making. Without a large number of qualified data, it is impossible to develop forecasting methods and study prevention and control decisions. Farmland ecosystem and its plant disease system are still evolving, so it is necessary to insist on disease monitoring to deepen the understanding of epidemic law and improve the prediction method. The contents of monitoring include disease status (number or degree of disease), pathogen development process and population number, pathogenic physiological races, vectors, crops and environment. Monitoring methods vary from content item to content item, but the common requirement is to ensure accuracy and simplicity. Some special projects need special technology and equipment.

Difference between disease epidemic monitoring and systematic observation

Systematic observation is to investigate the number or density of diseases in a fixed field or its sample points every certain number of days, and grasp the dynamic changes of the number of diseases with time, and its object is often limited to a certain disease or disease itself. At present, systematic observation of diseases has become an important part of "systematic monitoring" of pests (diseases, insects, grasses and rats) in farmland. Monitoring is based on the systematic investigation of several major diseases, and at the same time, it conducts a comprehensive and systematic investigation of meteorological factors, cultivation conditions and host conditions, and also obtains information about exotic diseases related to endemic diseases. For some diseases, it is necessary to increase projects, such as understanding the race composition of pathogens with physiological races, investigating the vector population dynamics and virus-carrying rate of arbovirus diseases, monitoring the resistance of pathogens to certain pesticides, and so on. In a word, the component synchronization of "disease system" is a comprehensive and continuous investigation, and the monitoring object is the whole "disease system", not the time series data of a certain disease condition. Therefore, the "system" in "system monitoring" is not the general meaning of the usual term "system investigation", but the meaning of the scientific term "system" in system science. Only by monitoring this system can the information obtained meet the needs of forecasting and decision-making.

The basic principles and contents of service production and monitoring research are the same, but the specific practices and requirements are different. What serves production should be simple and practical. As long as it can meet the needs of local forecasting and decision-making at that time, there should not be too many projects to avoid difficulties. In order to serve the research, the project needs to be comprehensive and meticulous, with high data quality requirements, so as to deepen the understanding of the law and improve the forecasting method. Try to be economical and feasible, and get the most information with the least manpower and material resources. The two must not be separated, and we must work hard to combine them, so as to kill two birds with one stone, use one data for two purposes, or seek common ground while reserving differences and exchange information with each other as much as possible. Whether it is for production or research, systematic monitoring must be lasting and stable, including the relative stability of organization, system and technical methods. Like meteorological work system, without long-term, massive and consistent reliable data, it is unthinkable to understand the law and improve the forecast level. The monitoring work is in the primary stage, and the benefit is not great, but the longer it persists, the greater its effect will be. From the development point of view, the systematic monitoring of farmland pests will gradually develop to the height of meteorological observation.

Disease surveillance

Refers to the regular continuous investigation of diseases (degree or quantity of diseases). Estimate the situation of each survey, or visually estimate a certain area, or estimate the whole by sampling and counting. Disease estimation is the cornerstone of epidemiological research. Without quantitative methods and data, there will be no quantitative epidemiology, and it will be difficult to predict diseases, identify disease resistance of varieties, determine efficacy and control effect.

Disease estimation

One of the important and difficult tasks in epidemiology. Diseases are usually expressed by their prevalence, severity and disease index. The prevalence rate is the percentage of the number of diseased plant units to the total number of investigated plant units. Plant body units can be plants, stems, leaves, fruits, spikes, etc. The corresponding common rates are actually diseased plants, diseased stems, diseased leaves, diseased fruits and diseased ears. The prevalence rate indicates the prevalence of the disease. Severity refers to the percentage of the diseased area or volume of a diseased plant unit to the total area or volume of the unit, indicating the severity of the disease. The combination of prevalence and severity comprehensively shows the degree of disease occurrence. The disease index is the product of the above two, or the weighted average of the severity of each level. In fact, it is the average severity of plant groups (including sick and not sick). The general calculation method of disease index is as follows:

When the severity is average, the prevalence and severity are in the form of decimals of 0.00 ~ 1.00:

DI (disease index) = I (prevalence) s (severity)

When investigating the severity values of each unit respectively, the severity values are integers of 0,/kloc-0, 2, 3…, and the prevalence rate is still a decimal:

In the above formula, Xi is the number of survey units of each severity level, ai is the level value of each level, and amax is the highest level value.

(When calculating, all percentages are converted into decimals, 0- 1).

Severity grading

It varies according to the type of disease. The investigation unit of endemic diseases is the diseased organ, and the most common leaf spot is the leaf. Each leaf is classified according to the percentage of diseased spot area to total leaf area, such as wheat rust 0, 1, 2.5, 5, 10, 25, 40, 65, 100%. When a single plant is taken as the investigation unit of systemic diseases or local diseases, it is divided into several grades according to the severity of symptoms of the whole plant, and the grade values are given as a = 0, 1, 2,3 … amax respectively. The overall grade varies according to the types of diseases and work needs, and is generally divided into 4-6 grades. In recent years, in order to facilitate the storage and processing of computer data, whether the investigation unit is leaves, fruits, branches or whole plants, the severity is divided into 9 grades, and those without diseases are given ten grades from light to heavy, namely 0, 1, 2, 3, 4…9. In order to unify standards and facilitate information exchange, some standard maps of severity grading have been gradually formed for many common diseases. In some diseases, the lesions can be densely connected, and the severity can really reach 100%. However, in some diseases, such as wheat leaf rust, the given value of 100% severity is often greater than its true value, and the total area of diseased leaf spore piles marked as 100% on the severity classification map only accounts for 37% of the leaf area (madden, 1965438+), because each spore pile occupies a sterile area. How to grade and grade the severity of the disease depends on the rationality and practicability according to the use of the disease data, which not only ensures the necessary accuracy, but also facilitates the rapid determination of the grade. If the classification is too small and too thick, the result is not accurate enough; If it is too fine, the grading difference is very small, and it is not easy to quickly determine the grading. If the survey data are only used to compare the severity of diseases or analyze the development of diseases, there is no need to classify them too carefully in the range of high severity, especially for multi-cycle and local diseases, the development of diseases is a logarithmic series, not an arithmetic arithmetic progression. However, if the disease data in the critical period (referring to loss estimation) is used for loss estimation, the disease classification should take into account that the disease grade and loss degree are generally consistent, and strive to have a certain functional relationship between them. In addition, the classification of severity should generally conform to the difference of human visual sensitivity.

Weber-fechner law

Weber-Fechner law

The law of visual sensitivity. Horsfall and Barrat (1945) pointed out that the severity should be classified according to Weber-fechner law. The law points out that in visual inspection, the sensitivity of the naked eye to signal stimulation is directly proportional to the logarithm of stimulation intensity. Because when the proportion of diseased tissues in the total area is less than 50%, people pay attention to diseased tissues, while when the proportion of diseased tissues in the total area is more than 50%, people pay attention to healthy tissues. Therefore, the severity below 50% should be divided into 25%, 12%, 6%, 3%, etc., and those above 50% should be divided into 75%, 88%, 94% and 97%. Some people call this division HB system. The first half of HB system is consistent with the exponential growth of disease, and the second half is similar to the self-inhibition of disease growth. Therefore, it is reasonable and feasible for HB system to study the increase of disease number. If it is used for loss estimation, the top two levels need not be divided too finely, because there is no difference between 94% or 97% yield loss.

Relationship between prevalence and severity

This is a question of practical significance. In the field investigation, the prevalence rate is easy to investigate, the error is small, the severity is difficult to investigate, and the error is large. Under certain conditions, the prevalence rate has a certain relationship with the severity. For most leaf spot diseases, the area expansion of a single spot is limited, and its prevalence and severity are mainly determined by the number of infection sites. Before the prevalence is close to saturation, there is a positive correlation between the prevalence I and the severity S, which is called I-S relationship. It can simplify the field investigation, only investigate the prevalence rate, and then calculate the severity according to the prevalence rate. Campbell et al. (1990) proposed the following three theoretical formulas:

When the prevalence rate is very low, the distribution of lesions is random (Poisson distribution), then

Where: m is the maximum number of possible disease spots on the plant investigation unit.

When the prevalence rate is high, the lesions may be binomial distribution, and the lesions often appear in groups, then

Where: b is the maximum value of the number of lesions in each lesion group divided by the number of possible lesions in each plant investigation unit.

If the lesion shows negative binomial distribution, then:

In the above formula, the definition of m is the same as that of formula (1), and k is the aggregation parameter of negative binomial.

The values of parameters M, K and B in the above three formulas vary with the types of diseases and need to be determined through years of field investigation. The empirical prediction formula can also be obtained on the statistical basis of a large number of measured values. However, whether using theoretical formula or empirical formula, when the prevalence is close to saturation, the severity can no longer be calculated from the prevalence.

Pathogen monitoring

Regularly and continuously investigate the development process and quantitative development of pathogens. It is necessary to monitor the development progress of pathogenic bacteria, such as the maturity progress of ascomycetes, which can be used as the basis for short-term and medium-term forecasting of wheat scab, pear scab and other diseases. What is more important is the number of pathogens. It is difficult to estimate the number of pathogenic species. Except nematodes and higher parasitic plants, viruses and bacteria are too small to be observed by naked eyes, and the "individual" counting unit of fungal population cannot be defined. Although sclerotia and spores can be counted, it is difficult to determine the biomass and reproductive potential of mycelium. In most cases, the absolute number of pathogen populations in a certain spatial range, including biomass and individuals, cannot be measured and estimated. Even if a method can be worked out in theory, it cannot be implemented in practice. In fact, it is to monitor the change of the relative number of vectors, compare the disease epidemic systems under different time and space conditions, or as a representative value of the absolute number of pathogen groups under specific conditions. The capture of spores in the air and the determination of the number of bacteria in the soil belong to this property.

Quantitative determination of airborne spore density and soil pathogens The airborne spore density at or before the beginning of the epidemic of airborne diseases is an important basis for forecasting. There are many ways to capture spores: the simplest method is vaseline slide method or culture medium plane method to receive airborne spores; There are also rotor rod samplers and so on. Soil pathogens, such as sclerotia, nematodes and fungal spores. , can be directly counted by visual inspection or microscopic examination; Can't directly count, need to choose selective medium for quantitative separation (