Module 3. Monitoring, Tracking and Identification of Phytosanitary Organisms
Lesson 3.1: Diseases
3.1.1 Overview & Concepts
Biotic diseases in wheat are generally caused by microorganisms, mostly parasites. Parasitic is considered every living (animal or plant) that grows and multiplies inside or on other living things. The relationship that develops between a parasite and its host it is called parasitism. Plant parasites or plant pathogens are organisms that link closely to a plant and reproduce at his expense.
Pathogenicity is the ability of the pathogen itself to invade and live in the host causing the development of a disease. When the host is a crop such as wheat and it is susceptible to the pathogen, widespread development of the disease may occur, commonly known as epiphytia, provided that the environmental conditions are favorable for the fast and continuous development of the pathogen.
There are obligate or biotrophic pathogens, which are characterized by living, growing and reproducing in their host. Some of them are: viruses, viroids, mycoplasma, bacteria, fungi, nematodes, protozoa. In wheat it is common to have fungi producing diseases known as downy mildew, powdery mildew, rust, smut, etc. On the other hand, there are necrotrophic or facultative saprophytic pathogens characterized by the development of part of their cycle on dead organic matter, but have also the ability to parasitize live plants.
3.1.2 Importance of diseases
In wheat there are diseases of regional and / or global importance. Biotrophic pathogens such as rust (Puccinia spp.), powdery mildew (Erysiphe spp.) and smuts (Tilletia spp. and Ustilago spp.) affect the crop across the board when the environmental conditions and crop susceptibility favor them. Their development pattern is generalized, which facilitates monitoring of the crop. It does not depend on the structural conditions of the crop such as a no-till or conventional planting, crop rotation, crop cycle and planting date. They are those that have a single phase in their life cycle, parasitic or pathogenic. Their progress may be affected by volunteer plants and weed which act as reservoirs of inoculum, increasing their potential for harm. When the disease development is optimal, they reach the economic thresholds rapidly and the time for taking control strategies is reduced. The close genetic relationship between host and pathogen is apparent which causes a faster or slower development of the epidemic or field epiphytia.
In wheat there are also saprophytes or necrotrophic pathogenic parasites of regional and global importance. They spend much of their life cycle in the stubble at the expense of nutrients in the dead host tissues. They can be divided into two groups, namely those who depend on the stubble for their survival and those forming longevity structures that allow them independence from the stubble. Most foliar wheat pathogens are dependent on the stubble to perpetuate between years. Brown or yellow spot (Drechslera tritici-repentis) and septoria (Septoria tritici) are in this category.
These diseases can affect the crop across the board when the environmental conditions and the cultivar susceptibility favor them; but they can also affect the crop more severely, in a localized manner, depending on soil fertility and leveling, which are seldom homogeneous in a field for cultivation. Their importance is related to the structural conditions of the crop such as a no-till or conventional sowing, where in the first one the severity increases substantially, because they are perpetuated from one year to another by the crop stubble. Their establishment in the crops is also dependent on the health of the seed used (initial inoculum), the crop cycle, crop rotation with different ancestors, planting date, etc.
Under normal conditions, the development of these saprophytic diseases is slower compared to biotrophic pathogens. The economic damage thresholds are reached more slowly so the time for taking control strategies is expanded, which will be in close relation to the stage of crop development. As in the previous case, it is also apparent the genetic relationship between the host and the pathogen, which causes a faster or slower development of the epiphytia in the field. These pathogens do not survive in buried stubble, have little saprophytic ability and because they do not produce endurance structures in the ground, their viability is contingent on the colonization of crop tissues. On the other hand, cultural practices that promote the movement of soil and burial of plant residues do not substantially affect the inoculum for root pathogens as they live in the soil. Causal agents of Take-all and root rot as Gaeumannomyces graminis, Fusarium graminearum and Bipolaris sorokiniana belong to this group of pests.
Diseases of Wheat by the American Psychopathological Society (APS, 2015)
Selection of pests, diseases and weeds on Wheat (UC/IPM, 2014)
3.1.3 Foliar diseases in wheat. Biotic and abiotic factors
The severity of a disease in wheat depends on the inoculum present. Pathogens that perpetuate in the stubble will have greater or lesser importance depending on the existing stubble when installing the crop. The severity of the disease will be in direct relation to the amount of crop residue and the pathogenicity, all directly influenced by the prevailing weather conditions during the growing season.
Infected seed assures the initial inoculum facilitating the crop infection and the perpetuation of pathogens from one year to another. The transmission rate can range from 10% to 40% depending on the pathogen and the productive region being considered. This parameter is important in crops grown under conventional planting; but it is essential in terms of direct seeding since it adds to the source of stored inoculum in plant residues.
Rotation with non-susceptible crops can break the cycle of pathogens, removing their food source, although this practice, highly recommended in integrated pest management (IPM), loses support when pathogens are capable of subsisting in several species or common weeds in the field or are forced biotrophic, which need to invade living tissues to fulfill their lifespan.
Surface residues are an immense reservoir of spores in the field. Additionally they induce certain pathogens to sexual reproduction resulting in new genotypes thereof which can increase their pathogenicity. Among the pathogens that produce leaf spots in wheat, only Bipolaris sorokiniana is able to persist in the soil.
In crop structures involving conventional planting, crop residues are decomposed by soil microorganisms causing damage to pathogens because they cannot feed and form sexual structures. By contrast, in direct seeding, the rate of decomposition of the stubble is much slower or almost zero under certain (cold-dry) weather conditions. This is very important because it influences the amount of time that pathogens can survive in crop residues. The crop management strategies that promote disintegration and / or reduction of the stubble on the soil surface will directly result in a shorter period of time in which the pathogens can survive in it. Chemical fallow can reduce the level of inoculum in the stubble. The greater the severity of a foliar disease is in a wheat crop, the greater will be the contamination of the remaining crop residue.
3.1.4 Monitoring and action thresholds for leaf diseases in wheat
It is necessary to monitor the crop from its emergence until the seedling stage in order to detect early infections which may come from infested seed. At this stage, under normal conditions, additional control measures are not necessary, as the lost leaves of infected seedlings will be replenished during tillering.
Nevertheless, if weather conditions are favorable, the early detection of infections will indicate the need for additional chemical control measures in more advanced stages of the crop.
To clearly determine the correct timing for the application of a foliar chemical treatment it is necessary to systematically monitor the crop during all stages of vegetative and reproductive development, which are favorable for disease development.
The appropriate thresholds to take action for the application of foliar fungicides are 40% incidence in the case of foliar diseases caused by necrotrophic organisms (septoria or brown-yellow spot) and 10-15% severity in the case of foliar diseases caused by biotrophic (rusts and mildews) organisms.
These action thresholds may fluctuate according to other specific parameters, such as the location of the crop, sanitary behavior of the variety, yield expectation, environmental conditions, critical state of crop development, level of symptom development (incidence / severity), emergence of new strains of pathogens and, obviously, the international price of the cereal.
Head blight of wheat caused by Fusarium graminearum deserves a separate paragraph. This disease is typical of the reproductive period of the crop, being very susceptible in full bloom. Environmental conditions (high humidity and high temperature) favor its development. There is no way of monitoring the disease since when symptoms become visible on the field, the damage is done and control measures cannot be taken. Therefore, in many areas where it is common to be present, preventive management should be done.
Almost all wheat cultivars have greater or lesser susceptibility to diseases. Nevertheless, some control strategies (IPM) including rotation with crops other than wheat or corn and moderate conventional planting, are more or less important, depending on the environmental conditions and the susceptibility of the cultivar. The disease is severe in 5-7 year intervals, depending on the region analyzed. The most susceptible cultivars are durum wheat (Triticum durum), although some varieties of bread wheat (Triticum aestivum) have high susceptibility. T. durum crops are sprayed preventively with foliar fungicides at the beginning of the flowering.
Fungal Leaf Spot Diseases of Wheat (NDSU, 2009)
3.1.5 Sample collection in the field
The most recommended situation is that the sampling in the field is carried out by the person who will make the diagnosis in the laboratory. Before taking samples it is very important to highlight some observations that could facilitate the diagnosis. These observations may be heterogeneous symptoms in some plants such as yellowing and / or discoloration over time. It is also important to mark the date of onset of symptoms, age and form of their distribution in the field. The estimate of field damage is an important fact for later to also estimate the potential losses that the disease could cause in the crop.
As for the crop, it is important to know the variety or cultivar, planting date, rainfall and temperature data for possible extreme heat or frost, prevailing winds, if irrigated and if so with what system, walk the lot to identify possible areas of flooding, predecessor crops, fertilization, cultivation and phytosanitary treatments. All these gathered data will facilitate making a presumptive diagnosis of the problem.
With regard to sampling, it is important that they are representative of the crop. It is only necessary to obtain few samples but enough to make the diagnosis. In this item, there are no set rules. Generally it depends on the discretion of the person running the sampling. The samples should not be totally dead. If a problem of soil pathogens is suspected, sampled should be taken from the edges where the problem is most severe. Therefore, samples should be taken from the places where the problem begins to be severe. If possible they should include affected plants and healthy plants. If the problem is a pest and it has been detected, it must be attached and sent together with the samples of plant material. Wet samples should not be taken or otherwise they should be dried before being sent to the laboratory. Root assessment must be made from the same with soil adhering to them to avoid their desiccation. It is important to disinfect the equipment used for sampling between the different samples. Samples should be wrapped in moist paper and introduced in a new unsealed plastic bag and immediately sent to the laboratory for analysis. If the shipping time is extended, the samples must be kept in refrigerated conditions. All these activities are necessary to perform the traditional diagnosis and / or of confirmation of the problem.
The whole process of making a diagnosis of the disease and pest problems in a crop includes the accurate identification of the causal agent, importance and magnitude of the problem, complementing it with the appropriate control strategy to solve the problem.
3.1.6 Sampling methodology
There are diverse methodological techniques recommended for disease sampling in wheat depending on whether the distribution of the disease in the crop is random or homogeneous and heterogeneous.
In both cases, the methodology can be random and systematic. Both can be done in strata; for which we can mention the following:
- Entirely random: it involves random sampling of the field, where the crop is walked and samples are taken at randomly selected sites.
- Fully random but stratified: it involves the determination of strata within the lot that can show substantial differences in the incidence / severity of diseases, so that within each stratum the sampling sites are randomly selected later.
- Systematic diagonal: it involves walking the lot diagonally and taking samples at preset sites.
- Systematic ladder: it involves walking the lot in zigzag and sampling at predetermined locations.
- Systematic: travelling in the form of a V or M, it involves walking the lot outlining either letter and sampling at preset sites.
The size and number of samples to be obtained depends primarily on the target, the availability of time and resources, the distribution pattern of the disease or phytopathological problem and the level of accuracy or certainty expected with the gathered data. Generally for each disease there is a number of samples that must be defined statistically.
3.1.7 Control strategies
Rotation with crops that are not susceptible to common wheat diseases is a very good control strategy to reduce the incidence and severity of leaf diseases. By contrast this strategy is of no importance when linked with obligate or biotrophic pathogens.
- Selection of the variety or cultivar
The development of a disease in a crop is the interaction of two entities: the host and the pathogen. The genetic characteristics of both in their union, under certain favorable environmental conditions, cause the development of an epidemic to occur. The genetic composition of the cultivars makes them behave more or less susceptible to the pathogens present in the crop.While full genetic resistance to all leaf diseases of wheat is a utopia, it is very important to plant cultivars with good resistance to most diseases. This will result in lower (less severe) levels of pathological complications in the crop. All with the possibility of avoiding, if weather conditions allow it, the need for fungicide applications with the corresponding increasing in operating costs of the crop.If it is a sizeable farm where several lots of wheat are sown, it is very important to diversify the cultivars that are planted from the point of view of optimum planting date and phytosanitary behavior. This practice will contribute significantly to avoid severe epidemics throughout the property and / or reduce the importance of diseases in all the crops.
- Chemical control
The use of seed protection fungicides aims to avoid the development of pathogens that come with it and thus eliminate the possible initial inoculum of leaf diseases. Actually this practice reduces early infections to the seedling stage, thus delaying the initiation of infections to the crop.In general, the periods of protection achieved with fungicide treatments in the seed are temporary; they do not prevent infections from the inoculum that is present in the stubble and significant yield increases occur rarely, both in quantity and sanitation of harvested seed.Chemical control strategies with foliar applications of fungicides have given very good results. Actually, chemical control aims to complement the protection afforded by the genetic behavior of the cultivar against a disease.Occasionally when a cultivar is very susceptible, chemical control is the only tool that works trying to reduce the severity of the disease. For this reason, among others such as the choice of fungicide, its rate, timing and quality of the application; the efficacy of leaf disease control by chemical sprays can vary and display random result.
Lesson 3.2: Weeds
This activity is to obtain information related to some ecological aspects in each agricultural region, defining the presence and dominance of each weed species in wheat and their relationship with: state of crop development, soil characteristics, planting method and tillage method, among others.
The influence of climatic and soil factors on the presence of weeds in wheat will be established with this information and it will allow mapping the distribution and ranges of infestation of the detected species.
Concurrently, this work will collect the weeds and respective seeds and result in a local herbarium and the material to make the relevant biological studies.
An example of a regional scouting guide is presented in the table below:
|Field||Identification name or number||Identifies the location of the scouting|
|Variety||Planted wheat variety||Gives an overview of crop coverage and vigor|
|Crop height||Height in cm|
|Date 1||When most weeds are between 20 and 30 cm high||It is the proper time to control to prevent competition|
|Date 2||Close to harvest||It is the proper time to control to avoid difficulties at harvest|
|Soil texture||Sand, silt, clay||Link the presence of weeds with the texture|
|Tillage||Conventional, reduced or no-till||Link the presence of weeds with tillage|
|Crop rotations||Preferably for the last 10 years||The present flora is related to the established crops|
|Weeds present||Common name, technical name, growth habit, height, apparent dominance, percent coverage||This data is the essence of the activity|
3.2.2 Weed monitoring
It is done in the early stages of crop development in order to conduct the implementation of weed management measures in a timely manner.
The monitoring is programed based on information provided by local research institutions, or by the actual growers, regarding the species that are normally observed.
This activity seeks to obtain information related to the appearance and / or disappearance of weed species, which will help define the behavior of populations over time depending on the management activities of the wheat crop.
It is the first step to take to tackle the issue assertively and it goes with the correct identification of the species, since each one of them responds differently to the management alternatives. In this activity the knowledge of their development phenology and reproductive capacity is also very important.
To define the extent of weed infestation the field should be e studied following a recording pattern depending on the shape and size of the property, using a square of 50 cm per side, quantifying the number of seedlings within it and multiplying the result by 4 to obtain the population of plants in 1 m².
If the field is less than or equal to 5 hectares, it is suggested to take a minimum of 5 samples, each at the point where it is marked “X” following the route shown below:
If the filed is more than 5 hectares but less than or equal to 10 ha, 8 points of sample are suggested and the following route:
If the property is greater than 10 hectares, 10 samples should be taken distributed strategically in the field seeking its complete evaluation. To group weeds it is necessary to form two groups: of narrow leaf (grasses and sedges) and broadleaf (multiple species), which in turn can be grouped from the viewpoint of their lifecycle as:
Annuals. They are weeds that complete their life cycle (they are born, grow, reproduce and die) in a year. Such species spread and reproduce only by seed.
Winter annuals. They are plants whose seeds germinate in the fall and complete their life cycle in the spring of the following year. Examples:
|Narrow leaf (grasses)||Broad leaf|
|Wild Oats||Avena fatua||Lambsquarters||Chenopodium album
|London Rocket||Sisymbrium irio|
Summer annuals. Their seeds germinate in the spring and summer and complete their life cycle in the fall of that year. Examples:
|Narrow leaf (grasses)||Broad leaf|
|Barnyardgrass||Echinochloa crus-galli||Pigweed||Amaranthus spp|
|Morning Glory||Ipomoea purpurea|
Perennials. They are plants that live for more than two years which are reproduced both by seed and by vegetative reproductive structures like rhizomes, stolons, tubers, bulbs, crowns, radical buds, etc.
Simple perennials. Plants which do not disperse vegetatively in the soil. They disperse and reproduce through the production of seeds, crown buds and root cutting segments. Examples:
|Narrow leaf (grasses)||Broad leaf|
Complex perennials. Plants that disperse vegetatively through tubers, rhizomes, radical buds, etc. as well as by seeds. Examples:
|Narrow leaf (grasses)||Broad leaf|
|Bermudagrass||Cynodon dactylon||Field bindweed||Convolvulus arvensis|
|Johnsongrass||Sorghum halepense||Silver-leaf Nightshade||Solanum eleagnifolium|
|Narrow leaf (Sedges)|
|Yellow nutsedge||Cyperus esculentus|
|Purple nutsedge||C. rotundus|
In the scouting, monitoring and sampling activities, weed species that are regionally unknown can appear; there is the importance of such activities.
By not knowing the weeds that are present, there is much probability to fail in their control since there will also be a lack of information on their susceptibility to the treatments.
In each country there are regulatory institutions in the plant health area, which conduct support activities for growers in general, and usually have Specialized Reference departments for each group of existing pests. One example is the “National Reference Center of weeds, birds and rodents” in Mexico, where species detected by growers in their properties and not previously found are identified.
Lesson 3.3: Insect pests
In wheat there are over 47 species of insect pests but not all are harmful to the crop with the same intensity and at the same time. Possibly in other commercial crops such as corn and cotton they are more abundant, this is why IPM was launched in cotton, which is more affected by sucking, leaf eaters, borers, and cutworms. The major pests in wheat in the 60’s were white worms and the larvae of leather jacket or crane fly (Tipula sp.), and later in the 70’s appeared the aphids. The management and biological control experiences developed in cotton were adopted.
There are several monitoring network services providers that facilitate information on main pests attacking wheat. One of them is the Alberta Insect Pest Monitoring Network which you can check here: 2015 Wheat Stem Sawfly Forecast
Several species of oat aphids are reported in wheat, Rhopalosiphum padi, R. maidis, R. rufiabdominalis, pale green aphids, Metopolophium dirhodum, M. festucae cerealium, dark green aphids of the spike, Sitobion avenae, S. fragariae, grain aphid, Schizaphis graminium and the wheat aphid Diuraphis noxia; these are sucking insects and were probably the main crop problem that forced the adoption of IPM using the micro parasitoid wasps of the genus Aphidius spp., and the release of several species of predators such coccinellids, which both in the larvae and adult stage prefer to prey on aphids. The oat aphid R. padi is recognized as the vector of the barley yellow dwarf virus, BYDV.
The most critical time of the attack by aphids, is on the stems and during the formation of the tiller when two (2) aphids / stem is considered a critical density, during jointing it is 10 aphids / stem and during flowering and grain filling 20 to 25 aphids / leaf and spike.
Other insect reported as wheat pests are some beetles or several species of scarabaeid beetles known as white grubs of the genus Hylamorpha, Phytoloema and Athlia that are in the larval stage, are white, move in the soil vertically damaging the roots and can act as cutworms cutting wheat plants during emergence. The adults are green beetles and fly during the spring and summer placing the eggs in the soil, where the larvae develop and begin feeding on the rootlets.
In wheat there are also reports of attacks by wireworms: Medonia deromecoides and Conoderus rufagulus; the adults are dark grey elongated beetles and the larvae are creamy white, elongated with three pairs of true legs and well-marked segments; they live in the soil and damage the roots. There may also be sporadic attacks of the wheat crane fly, Tipula apterogyne that while sporadic it can cause large losses, as it happened in the 60’s; it is a large adult mosquito living under leaf litter in damp, shady places and are a problem during the winter. The larvae are lead-colored, legless and live buried in the ground; they can kill plants by damaging the collar.
The main damage from insects in wheat occurs during storage, where almost all the primary and secondary pests affecting warehouses impact it damaging and contaminating the grain. Coleoptera and Lepidoptera insects are the most common, and when no appropriate control measures are taken can affect up to 30% of the production. The most important thing is to implement IPM practices in the warehouses where the wheat grains are stored, applying biological, physical and chemical mechanisms (mainly fumigants) for their control.
Lesson 3.4: Harvest, storage and processing
For having some similarity in symptomatology with the phytosanitary problems, we will refer to the various physiological disorders caused by abiotic factors, which can become very important and are attributable to unfavorable environmental conditions.
When the temperature drops below zero (0) °C, depending on the variety, wheat in general is resistant to frost from tillering to flowering, but from flowering until the formation of the grain in the dough stage, it is very sensitive to temperatures below 5 ° C, causing abortion of the flowers and sterility of the spike. Therefore, planting dates need to be adjusted so that the emission of spikes and flowering occur when the chance of frost is low. Additionally, good potassium fertilization makes plants resistant to cold.
They are crystals of water that freeze and plunge to the ground, the smaller ones can become water and the larger ones reach the ground solid; if the crop has flowered it strikes the plants violently, causing grain drop and white glumes. These events are of short duration, occur in winter or early spring and are very difficult to predict.
This happens when the leaves exhibit yellow or reddish-white horizontal bands alternating with the green of the leaves; they are mainly due to fluctuations in soil temperature, and high relative humidity at night. Thus, the leaves are saturated with water overnight and with the temperature increase in the soil surface or sunlight, they dry quickly and the bands are formed in the youngest leaves. It does not kill the plants and it disappears when the fourth leaf is formed.
It is the lack of chlorophyll in the leaves and is evident due to the lack of pigmentation; it is observed in some isolated plants as a result of certain temperature conditions which affect the formation of the precursors of chlorophyll, such as porphyrin involved in the formation of plastids; it is considered an irreversible genetic condition.
It may be caused by disease, bad weather, poor crop management and poor fertilization. Strong winds can knock down many plants following a persistent rain, and if it happens at the time of anthesis or flowering, it prevents pollination and fertilization, causing serious losses that affect yield and make harvesting more difficult. The management practices that favor lodging are high seeding rates, high doses of nitrogen and irrigation applied on windy days.
3.4.7 Soil acidity
It occurs when there is loss of bases in the soil, such as calcium and sodium, the lot is managed for high nutrient extraction without proper fertilization or there is constant use of ammonium fertilizers. Acidity can cause an increase in the percentage of exchangeable aluminum that prevents good root development and presents yellowing of the leaves and a tendency to dry prematurely.
It occurs when a low rainfall spring is followed by a cold and wet period prior to the emission of tillers and spikes, resulting in much disparity in the state of development of the plants, leading to a lack of uniformity at the time of maturity. The damage is severe because a lot of small grains are produced and harvest is delayed.
Is the lack of available moisture in the soil; if it occurs in spring, the summer is usually associated with high temperatures and much water lost through transpiration. The plants appear limp, chlorotic and the leaves are wilted. If the drought happens 20 days before or 10 days after anthesis, it causes large damage in wheat yield, when it occurs in times other than those mentioned, the plants recover. When it occurs during grain filling, the grain turns out shriveled with low starch and high protein content.
It occurs in flat, low infiltration clay soils. Root respiration is affected and it hinders the absorption of nitrogen. Many of the important microorganisms such as the decomposers of organic matter and nitrification die for lack of oxygen. The solution is to drain the soil surface and internally; surface drainage can be performed even after seeding and it is recommended to apply nitrogen afterwards.
3.4.11 Sun damage
It occurs in areas of high luminosity and solar radiation, causing the leaves to dry and shrivel; the damage depends on the availability of water, with it they can even withstand temperatures of 40°C without damage, but when the water is lacking it can cause death as they completely close their stomata and transpiration is suspended.
3.4.12 Inappropriate use of herbicides
Inappropriate use of herbicides can produce twisted stems, curved leaves, necrotic spots on the foliage and emission of abnormal spikes. In plantations where excessive doses are applied or high concentration applications are repeated, damage occurs in the foliage and affects the development of the leaves. Leaf deformations are produced mainly when applications are made at the wrong time, for example with hormonal herbicide when the plants have less than 4 leaves.