Bacteria are discreet enemies that usually remain on the surface of wheat plants, without producing any visible symptoms, but can take advantage of some type of favorable environmental condition to penetrate the plants and begin an infectious process. It is the case that Pseudomonas syringae quality Xanthomonas translucens, which require attention and care from producers
Bacteria are always present on the surface of plants and animals. Excluding the necessary exceptions, this situation represents a natural risk condition that agricultural crop plants are subject to when installed in the field. It turns out that one of the most important characteristics that bacteria associated with plants present is the fact that they develop in an epiphytic way, that is, they remain on the surface of plants, nourishing themselves with their natural exudates, without producing any visible symptoms, but they can take advantage of some type of favorable environmental condition, such as excess humidity, to penetrate the plants and begin an infectious process. This is a particularly relevant feature for Pseudomonas syringae p.v. syringae, the causal agent of the disease known in Brazil as wheat leaf blight, one of the two bacterial diseases reported in the country as causing significant damage to crops.
The other bacterial species described as causing high damage to wheat crops in Brazil is Xanthomonas translucens p.v. translucens, causal agent of a different symptomatological condition, that is, it produces symptoms in both leaves and ears of wheat. When it reaches the leaf, it causes the symptom known as bacterial streak or streaked spot. When it attacks the cobs, it produces the symptom called black cob, and there is the term black chaff, in English, which is internationally recognized to refer to this type of cob symptom. Although fewer than those already carried out with P. syringae p.v. syringae, there are well-established studies that demonstrate that X. translucens p.v. translucens also develops epiphytically and that this resource and/or strategy is important for its survival and subsequent infection in wheat plants. In Brazilian conditions, it has been observed that symptoms caused by X. translucens p.v. translucens in wheat crops they have occurred more frequently in warmer regions of the country. However, even in Rio Grande do Sul, epidemic outbreaks of the disease occur sporadically, although it is a state with a colder climate compared to regions such as the north of Paraná and the south of São Paulo, where the occurrence of the disease has been much more frequent. frequent.
Pseudomonas syringae p.v. syringae is a bacterium that is practically always present on the surface of wheat plant leaves. It lives epiphytically, especially on the leaves located at the top of the plants. Therefore, the simple isolation of this bacterium from wheat plants does not mean that an infectious process is occurring. On the other hand, epidemic outbreaks of wheat leaf blight are sporadic, and this condition must be one of the reasons why there is no consistent information about the damage that the disease causes to wheat crops.
The initial symptoms of wheat leaf blight generally appear close to the beginning of heading, on the upper leaves, and in the form of numerous small watery spots. After two or three days, these lesions expand and coalesce (come together), forming larger, grayish lesions with a dehydrated appearance. In Figures 1A and 1B it is possible to see this type of symptom. When humidity is high, bacterial exudates may eventually be observed in such lesions.
The bacterium has a very large number of host species, which includes corn, sorghum, apple trees, peach trees, several weeds and many other species. It survives in these plants, predominantly, in an epiphytic form. The environmental conditions that favor the disease are mild temperatures (15°C to 25°C) and high relative humidity. Under temperatures between 15ºC and 18°C, combined with high humidity, in 2010, an epidemic outbreak of the disease was observed in Rio Grande do Sul. Figure 2 shows the temperature and humidity data recorded in Passo Fundo during that period. All these circumstances confirm the indications that the presence of the pathogen does not represent the most important aspect for the development of infections caused by P. syringae p.v. syringae. Favorable environmental conditions, represented especially by temperature and humidity, are decisive for the development of bacteria and the emergence of epidemic outbreaks.
An alternative to control the disease is the use of resistant cultivars. However, due to its sporadic occurrence, wheat breeding programs in Brazil have not used routine procedures to evaluate their genotypes in response to the disease. Therefore, there is no consistent information on the reaction of Brazilian wheat cultivars to leaf burning. Despite this, during the epidemic outbreak that occurred in 2010, in Rio Grande do Sul, it was found that there are considerable differences between the cultivars currently used in Brazil in relation to the reaction to the disease.
Bacterial streak or streaked leaf spot Xanthomonas translucens p.v. translucens
This bacterium survives in seeds, which are the main agents that spread this pathogen, a condition that represents a problem for international germplasm exchange. Damage to wheat crop yield caused by X. translucens p.v. translucens can vary from 8% to 40%.
On the leaves, the first symptoms appear as small, light brown, watery linear spots or streaks. In the initial stages, the lesions develop longitudinally between the veins, after which they can expand and coalesce, producing irregularly shaped grayish-brown spots. In Figures 3A and 3B it is possible to observe the symptoms caused by bacterial streak on wheat leaves. In conditions of high humidity, abundant bacterial growth appears in the form of exudate (viscous droplets).
The black ear (black chaff) refers to the browning of ears and cob stems. Dark, linear, watery streaks on the glumes are the main symptoms. These glume lesions may coalesce. In severe attacks, the kernels may become stained and shriveled. There are at least two types of abiotic symptoms that can be confused with black tang symptoms caused by X. translucens p.v. translucens. One is melanism and the other is “pseudo-black chaff”. Melanism is a process of concentration of black pigment in ears and their stems, most likely caused by a combination of genetic factors and environmental adaptation. O pseudo-black chaff represents the phenotypic expression of stem rust resistance genes (Sr2) when the plant is subjected to some type of stress.
The pathogen survives both inside and outside the seeds, but its main location is external. It is known that the transmission rate of the pathogen from seeds to aerial parts is low, although the extent to which this low transmission rate may represent in the development of epidemic outbreaks of the disease is not known. The bacteria can survive for a long time in stored seeds. There is some controversy about exactly how long this period is. The variation in this sense is from 36 months to 81 months, but all studies confirm that the bacterial population in seeds is reduced as storage time increases.
Xanthomonas translucens p.v. translucens It survives in the soil, in several alternative hosts, seeds and cultural remains. However, seeds are considered the most important source of primary inoculum for the disease. Free water on the surface of plants allows the pathogen to disperse on the leaf, increasing the number of lesions. The bacteria enter through the stomata and multiply in the parenchyma tissue. Rain and wind are responsible for dispersing it throughout the crop, from the bacterial exudates formed in the lesions of the leaves and glumes. The bacteria also tolerate a wide range of temperatures, ranging from 15ºC to 300C, but, within this range, it develops better when the temperature is above 260C. Temperature is known to have a significant impact on disease epidemics. The multiplication of the pathogen in leaf tissues is directly dependent on temperature, and dry air conditions (Relative Humidity <30%) do not limit the progress of the disease.
The use of bacteria-free seeds should be seen as the main measure to prevent the entry of X. translucens p.v. translucens on the farm. The success of this measure depends on controlling the origin of the seed, which must have been produced in crops where the presence of symptoms of bacteriosis was not observed. Furthermore, the seeds produced in such crops must be subjected to health tests that also prove the absence of contamination with X. translucens p.v. translucens. Rotation with crops that are not susceptible to the bacteria is an important strategy for controlling the disease. In this sense, the plant species of agricultural crops susceptible to virulent infection by the bacteria, whose confirmation has already occurred, are the following: oats, rye, barley, wheat and triticale. The use of cultivars resistant to bacteriosis is also an recommended practice, although it is considered that most of the information available on the reaction of cultivars to the disease is not consistent.
The low temperatures recorded during winter can also produce negative effects on the wheat crop, not only in the form of frosts during flowering. These effects can be seen especially on leaves where light and translucent spots can be seen. Although this type of damage is essentially of abiotic origin, related to a physical phenomenon that occurs in nature, it has already been proven that its occurrence involves the effective participation of some microorganisms. In wheat, the main biological agents associated with this symptomatological condition are the so-called ice-nucleating bacteria. Such bacteria represent a portion of the community of epiphytic bacteria that naturally inhabit the surface of leaves in regions with a temperate climate, generally without parasitizing the plants, but which enhance the harmful action of cold and/or frost. It turns out that, although ice melts at 00C, pure liquid water, under certain conditions, can reach temperatures of up to –400C without solidifying. Normally water remains liquid up to –100C, however, there are several types of particles in the environment, including epiphytic bacteria, which can serve as a nucleus to guide water molecules and provide their alignment in ice crystals at temperatures that are not so low. The harmful effect caused by ice crystals, formed by the freezing of water present in the internal and external spaces of plant cells, is due to the rupture that these structures cause in cell membranes, causing the loss of internal content and cell death. In this sense, the nucleating action of bacteria such as Pseudomonas fluorescens, Erwinia herbicola and variants (pathovars) of P. syringae, that can catalyze the formation of ice at temperatures close to –20C. Some bacteria, such as P. syringae p.v. syringae e X. translucens p.v. translucens, in addition to exerting parasitic actions on plants, they are active in the ice nucleation process. Furthermore, there is the possibility of these bacteria taking advantage of wounds in plant cells, caused by the formation of ice crystals, as a means of accessing the interior of plants, thus starting to act as infectious agents. All these circumstances indicate that the greater sensitivity of wheat plants to the action of frost or low temperatures depends on the density and diversity of bacteria present on their surface.
João L. Nunes Maciel Flávio Martins Santana, Embrapa Trigo
Article published in issue 189 of Cultivar Grandes Culturas.
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