​Climate favors bacteriosis at the end of the wheat harvest cycle

For this wheat crop, the outlook is favorable regarding grain productivity and quality. However, just like every year, producers need to live with and control various diseases that attack the crop on a daily basis. This year, for example, the occurrence of rain for consecutive days and the low temperatures recorded in the same period allowed the development of bacteria to the point of causing symptoms in wheat. The reports are mainly from the Central, West and Southwest regions of Paraná. In the Northwest of Rio Grande do Sul the appearance was not as widespread, with more specific cases. In the south of Paraná, Planalto Catarinense and in the regions of Passo Fundo and Vacaria, in Rio Grande do Sul, there were no significant records of bacteriosis.

The first occurrence was identified at the beginning of September, in the West and Southwest of Paraná, when there was a period with low temperatures, which did not exceed 20°C, associated with rain and relative humidity above 90%. According to the phytopathologist at Biotrigo Genética, Paulo Kuhnem, these climatic conditions, associated with the phenological stage of the crop, created the ideal scenario for the bacteria. "The bleaching symptoms observed on wheat leaves suggest that the causal agent is a bacterium of the genus Pseudomonas, probably P. syringae pv. syringae, but other species and pathovars may also occur.” This genus of bacteria is considered an epiphytic bacterium, in other words, it lives in the environment naturally, including in the wheat leaf. “The disease appears when periods of cold and rain occur during the heading phase, which is the phase of greatest susceptibility of the crop, rarely occurring before this phase”, he explains. bacteria causes a whitish lesion because it releases a toxin that quickly burns the plant tissue. “So, three to four days in these climatic conditions allow that bacteria that lives naturally in the wheat leaf to multiply quickly and be able to overcome the barriers of plant, penetrate and cause bleaching”, warns Kuhnem.

What to do?

Once symptoms appear, there is not much that can be done. The phytopathologist warns that it is necessary to be very careful when using fungicides, as the plant's metabolism drops during this cold period and when a fungicide is applied it is unable to properly metabolize this product, which ends up stressing the plant and potentially causing phytotoxicity, which can worsen the problem of bacteriosis. “It's difficult because we don't have a bactericide to apply and control after infection. As the producer had not encountered bacteriosis for a few years, he ended up getting scared, but with the appearance of dry and sunny days the symptoms did not evolve”, he reports.

Another guideline is to avoid crowding the plants so as not to prolong leaf wetness. “In crops where many seeds are placed, the density of plants maintains this leaf wetness for much longer, which can cause a greater severity of bacteriosis”, he advises.

Crops where other foliar diseases occurred, such as yellow spot, generally showed a greater intensity of bacteriosis. Other factors that may also have favored the appearance of the bacteria are injuries caused by adverse weather conditions such as frost, hail or places where there is a lot of wind. This is because the bacteria, by itself, cannot penetrate plant tissue. It enters through natural openings or through these wounds. “Producers may notice the greater frequency of symptoms in the curvature of the leaf, because normally where it curves it ends up causing microcracks into which the bacteria can penetrate, in this high humidity environment”, he explains.

Given this scenario, the business director and breeder at Biotrigo Genética, André Cunha Rosa, highlights that, among prevention measures, genetics is still the only one that is truly effective. Although the damage from bacteriosis is absolutely superficial - unlike leaf blight and leaf spots which affect metabolism more broadly - and the damage is therefore less than often expected based on a visual assessment, it is important to know that there is much difference between cultivars for resistance to this disease. “The cultivars TBIO Sossego, TBIO Iguaçu and TBIO Sintonia are those that rarely show any symptoms. TBIO Mestre and TBIO Sinuelo presented intermediate levels. TBIO Toruk and TBIO Tibagi present symptoms more easily”, he explains. The complete ranking of Biotrigo cultivars for resistance to this disease can be found both in printed promotional material and on the company's website (www.biotrigo.com.br). The breeder also considers it important to emphasize that, over the years, the company's forecast is that this problem will become less and less important due to the launch of more cultivars resistant to the problem.

other diseases

Most crops are beginning to enter the heading and flowering phase. During this period, the main disease is Fusarium head blight, which affects grain quality and yield. “To maintain the good quality of crops that we have so far, it is necessary to apply a specific fungicide to try to protect the aerial organ, which is the flower, the site of FHB infection”, recommends the phytopathologist. The producer must pay attention and apply the fungicide when his crop shows 50% flowering. According to Kuhnem, it is also necessary to keep an eye on the region's climatic conditions for a possible second application of fungicide.

Crops in RS and PR

According to the economic information from Emater/RS, released this Thursday (22), wheat crops in Rio Grande do Sul are in the vegetative development phase with 34%, in heading and flowering, reaching 40% of the area. With low temperatures, days with high light and short periods of rain, the crop quickly advanced to the granation stage, reaching 25%, with 1% beginning to mature. The crop standard is very good, with excellent production potential.

In Paraná, according to the bulletin from the Department of Rural Economy (Deral), released on September 20, 22% of the wheat sown has already been harvested and almost 90% of the total area for this harvest is considered to be of good quality. Regarding development, 54% is in the maturation phase, 28% in fruiting, 11% in flowering and 7% in vegetative development.