The wheat crop (Triticum spp.) is one of the most cultivated in the world, being the third most produced grain, with estimates for the 2023 harvest according to Conab (2023), of 789 million tons, with an increase of 1. 24% of production compared to the 2022 harvest. Conab also points out that Paraná and Rio Grande do Sul concentrate most of the wheat production, accounting for about 86% of Brazilian production. Even with an estimate of wheat sowed area smaller than last year, the expectation of production is higher.
Brazil, for various reasons, is not among the countries that produce the most wheat in the world. As much as there is a large grain producing region in the country, due to climatic factors, there are complications at various times in the development of the crop. Despite this, triticulture has been an extra income for producers in southern Brazil, due to the development of genetic improvement and increasingly effective management strategies.
Considering linking productivity with profitability, the reduction of some aspects that limit the development of culture becomes fundamental for the expression of productive potential. One of the factors that affect the development of wheat and the final grain yield is the infestation of diseases during the production cycle, which, in the vast majority, are caused by fungi, although diseases caused by viruses and bacteria also cause serious damage. Therefore, monitoring is extremely important for successful management and final productivity of the crop.
Leaf spots have been identified as the main diseases in the wheat crop. Among them, we can highlight the Yellow Spot (Drechslera tritici-repentis), Glum Spot (Septoria nodorum), Brown Spot (Cochliobolus sativus) and Speckled Spot (Septoria tritici).
symptom and control
YELLOW SPOT (Drechslera tritici-repentis): it is the main leaf spot found in wheat crops in southern Brazil. For this disease, “the main sources of inoculum are infected seeds, crop residues and ryegrass plants, an alternative host, present in or close to the crop” (REIS, et al., 2011).
Symptom: initial symptoms can be observed on wheat leaves, having spots initially in small dark spots with yellowish halos, which expand into oval or diamond-shaped spots. As they coalesce, they result in large necrotic leaf areas.
Control: as it spreads in different ways, it is recommended to use rotation with crops that are not susceptible to this pathogen, such as vetch, fodder turnip, rapeseed, flax, among others. It is advisable to use healthy seeds or seed treatment with a mixture of iprodione + thiram and guazatine. According to Forcelini (2013), “the curative application of fungicides has limited action on the expansion of the lesion, since this process is mediated by the production of toxins. Partial control of this component of the epidemic is only possible with triazole fungicides applied in the first days after infection.” In other words, the importance of monitoring and applying the fungicide at the beginningm of the appearance of the first symptoms is highlighted,
Glume spot (Septoria nodorum): This disease is also known as leaf spot, node septoria and knot spot. It predominates in no-tillage areas, as it survives in crop residues and monocultures, in addition to the ability of the fungus to survive in infected seeds. Damage can cause losses of 30% to 60% in grain yield.
Symptom: the first symptoms are small chlorotic spots, which appear on the lower leaves. After expanding, they coalesce with other spots and cause leaf death. When the disease attacks the nodes, they darken, become almost black, and there may be complete strangulation of the nodes that break easily. Therefore, when the fungus attacks this location, losses can be higher in grain yield.
Control: as it has its survival in crop residues, the main control strategy is crop rotation with non-host species, in addition to using healthy seeds and/or seeds treated with fungicides (iprodione+thiram, triadimenol and guazatine), cultivars resistant, and in shoots with systemic triazoles, such as propiconazole and tebuconazole.
For Lau et al. (2020), “fungicides from the triazole and strobilurin groups are indicated for control. However, the use of isolated strobilurins should be avoided, as there are reports of isolates with lower sensitivity”.
BROWN SPOT (Cochliobolus sativus): this disease has sporadic occurrence and occurs in wheat growing regions with higher temperatures. The main sources of inoculum are infected seeds and crop residues colonized by the fungus, which remain on the soil surface in direct seeding and monoculture systems.
Symptom: the main symptoms are observed on the leaves, with small necrotic spots, producing a lesion with a dark brown center and a rounded edge. Lesions are usually surrounded by a yellow halo. In infected grains, the symptom of “black tip” can be observed.
Control: the main effective control practices are seed treatment with fungicide; crop rotation with non-susceptible species, reducing the inoculum present in infected crop residues; sowing time and genetic resistance. For the control of leaf spots in wheat, management strategies are essential, with the objective of keeping the pathogen population below the threshold of economic damage.
SPOTTED SPOT: (Septoria tritici): it is the secondary disease in the cultivation of wheat in Brazil, its occurrence occurs in colder regions. In years with strong disease pressure, crop productivity may decrease by 35% to 50%. The initial inoculum occurs through airborne ascospores, through infected seeds and in crop residues.
Symptom: the initial symptoms are manifested on the lower leaves, with small irregular spots between the leaf veins of a reddish brown color. With the development of the disease, it starts to become grayish, with the increase in the size of the lesions expanding throughout the leaf, having small dark punctuations in the center of the spot.
Control: the main form of control, aiming to eliminate the inoculum present in the crop residues, is the crop rotation with non-host species. It is also worth mentioning the importance of using resistant cultivars and seed treatment with fungicides (iprodione+thiram, triadimenol and guazatine). The recommendations for this leaf spot are the same as those used to control glume spot, as there are still no resistant varieties.
Thus, it is evident that the use of good quality seeds, seed treatments, crop rotation and early intervention with fungicides when the first symptoms appear are important strategies for good management of diseases in wheat.
By: Grazieli Greth Sperling and Pedro Rockenbach, students of the Agronomy course at UFSM, Frederico Westphalen campus, members of the Tutorial Education Program – PET Agricultural Sciences, under the supervision of the tutor, Professor Dr. Claudio Jose Basso.
Cover image: Pedro Bonini
