Symptoms of Common Mosaic in Wheat

The common mosaic, a virus that is difficult to control, has a vector with a long period of survival in the soil and a wide range of host plants. As genetic resistance is one of the main alternatives available against the disease, knowing the behavior of wheat cultivars and their incidence becomes fundamental.

The virus known as common wheat mosaic, in Brazil, occurs mainly in the states of Rio Grande do Sul, Santa Catarina and southern Paraná. The virus that causes this disease has been identified as being Soil-borne wheat mosaic virus (SBWMV). SBWMV belongs to the genre Furovirus. The virus particles have a rigid rod shape (Figure 1), and inside them is the genetic material (RNA) which, once transmitted to the plant cell, begins the infectious process that results in the expression of mosaic symptoms.

This virus, capable of infecting wheat, triticale, rye, barley and other grasses, is transmitted by a eukaryotic organism, residing in the soil and obligatory parasite of plant roots, called Polymyxa graminis. Damage to production caused by mosaic is usually limited to areas of the crop where the vector is concentrated, but under favorable environmental conditions (cold and humidity), large areas with susceptible cultivars can be compromised. The long period of survival of the vector in the soil and the wide range of host plants make it difficult to control this virus in any way other than through genetic resistance. Therefore, it is essential to characterize the level of resistance and potential damage of cultivars available on the market to assist in decision-making regarding their use in areas with a history and/or risk of common mosaic. In this sense, this work presents the characterization, under standardized conditions, of the reaction of wheat cultivars that make up the State Trial of Wheat Cultivars of Rio Grande do Sul to the common mosaic. The results of this characterization, carried out annually, are presented at the Meeting of the Brazilian Wheat and Triticale Research Commission The test is carried out in the field, in an experimental area at Embrapa Trigo (Passo Fundo, Rio Grande do Sul) with a history of the occurrence of common mosaic (S28°13'; O52°24'). The reactions to the common mosaic of the 30 cultivars of the State Trial of Wheat Cultivars of Rio Grande do Sul (EECT-RS) are characterized. The test cultivars are Embrapa 16, resistant, and BRS Guamirim, susceptible. The test area contains soil managed without plowing, in a direct planting system. Manual sowing is carried out at the beginning of July, opening planting furrows and fertilizing with N-P-K (5-25-25) in an estimated quantity of 260kg/ha. 

The trial is conducted in randomized blocks with five replications for each wheat cultivar. Each experimental unit (plot) consists of a 0,5 m long line with approximately 30 seeds/line. To map the area regarding the occurrence of mosaic, lines with the cultivars Embrapa 16 and BRS Guamirim are interspersed every five lines of the test cultivars. To avoid the effects of other diseases, pests and weeds, cultural treatments are carried out with fungicides, insecticides and herbicides at doses indicated for wheat crops. Top dressing nitrogen is applied at a dose of 40kg N/ha, at the tillering stage, in the form of urea. 

For each plot, the symptoms are evaluated and the plants are classified into the following categories: 1 = absence of common mosaic symptoms, 2 = plants rarely show symptoms, which are not very evident, 3 = more frequent and evident mosaic symptoms, without apparent impairment of plant development, 4 = plants always with typical mosaic symptoms with evident yellowish streaks on the leaves and stems, but without evident reduction in plant height and ear size, and 5 = plants with mosaic symptoms evident on the leaves and stalks. stems and compromising normal growth with reduction in plant height and cob size. Visual assessments are carried out at the elongation and heading stage for most cultivars. After harvesting, the total weight of grains for each plot is quantified. To compare cultivars, the average scores from visual evaluations and the total weight of grains produced per plot are used.

In 2014, the distribution of common mosaic was uniform in the area. Of the 35 susceptible control plots (BRS Guamirim), only five (14%) did not receive the maximum score (5) that is expected for this cultivar. The effects of the virus on the BRS Guamirim cultivar were significant, resulting in a grain yield of 26,5g/plot. The Embrapa 16 cultivar (resistant control) rarely presented any plants with mosaic symptoms and the grain yield was 56,1g/plot. Therefore, the grain yield of the BRS Guamirim cultivar was 52,8% lower than Embrapa 16. The homogeneous distribution of the disease in the area and the effects on the controls indicate that the characterization results are safe with a low occurrence of escape. Among the wheat cultivars analyzed, differences were observed regarding the reaction to the common mosaic virus (Table 1), ranging from clear symptoms on the leaves, with an evident reduction in the normal development of the plant, to asymptomatic plants with normal development. The correlation between the visual grade and the grain yield of the plots was -0,83. The cultivars BRS Guamirim, TEC 10, FPS Nitron, Marfim, TEC Vigore, Fundacep Horizonte, LG Oro frequently exhibited symptoms. More rarely, symptoms were observed in Quartzo and BRS Marcante. Of this group, BRS Guamirim, TEC 10, FPS Nitron, Marfim and Fundacep Horizonte showed low grain yields on the plots (below the test average – 1 standard deviation), suggesting a negative effect of the virus on grain yield. This effect was most evident in TEC 10 and BRS Guamirim, which received a score of 5 on the scale in all plots in the second evaluation. The TEC Vigore cultivar, although it always exhibited evident foliar symptoms (grade 4 in the second evaluation), presented a grain yield close to the test average. LG Oro had a maximum score of 4, with 3 being the most frequent, and its plots presented grain yields slightly above the test average.

Among the cultivars evaluated, many did not exhibit symptoms and presented excellent yield in areas with strong disease pressure, making them an option for cultivation in areas with a history of mosaics.

Douglas Lau, Paulo Roberto Valle da Silva Pereira, Ricardo Lima de Castro, Embrapa Trigo

Article published in issue 198 of Cultivar Grandes Culturas.