Control of wheat rust in different water regimes and simulated rainfall

Knowing the behavior of fungicide applications under different water regimes and rainfall incidence is important to establish sustainable management of wheat rust in search of high productivity.

Wheat cultivation in the south of the country is the oldest, precisely because of the climate that is closer to temperate and mild temperatures. The largest production of wheat is located in the states of the South region, but São Paulo and Minas Gerais are also significant in the production of the grain. In recent years, the planting of the crop in the Center-West has been motivated by the good results achieved in productivity, the market prices of the product, and also because it constitutes a good alternative for using the soil in the winter period. According to Conab (2013), the planting of rainfed wheat in the central and southern regions of Minas Gerais accounted, in the 2011/2012 harvest, for more than 70% of the crops planted in the state.

Wheat may suffer from possible negative impacts resulting from climate variations, such as, for example, drought in the Central-West regions and excessive rain in crops in the South of the country, making it necessary to adjust its production system. Given this scenario, it is important to conduct studies that evaluate the interaction between agricultural products applied to aerial parts and plants. It is known that plants subjected to water limitations present some adaptive morphological characteristics, such as a reduction in the relationship between surface and volume, thickened cuticle and cell walls; presence of waxes; protected stomata; sclerenchyma caps; water-storing fabrics; well-developed palisade parenchyma and idioblasts with phenolic compounds and crystals (Burrows, 2001; Fahmy, 1997; Fahn; Cutler, 1992; Rotondi et al.

The development of adaptive morphophysiological mechanisms in plants under water deficit reduces water loss to the atmosphere, preventing dehydration. This can also affect the absorption of agricultural products, such as fungicides. In this sense, a trial was conducted at the experimental station of the Phytus Institute, in Itaara, Rio Grande do Sul, with the objective of verifying the behavior of fungicides on wheat plants, cultivar Quartzo, subjected to different water regimes and rainfall simulations after harvesting. application.

The test was conducted in a greenhouse, consisting of two water regimes: with water deficit (50%-60% of field capacity) and without water deficit (90%-100% of field capacity), in addition to five intervals of time between fungicide application and rain simulation (0 minutes, 30 minutes, 60 minutes, 120 minutes and 240 minutes), one control without rain and the other without fungicide application. The water deficit was established over a period of 25 days, between the stages of flag leaf emission and heading, with irrigation control carried out by the vessel weighing method. After water restriction, the fungicide composed of a mixture of the active ingredients trifloxystrobin + prothioconazole (60g a.i./ha + 70g a.i./ha) with the addition of Aureo at a dose of 0,375L.p.c./ha was applied. After application of the fungicide, rain was simulated as described. Wheat plants were inoculated with uredospores of Puccinia triticina 12 hours after application of the fungicide.

Firstly, the Number of Days for the Appearance of First Pustules (NDAPP) (residual of treatment) was evaluated. To this end, daily assessments began from the second day after inoculation to visualize the first symptoms with the aid of a 20x magnifying glass.

To assess the severity of P. triticin Only the flag leaf of each plant was considered. The severity of the disease was evaluated at 25 Days After Application (DAA), when all wheat flag leaves showed symptoms of the disease. To determine severity, visual notes were assigned to the percentage of leaf area with disease symptoms in relation to the healthy area of ​​the leaf.

Data analysis revealed a significant interaction between the factors analyzed for the variables Number of Days to Appearance of the First Pimple (NDAPP) and severity of Puccinia triticina on the flag sheet (Figures 1 and 2).

NDAPP indirectly demonstrates the speed of fungicide absorption in applications at different intervals of rainfall simulations, influencing the fungicide/plant interaction in aerial part applications. The shorter the time interval between the application of the fungicide and the rain simulations, the shorter the time for the appearance of the first pustule (Figure 1). Thus, it is inferred that with rain, washing occurs and all fungicide not yet absorbed by the leaf tissue is removed.

It is noted in wheat plants under favorable conditions (without water deficit) that the rain simulation interfered with the retention of product in the leaf even 240 minutes (four hours) after application. As a consequence, it can be said that the treatment presented one day less chemical residual to control the disease than the fungicide treatment without subsequent rainfall simulation. Debortoli (2008), in a study with the objective of evaluating the effect of rain on the residual of azoxystrobin + cyproconazole in seven soybean cultivars, observed that simulated rain 240 minutes after application influenced the absorption rate of the fungicide.

In the simulated rain zero minutes after applying the fungicide, regardless of the water regime imposed on the plants, the fungicide was quickly absorbed by the wheat leaves. The delay in the appearance of the first pustule of Puccinia triticina in the flag sheets in relation to the witness allowed this conclusion.

There is a delay in the appearance of the first pustule and a significant reduction in the severity of P. triticin in plants under water deficit, when compared to plants in favorable conditions (without water deficit) (Figures 1 and 2). It can also be observed that the controls of plants under water deficit presented the first pustule two days later compared to plants under favorable conditions. Taiz and Zeiger (2013) suggest that plants under water deficiency respond against dehydration by thickening the cuticle to reduce transpiration, as well as the penetration of pathogens. According to Paiva and Oliveira (2006), several strategies are developed by plants in water deficit to reduce water loss and optimize the use of the small amount that can still be found in the soil.

The application of fungicide without subsequent simulation of rain on plants under water deficit represented a five-day delay in the appearance of the first pustule and a 3,2% reduction in the severity of P. triticin, in relation to plants without water deficit. The data suggest that the concentration of fungicidal active ingredient inside the cell is higher in plants under water deficit due to the reduction in leaf water potential and turgidity. The greater amount of active fungicide in cells and the lesser degradation of the fungicide in plants under water deficit, due to the lower physiological activity, the drop in stomatal conductance and the closure of the stomata (Stuhfauth et al, nineteen ninety; Ohashi et al, 2006; Law et al, 2006) may explain the reduction in severity values ​​and delay in the appearance of the first pustules.

Under stress conditions, the plant reduces the demand for ATP as a consequence of decreased growth and photosynthesis (Flexas; Medrano, 2002) and, therefore, reduces the degradation of the fungicide in plants under this condition.

The application of fungicide to wheat plants subjected to water deficit resulted in an increase in the disease control residual, as well as lower severity values. Regardless of the water regime, 240 minutes (four hours) is the minimum time interval between the application of the fungicide and the occurrence of rain for efficient control of the disease. Therefore, it is important to know the behavior of fungicide applications in wheat under different water regimes and rainfall simulations, with the aim of establishing sustainable management of the disease in search of high productivity.

Marlon Tagliapietra Stefanello, Leandro Nascimento Marques, Marcos Belinazzo Tomazetti, Renan Viero Dal Sotto and Ricardo Silveiro Balardin, Federal University of Santa Maria

Article published in issue 193 of Cultivar Grandes Culturas.