Management of phytophthora leaf spot in soybeans

A root rot of phytophthora, caused by Phytophthora soyae, is disease that causes damage at any stage of development of soybean plants, with the power to put them to death. Rainy years and long periods of Waterlogging favors the disease. The degree of genetic tolerance of the cultivar chosen for planting plays an important role in preventing this pathogen.

A root rot of phytophthora, caused by Phytophthora soyae, is disease that causes damage at any stage of development of soybean plants, with the power to put them to death. Rainy years and long periods of Waterlogging favors the disease. The degree of genetic tolerance of the cultivar chosen for planting plays an important role in preventing this pathogen

A Soy is one of the most important crops in Brazil. However, annually the loss productivity caused by diseases can reach 20%. In the country, they have already been identified approximately 40 diseases caused by fungi, bacteria, nematodes and viruses, and this number tends to increase due to the expansion of areas soybean crops and also through the practice of monoculture (HENNING, 2009).

At the beginning of the 2016/2017 harvest were recorded in areas in the interior of the region Midwestern Paraná reports from producers about deaths of soybean seedlings in reboleiras. Visually in the field, many of these symptoms can be characteristic of pest attacks, physical soil problems and injuries caused by diseases, however, in collections of seedlings that showed wilting and death, it was observed through slides in an optical electron microscope, the occurrence of a diversity of fungi colonizing these seedlings. Among these pathogens, what prevailed was Phytophthora soyae, causal agent of the disease popularly known as root rot of phytophthora, which causes damage at any stage of plant development, which can lead to death depending on the degree of genetic tolerance of the cultivar.

A phytophthora root rot occurs in rainier years with periods prolonged waterlogging. In clayey soils the aggravating factor lies in the compaction, making water drainage difficult.

In according to Godoy, et al., (2016), the optimal temperature for pathogen proliferation is 25ºC, which can attack seeds in the soil causing rot. Young plants that are attacked may show yellowing and wilting of the leaves, and it is possible to find dead seedlings in the line of sowing both isolated and in small rows. In adult plants the Symptoms are leaf chlorosis (they dry out and remain attached to the stem, facing low), plant wilting and root rot.

Per present genetic mutation, some distinct breeds of Phytophthora soyae, like race 1 (which presents a compatible reaction to the gene Rps7) and race 3 (compatible reaction to genes Rps1a It is Rps7). Because it is a disease monocyclic oospores (reproduction structures) of the pathogen can survive for many years in the soil without the presence of the host (COSTAMILAN et al. 2007).

In due to the frequent occurrence of this pathogen, the climatic conditions in the sowing time and the type of soil in the region, the Research Center team Agricultural (CPA), evaluated the resistance or susceptibility of different soybean cultivars and isolates of Phytopthora soybean. O experiment was conducted between September 2016 and March 2017, in a greenhouse with a controlled environment, using the 20 most sown cultivars in region, with three replications in a completely randomized design.

For the isolation of the pathogen from the western region of Paraná used soybean plants of the 2016/2017 harvest, with symptoms of the disease (Figure 1), removing small cuttings from the infected parts of the plant, placing them in Petri dishes with specific and sterilized culture medium.

For To carry out the experiment, the toothpick methodology was used, adapted from Keeling (1982), consisting of the subculture of the Phytophthora soyae provided by Embrapa Wheat, Passo Fundo, Rio Grande do Sul, race 4, and isolated from crops in the western region of Paraná, which is in the process of identification. You isolates were kept in a B.O.D chamber at 25°C for 15 days, in Petri dishes, containing carrot-agar culture medium and toothpicks (cut into quarters size and with sharpened ends), as shown in figure 2, previously autoclaved at 127°C for 20 minutes.

To the soybean cultivars used in the test were sown in plastic pots with substrate based on organic compounds, with 10 plants per pot. Remained for 15 days in a greenhouse with controlled irrigation, and after this period, the inoculation of the pathogen into plants, introducing the toothpick containing mycelium approximately one centimeter below the cotyledonary node, carefully to avoid crossing the hypocotyl. The inoculated plants received irrigation controlled, for seven days, every two hours, for 20 minutes (Figure 3), and after During this period the evaluation was carried out.

A evaluation observed the incidence of dead, infected and healthy plants. Plants healthy are those that presented slight or no necrosis above or below the point of inoculation. Infected plants showed necrosis extending up to approximately one centimeter above and below the inoculation point. And, dead plants showed extensive necrosis above and below the point of inoculation, leading to death of the plant.

For the classification of cultivar reactions was taken into account percentage of dead plants (PM) (number of dead plants, added to number of infected plants divided by 2, multiplied by 100 divided by the total number of plants used (%PM=PM+(PI/2)*100/TP). Therefore, resistant plants (R) were considered with 0% PM; plants moderately resistant (MR) with 1% to 20% PM; moderately susceptible plants (MS) with 21% 50% PM; and, susceptible plants (S) with more than 50% PM.

Through of this experiment, using two isolates of P. sojae different, it was possible to observe that there are differences in the reaction of the cultivars, confirming that the P. sojae directly influence plant infection. When analyzed the incidence of resistant plants, which do not show signs of injury (Figure 4), the isolate provided by Embrapa Trigo showed a higher incidence (Table 1, Ps1), when compared to the isolate from the western region of Paraná, which presented a lower number of resistant plants (Table 1, Ps2), which makes It is possible to conclude that the isolate from the West region has greater severity.

Through observation of plants that presented moderate resistance and moderate susceptibility, it was possible differentiate them by the size of the lesion and number of plants infected by repetition. In plants with moderate resistance, a lower amount of infected plants, and the lesions close to the inoculation point were smaller (Figure 5), when compared to plants with moderate susceptibility, which showed a greater number of plants infected per repetition and a greater lesion size close to the inoculation point (Figure 6).

The susceptible plants were those that presented larger lesions close to the insertion point, which could in some cases they reach close to the insertion point of the first leaves. Consequently these plants had a higher number of plants infected and/or killed by repetition (Figure 7).

A Based on the results of this work, it was possible to identify the reactions of main cultivars sown in the region regarding this pathogen. AND also advance in the search for different resources for management, such as decompression soil, avoid sowing in times of excessive rainfall, use seed treatment with metalaxyl and mainly choose cultivars that tolerance to be used in areas that have already presented or may show problems with Phytophthora soybean.

Article published in issue 219 of Cultivar Grandes Culturas, August, 2017.