Asian rust challenges in the harvest

Panorama of incidence of Asian rust in Brazilian soybean-producing states points to the risks of worsening the disease, especially in areas sown late. Producers must pay attention to weather conditions and protect crops to avoid reductions in productivity. Rotation of fungicides and multisite association stand out among the necessary measures to increase the efficiency of site-specific products...

A soybean harvest in Brazil begins with the end of the periods of sanitary emptiness, which vary by state. This measure is currently one of the main strategies management of Asian rust and aims to reduce the inoculum of fungus Phakopsora pachyrhizi at off season. Crops sown shortly after the end of the sanitary void tend to have the disease at more advanced stages of crop development, but they begin the multiplication of the fungus, which is spread by wind, to more sown areas afternoon.

To the occurrences of Asian rust in the harvest can be checked on the site map of the Antirust Consortium (www.consorcioantiferrugem.net), also available in applications on iOS and Android platforms (Table 1). The main objective of map is to inform the first occurrences to alert the producer about the arrival of the disease. As the fungus is easily spread by wind, with alert, the producer can protect his crops, avoiding productivity losses. The Antirust Consortium is a public-private partnership and the information in website are inserted by researchers and technical assistance, but there is no 100% coverage in all producing regions. Other partnerships have been established to increase the scope of the site.

At 2017/18 harvest, the first occurrences of Asian rust were recorded on the website from the second half of November in São Paulo, Paraná, Rio Grande do Sul and Minas Gerais, in commercial crops, some irrigated, and in experimental areas used to monitor the fungus. More than 70% of the occurrences recorded on the website until the end of January were from crops in the grain filling phase (stage R5), which characterizes a calmer situation management for the first sowings. Although rust is one of the more severe diseases of the crop, its damage potential decreases as the incidence is late in the crop. This late incidence in the first sowings is one of the consequences of the inoculum reduction caused by the void Restroom.

In Rio Grande do Sul, Asian rust proved to be very severe in recent harvests. This disease continues to be the biggest concern of producers and technicians, due to the large amount of fungus inoculum from Bolivia, Paraguay and other regions of Brazil. Rio Grande do Sul is the only producing state that does not has a sanitary void and, in mild winter situations like 2017, there is maintenance of volunteer soybean with Asian rust in the field. Rainfall in November and January occurred above the historical average, with no major summers in the region until the end of January. At current harvest, the disease appeared 30 days earlier compared to the previous harvest (16/11/2017) and, despite being discreet in the vegetative phase of soybeans, now is on an upward curve, with the potential to become a major epidemic. The low number of occurrences in the state reported on the website of the Consortium can be explained by the fact that the producer knows the disease well and Do not send samples to laboratories to register the incident.

O state of Paraná has the highest number of reports on the Consortium website due to excellent coverage obtained through the collaboration of technical assistance. The disease has been reported in practically all producing regions. In the state, the Sowing was delayed by 10 days to 15 days, due to the delay in rain. Despite this delay, the first outbreak of Asian rust was reported in the same last harvest season (second half of November). The low rainfall in beginning of the harvest and then frequent rainfall at the end of December and the beginning January interfered with the development of crops, prolonging the crop cycle culture. As a result, the majority of harvests should occur from February. The frequency of rains made entry for water applications difficult. fungicides, causing delays and increased intervals. In some situations they are grain-filled crops reported with high severity of Asian rust in the downloader.

Rust problems tend to worsen in sown crops after November. Soybean crops sown in December and January, after beans, have presented symptoms already in the vegetative stage and will require a greater number of applications with the most efficient fungicides, increasing selection pressure resistance of the fungus to fungicides. These areas, although not significant in relation to the soybean area of ​​the main crop in the state, may accelerate the loss efficiency of fungicides. In this harvest, due to the delay in the start of the rains, the soybean sowing window was extended to January 14th in Paraná.

The majority of soybean crops in the state of Mato Grosso do Sul is in the filling phase of grains and in some areas the harvest is already taking place. Asian rust is present in practically all soy producing municipalities, but in general, Disease infestation is low and restricted to the lower reaches of the plants. In the region of Bonito and on the border with Paraguay, the situation is more serious and the severity of disease is high, with difficulty controlling the fungus. The intensity of the disease began to rise in the last week of January, in the state as a whole, indicating the possibility of a severe epidemic, which could compromise the productivity significantly in areas sown from November onwards.

In the southwest of Goiás, the occurrence of Asian rust has been unpredictable, starting more early in certain harvests or regions, even under less favorable conditions, or late under favorable weather conditions. These situations are directly related to the presence of the initial inoculum, reinforcing the need for a good sanitary void. In the southwest, the partnership between the Rural Union, the University of Rio Verde and collaborators have contributed to the monitoring the disease through foliar analysis for producers, having More than 1,3 samples had been analyzed by the end of January. Many of these Samples are collected in specific plots for monitoring (called sentinels), which do not receive fungicide application until the emergence of rust, when they are then eliminated to prevent the fungus from multiplying.

You first reports of Asian rust in this harvest occurred on 28/01/2018, in Rio Verde, by Agro Carrregal and on 29/01/2018, in Jataí, by the Federal University of Goiás, in soybeans at the R6 and R5 stages, respectively. In the 2016/17 harvest, the first report of Asian rust occurred on 11/01/2017, in the municipality of Jataí, followed by Rio Verde, on 19/01/2017. Both in the 2016/17 and current, all monitoring plots ended their cycle without symptoms of Asian rust detected, even with favorable climatic conditions for its occurrence. However, the extensive sowing window, exceeding 60 days, in southwest of Goiás has been a cause for concern, because crops sown from November onwards they can receive spores from other states already with occurrences of Asian rust.

At the Mato Grosso, in general, the 2017/18 harvest has been presenting rainfall above the normal average. In some locations, water restrictions were observed in the grain filling phase. The weather conditions in the months of December and January were favorable for the occurrence and development of Asian rust in the state.

The first occurrences were recorded in the municipalities of Campo Verde and Primavera do Leste, in the second half of December. Posteriorly, the regions of Parecis (Tangará da Serra, Deciolândia and Campo Novo do Parecis) and medium-morte (Nova Mutum, Lucas do Rio Verde and Sorriso) in the first half of January. In the region of Vale do Araguaia (Nova Xavantina to Vila Rica) there was no reports by the end of January.

Crops sown in September with Early cultivars (<105 days) have already been harvested. But in those sown with October onwards (around 60%), in some regions, applications of fungicides were delayed due to the high frequency of rain associated with operational capacity of properties. Considering the inoculum supply and the dispersion capacity of the fungus, significant losses of productivity in these areas.

A sowing in the western region of Bahia occurred within the scheduled calendar, of 08 from October to January 15th, as weather conditions were favorable to this operation. Crops sown in October, in areas irrigated, have already been or are being harvested, without problems with Asian rust.

The first report of the disease occurred on 03/01/2018, in the municipality of São Desidério, in a commercial area at the R3 stadium. With the weather conditions favorable to the development of the fungus, other foci were reported in this same municipality. If conditions remain favorable to the fungus, problems with Asian rust can worsen in the region, because there are crops in different phenological stages. To follow the region's outbreaks, Círculo Verde in partnership with the Rural Union of Luís Eduardo Magalhães (SRLEM) and employees from several agrochemical companies came together and started to offer the “Alerta Rust Laboratory” to the Rural Union for analysis free of charge of leaves with suspected disease.

In general, it is observed in all regions that crops sown earlier showed escape or late incidence of Asian rust and the situation is likely to worsen in late sowings. From January, one can observe, on the website, widespread occurrences in some states and may still increase in states that had late sowing. O producer must pay attention to weather conditions and protect crops to avoid reductions in productivity.

O number of fungicides with satisfactory efficiency for controlling Asian rust reduced in recent years with the resistance or less sensitivity of the fungus to three main groups of fungicides with site-specific mode of action: “triazoles” (demethylation inhibitor fungicides, IDM), “strobilurins” (External Quinone Inhibitors, IQe) and “carboxamides” (Inhibitors of Succinate Dehydrogenase, ISDH).

The rotation of action modes, recommended as anti-resistance strategy, it is not possible to be carried out in the culture once that fungicides are formulated in mixtures with two or three modes of action. Despite Therefore, the recommendation is to rotate fungicides. Although the resistance is crossed within the same mode of action, the active ingredients are affected differently by fungal mutations.

AND difficult to predict which mutations will be present in the fungus in the harvest and in the regions and, thus, to increase the efficiency of site-specific fungicides, once detected in the region, the producer must always seek to associate the multisite fungicide (based on copper, mancozeb and chlorothalonil) in applications for the control of Asian rust, due to the high potential for disease damage.

By Claudia V. Godoy, Embrapa Soya; José Fernando J. Grigolli, MS Foundation; Éder N. Moreira, Faculdade Centro Mato-grossense – FACEM; Hercules D. Campos, University of Rio Verde – UniRV; Carolina Deuner, University of Passo Fundo; Monica C. Martins, Green Circle

Article published in issue 225 of Cultivar Grandes Culturas, February, 2018.