Rust has arrived. And now?

The first outbreak of Asian rust in commercial soybean crops in Mato Grosso do Sul was confirmed on November 19, in Maracaju. This fact sparks great reflection. Someone might say that it was expected, and they are right. Another might say that now we have to be extra careful, and they are also right. But our reflection can go far beyond apathy and alarmism.

How to control rust?

To reduce losses caused by this disease, we must insist on monitoring crops. Even to carry out preventive fungicide applications, we need to evaluate the crops to see if the application is truly preventive or if the rust is already installed in the crop. This is very important because if rust is already present, it is recommended to reduce the time interval between applications and add multisite (contact) fungicides to the control program. Other important recommendations are to choose the fungicides with the best performance in the annual efficiency trials, published annually by Embrapa (click here to access), and alternate applications of fungicides with different active ingredients, avoiding more than two applications of the same fungicide in the same harvest.

We must warn that the earlier rust appears in the plant's cycle, the greater the difficulty of control and the greater the potential losses due to control failures. The occurrence of rust at the beginning of flowering or before, in years with a climate favorable to the disease, can cause a drastic reduction in productivity, making harvesting impossible when not efficiently controlled. Fortunately, cases like this have not occurred, and this is due to legislative (sanitary void and deadlines for sowing) and cultural controls (early cultivars and sowing at the beginning of the recommended season), in addition to preventive and chemical control when necessary.

What about resistant cultivars?

The search for rust-resistant soybean cultivars has required a great effort from agricultural research. There are some resistant cultivars on the market, but all of them are based on monogenic resistance, that is, very efficient but which can be easily supplanted (“broken”) by the fungus that causes the disease. Therefore, even in these cultivars, fungicide must be applied preventively. But why sow resistant cultivars if we have to apply fungicide? Because the resistant cultivar considerably reduces damage due to control failures (and these failures are much more frequent than we imagine!). Cultivars with resistance genes are not immune, that is, the disease occurs, but does not progress or evolves very slowly. A resistant cultivar is a guarantee, an insurance, perhaps the cheapest insurance there is. Furthermore, resistant cultivars reduce fungal selection pressure on fungicides, delaying the emergence of resistant fungal populations. The latest soybean cultivar resistant to Asian rust is BRS 511 (click here to learn about the cultivar), an early cycle variety recommended for different regions of the country. The resistance gene of this variety is different from that present in all other previously released varieties. And, in addition to being resistant to rust, it is also resistant to phytophthora rot (click here to find out more), a common disease in the Southern region of Brazil, whose relevance has been growing in the Central-West.

What does rust like?

The presence of water on the surface of the leaves is an essential condition for the establishment of rust. Therefore, the longer the time with leaf wetness, the better the conditions for its development and the more severe the damage.

The time during which a plant remains with wet leaves is much more dependent on the frequency of rain than on its volume. Dew and irrigation are also determining factors. Hot days and mild nights, with high air humidity, favor the formation of dew and thus create favorable conditions for rust.

The focus of rust in MS

It is very interesting to note that the first crop in which rust was detected in MS was already at the R5 stage, that is, when filling pods. This is extremely important information, because it shows the result of two very valuable control measures adopted by the rural producer, which are: 1) carrying out the sanitary vacuum (to reduce the survival of rust in the off-season) and 2) sowing the crop at the beginning recommended time (so that the plants escape rust). If it weren't for these two control measures, rust would probably appear much earlier, perhaps even before flowering begins.

Despite this, this first outbreak of MS was detected much earlier than last year, when the first reports occurred in January. The number of days with rain, from the first of October to the 15th of November, was very similar between the years 2017 and 2018. However, in 2018 the volume of rain in this period was almost 200 millimeters higher, which maintained humidity in the high air and created conditions for longer leaf wetness, favoring rust. These climate data are available in Guia Clima, Embrapa Agropecuária Oeste's meteorological stations page (click here to access).

Another important piece of information is that the fact that rust has been detected now does not mean that it was not present before. We must remember that the registration of rust outbreaks in the Antirust Consortium (www.consorcioantiferrugem.net) is voluntary. While it is very important to notify the Consortium and the competent authorities about the occurrence of the disease, as this serves as a warning to producers, we cannot blindly trust in the absence of rust in places without reports of its occurrence. This reinforces the need for constant monitoring of each crop, especially from the beginning of flowering or the closure of rows, as there is no way to reliably predict when rust will appear.