Soy Technical Notebook Part 2: Double copy

Soybean rust (Phakopsora pachyrhizi) has been present in Mato Grosso since the 2002/2003 harvest. In these 17 harvests, there were many lessons learned. Factors such as large areas of soybeans as the preferred host, a tropical environment with high humidity and prolonged leaf wetness; suitable temperature, rainy periods making fungicide applications difficult and a pathogen that multiplies at explosive rates have meant that rust has found Brazil to be one of the best places in the world to spread and cause great damage. As if this were not enough, the management tools for this disease are basically based on chemical control and escape, since there is practically no genetic resistance and other effective management tools.

Because of this, the exposure of the few efficient mechanisms of action to control the disease has been high, favoring the adaptation of the fungus to the site-specific fungicides currently used, as in the case of triazoles, strobilurins and carboxamides. The first cases of resistance were seen in 2006/2007, just five harvests after the start of product exposure, and confirmed in the following harvest, for triazoles. Currently, in Mato Grosso, there are populations with multiple resistance to the three previously mentioned mechanisms of action, and research observations show that the process of selecting resistant populations of the fungus to fungicides is not stabilized, that is, efficacies continue to decline year after year. after year.

Multisite fungicides have brought great comfort to the soybean business, contributing greatly to resistance management and improved disease control (Table 1).

However, this group of products also has its limitations, mainly due to the short residual effect (Figure 1) and the control spectrum. They work very well, as long as they are associated with site-specific fungicides. And in this context, for the site-specific + multisite combination to present good results, it is essential that the site-specific ones continue to deliver satisfactory efficacy, and thus have a complementary action to the multisites.

The biggest fears, especially in the regions where soybean rust has been more aggressive in Mato Grosso (Parecis, South and East of the State), are that the disease will manifest itself earlier than normal in recent years or that there will be a strong dry spell at the time of sowing, delaying planting, thus reaching the crop at younger stages. If this occurs, research has shown that the impact could be very large, as the disease can generate more cycles, causing more damage and losses. Furthermore, many producers have difficulty understanding the concept of multisites, claiming they do not see benefits, difficulty in application and constant increase in production costs. Finally, Mato Grosso also has a sowing system outside the normal planting window (December or February) to produce saved seeds, which enhances the process of selecting resistant fungi.

Target spot (Corynespora cassiicola), in Mato Grosso, began to gain greater importance at the end of the 1990s, with the launch of cultivars resistant to soybean cyst nematode that showed high susceptibility. Since then, a significant part of the materials released on the market have shown susceptibility and required attention to management. Despite being a less aggressive pathosystem than soybean rust, significant losses have been noted in some cultivars when the target spot is not efficiently controlled. The aggressiveness of the disease appears to have increased in recent years. Losses in susceptible materials, which until then were 5sc/ha to 8sc/ha, increased to 10sc/ha to 15 sc/ha in recent harvests, reaching up to 20sc/ha in the 2018/19 harvest. 

The process of selecting fungal populations that are less sensitive to fungicides is also present for the target spot. In 2009/2010, the reduced efficiency of benzimidazole fungicides against the disease was confirmed in the field. Unlike rust, current chemical control options for C. cassiicola are more restricted. Among the most effective fungicides, those containing prothioconazole, fluxapyroxad and bixafem stand out, that is, a very limited supply of active ingredients, which need to be used correctly in order to maintain effectiveness. A drop in performance for some carboxamides in the field has already been observed (confirmed by Frac in December/2018) and this raises awareness of management issues.

The fungus C. cassiicola can be present in other important crops in the state of Mato Grosso. One of them is cotton. There are more than 1 million hectares cultivated in the last harvest, where soybeans, under the remnants of this crop, present a greater severity of the disease and more difficulty in controlling it. Another species is Crotalaria spectabilis, very important in nematode management. However, it becomes a problem in the case of target spot, as, as a host, the severity of the disease has increased in soybeans grown in its stubble. Multisite fungicides have made an important contribution to controlling this disease, especially mancozeb. But, again, site-specific fungicides must continue to perform, so that multisites can complement control.

Considering that the target spot has increased in severity, that it has other hosts, that many soybean cultivars are susceptible and that chemical tools are limited, the suggestion is that the producer uses as many tools as possible in an integrated manner, knowing the history of the area, the choice of genetics, monitoring to define the correct moment of control, new fungicide formulations, the use of multisites and the rotation of mechanisms of action.

Cercospora leaf blight (Cercospora kikuchii) has been the most important End-of-Cycle Disease (CLD) in Mato Grosso, and has been present since the time fungicide application to soybeans began (around 1996 to 1998). This disease was initially controlled with benzimidazole fungicides at the end of the cycle, however, it was no longer seen due to the large number of applications of triazoles and strobilurins after the emergence of soybean rust. The launch of more susceptible cultivars and the reduction in the performance of some fungicides have brought back this disease, which has been present for the last three to four years, causing accelerated maturation of crops, shortening the cycle and reducing productivity by between 3sc/ha and 5sc/ha in some cultivars, in preliminary results. Identifying the most susceptible materials and treating them as needed has brought viable responses to this disease (Figure 2).

Finally, soybeans have experienced production ceilings never seen before. However, just as the increase in productivity has been rapid, challenges have arisen at the same speed (foliar and root diseases, nematodes, sensitivity to photoperiod, sowing times and others) and have caused the crop to become increasingly more technique and requires more knowledge to be able to explore the productive potential and guarantee profitability for the producer.

Check out part 1 here.

Check out part 3 here