Management of wound nematode in soybean-corn succession

The soybean-corn succession is extremely popular and accepted in Brazil as it allows for two harvests with crops that are easy to sell. Unfortunately, however, this practice caused a serious side effect with an increase in the incidence of nematode lesions. Pratylenchus brachyurus.

In 2003, this situation culminated in the first losses caused by the nematode in soybeans recorded in Brazil. Losses have evolved in an increasing spiral since then, motivating the adoption of control measures that are often drastic, such as replacing corn with Crotalaria spectabilis, C. ochroleuca or millet. Due to the importance of corn for the producer's income, there was great and understandable reluctance on the part of farmers to give up corn in the second harvest. In areas where losses caused by P. brachyurus are small or moderate, it is still possible to maintain the corn, using other control methods, such as synthetic and biological nematicides. There is still the alternative of the corn/C. spectabilis.

For a long time, especially in the years immediately after 2003, there was great investment in research to test corn hybrids and cultivars, looking for something that was resistant to P. brachyurus. As a result of these efforts, it is now known that there are corns resistant to P. brachyurus, but always with moderate resistance. As an example, the results presented in Table 1 can be seen. In this experiment, two reproduction variables were evaluated: FR (reproduction factor), which is the ratio between Pf (final population) and Pi (initial population), and estimates the population variation of the nematode during the experimental period; and Nem/g, which is the ratio between Pf and root mass, and estimates the density of nematodes present per unit mass.

Comparing the two variables in the six hybrids tested, P30K75 proved to be more resistant than the others, although less resistant than C. spectabilis. These results have been repeated in several other experiments, and the conclusion is that most materials in corn are susceptible to P. brachyurus (as the nematode reproduces a lot); C. spectabilis it is highly resistant (the nematode does not reproduce); and some hybrids, such as P30K75, are moderately resistant (the nematode reproduces, but less than in susceptible materials).

At this point, a brief comment is necessary. Perhaps the reader has already come across experimental results that lead us to believe in the existence of corn highly resistant to P. brachyurus. This is extremely unlikely. There are experiments that, although conducted honestly, yield debatable results, in which the FR values ​​for P. brachyurus in corn they were below 1, that is, they are experiments in which the nematode density decreased. However, caution is needed regarding these results. Low FR values ​​are probably obtained when experimental conditions are unfavorable for nematode development. For example, the most favorable temperature range for P. brachyurus is 25°C to 30°C. Therefore, experiments to obtain FR must be carried out in an environment within or close to this range. Table 2 presents the FR values ​​of P. brachyurus in corn roots maintained “in vitro” at six different temperatures. As results and conclusions, favorable temperatures (25°C and 30°C) = high reproduction; unfavorable temperatures (10°C and 35°C) = low reproduction and reduction in population density. Extrapolating these results to greenhouse and field conditions, it is possible to assume that FR values ​​below 1 for P. brachyurus in corn are probably obtained when experiments are carried out at temperatures unfavorable to the nematode (excessively low or high). Receiving such results, the farmer may believe that he can reduce the density of P. brachyurus, but this will only occur if conditions are very unfavorable to the nematode. If corn is sown and develops in times or places with favorable temperatures, P. brachyurus it will reproduce intensely in the corn roots, causing immense frustration for the farmer.

Therefore, what can be said with certainty is that there are corns that are moderately resistant to P. brachyurus. Is it worth using them to control nematodes? This answer was partially answered in two recently completed experiments. In the first, carried out in a greenhouse, the RF of 14 corn hybrids was estimated, which were classified into three categories: highly susceptible (one hybrid), susceptible (three) and moderately resistant (three), in addition to those intermediate between susceptible and moderately resistant according to the average test (seven hybrids). The objective of the first experiment was to choose the hybrids to be used in the second: two susceptible hybrids and two moderately resistant ones. These hybrids were sown in pots with soil containing P. brachyurus and maintained until the end of its production cycle. Soon after, soybeans were sown in the same pots, resulting in grain production. Table 3 shows the results of the two experiments, which indicate that soybean production is inversely proportional to the FR values ​​of the corn hybrids that preceded them. Although these are preliminary results, they are strong indications that the management of P. brachyurus can be done through the use of corn hybrids or cultivars that are moderately resistant to the nematode.

Even if these results are confirmed by subsequent experiments, sunn hemp C. spectabilis e C. ochroleuca will continue, for now, to be the main options for managing P. brachyurus in soybeans. Moderately resistant corn (it is important to highlight that they are moderately resistant!) should be recommended in areas infested by P. brachyurus, only for those farmers who definitely do not give up corn in the second harvest. An article published in 2017, whose results are summarized in Table 4, demonstrates that C. spectabilis e C. ochroleuca were superior in managing P. brachyurus in comparison with P30K75 corn, which is moderately resistant to the nematode (Table 1).

Conclusion

The most effective method for controlling P. brachyurus in soybeans it is the succession with C. spectabilis ou C. ochroleuca. However, for farmers who do not give up corn as a second crop, there are synthetic and biological nematicides, the corn consortium with C. spectabilis and, finally, corns moderately resistant to P. brachyurus. The methods suggested in item 2 should, whenever possible, be used in combination, to obtain an additive effect.

Mário Inomoto, Mariana Mailkut dos Santos, Esalq/USP