How to deal with the presence of the nematode Aphelenchoides sp in RS

Dozens of specimens of the nematode Aphelenchoides sp, causing the Crazy Soy 2 phenomenon, were identified by the Phytus Institute Nematology laboratory in the aerial part of symptomatic plants from soybean crops in Rio Grande do Sul.

In the 2015/16 harvest, the Phytus Institute, in Rio Grande do Sul, received several complaints from producers and consultants from some municipalities in the state of Rio Grande do Sul regarding the occurrence of a physiological disorder in soybean plants. Clear differences in relation to the maturation cycle, parts in advanced stages of senescence and others expressing strong leaf retention and absence of vegetables.

Symptoms linked to physiological disorders in soybean plants are considered extremely complex, as they generally do not have a logical explanation. Although some biotic and abiotic factors may cause some type of disturbance in soybean plants.

Some preliminary hypotheses have been created and need to be taken into consideration. One of the first and possible causes initially ruled out was the relationship with bedbug attacks, but these areas did not show high levels of this pest. It is known that an intense attack by bedbugs can trigger a physiological disorder due to their diet. Depending on the stage of development (nymphs), it can lead to poor grain formation, followed by the fall of legumes and a delay in the reproductive phase. This type of anomaly has already been reported by researchers and called “crazy soy”.   

Another possible cause for the problem is linked to the water regime (excess or lack of rain) during the crop development period. In the first aspect, in some crops plants with normal development were observed. However, they were not small in size, a characteristic that is normally seen in areas with excess water, weakening this theory. Furthermore, in some areas, the terrain had a certain degree of slope, which clearly interferes with the excessive accumulation of water, making the environment unsuitable for this issue.  

At the other extreme, water stress can cause flowers and vegetables to abort, especially when the stress occurs during the flowering and vegetable formation phase. A determining factor in discarding this theory was in relation to the different formats of symptoms in crops (stripe, spots). Furthermore, it is worth noting that, in the 2015/16 harvest, the rainfall regime was approximately 20% higher than the historical average in Rio Grande do Sul, due to the El Ñino phenomenon.

Two other possible causes fell on the soybean variety, and climatic conditions linked to temperature and photoperiod. At first, crossing the information given by producers, it was observed that it was unlikely, as such symptoms appeared in at least five different varieties of soybeans, showing that they did not have a direct correlation with any specific variety. The causes regarding light and temperature are also unlikely, due to the distance between the places where the symptoms appear, as well as the distribution of the affected plants in the crops. It is worth remembering that the photoperiod is a variable characteristic of cultivars, where each one has its critical photoperiod and temperature plays a fundamental role throughout the phenological development stage of the plant.

 Another possible cause was based on nutritional deficiency. Nutritional imbalance directly interferes with plant development processes and, depending on deficiency levels, may cause some disturbance in their development. Examples include leaf retention, green stem, poor vegetable setting and even the formation of parthenocarpic seeds, among others. Furthermore, another question focused on a problem that until now is very recent, reported and scientifically proven (Koch's Postulate), by Embrapa researchers Mauricio Meyer and Luciany Favoreto from Epamig, then called “Crazy Soy 2”, caused by the nematode of the genre Aphelenchoides sp. The symptoms on the plants were very similar to those already reported by researchers.

The main symptoms observed in the plants of these crops in Rio Grande Sul were stunted and tapered leaves, with a dark green color, very elongated “fluted” stems and vegetables with grains of different sizes. In these areas, a high rate of vegetable abortion was also observed during the reproductive stage (R3 – R6), followed by an excessive number of sprouts in the armpits.         

Samples of the aerial part of plants similar to the symptoms of Soybean mad 2 were analyzed in the Nematology laboratory of the Phytus Institute, Rio Grande do Sul, where dozens of specimens of these nematodes were identified (Aphelenchoides sp) present in the aerial part of the plants. Nematodes from this family are known worldwide as phytoparasites of the aerial part of plants, surviving as mycophages and feeding on fungi present in the soil. There are diverse reports of damage to crops such as rice, ornamentals and forestry. The characteristic of a migratory ectoparasite allows it to cause infection/damage in the host and, subsequently, move to the next site of infection. Although the presence of this nematode was found in soybean plants, there are still a series of questions to be answered regarding the biology of this nematode, such as its infection, colonization and reproduction in soybeans. These aspects are essential for formulating measures aimed at management and control. In this case, there is still nothing concrete to recommend to producers. However, some aspects must be taken into consideration to try to clarify as quickly as possible the causes involving this nematode in soybeans. This study must include other areas, as in the case of physiological events, any factors (biotic and abiotic) may be involved with this anomaly as well.   

Paulo Sérgio Santos, Marcelo Madalosso, Instituto Phytus

Article published in issue 204 of Cultivar Grandes Culturas.