Nightly application of fungicides

Spraying fungicides at night, when the weather is milder, can be a good alternative for combating diseases such as Asian rust in soybeans. From the end of the day, climatic extremes attenuate, which provides very interesting conditions for the production of drops and their positioning on leaf tissues. However, the operation should be avoided late in the morning and when rain is forecast the following day.

Brazil has a wide territorial strip, positioned in the tropical and subtropical regions of the globe. From a biological point of view, there is no other place on the planet that offers better conditions for a living being to shelter, feed and reproduce. In this way, agricultural crops are constant targets of the most intense biotic attacks ever recorded. Especially the soybean crop, due to its spatial and economic representation, suffers the negative impact of Asian rust, which accelerates leaf fall, damaging the formation and filling of grains. Faced with this, the alternative is to use fungicide applications to prevent the uncontrolled spread of this disease.

Spray application is a complex process and can be drastically influenced by weather elements such as temperature, relative humidity, wind speed and light. It is clear that these conditions influence the quality of the application technology, even more so with intense leaf development and inter-row closure, which consequently affect the effectiveness of the fungicides used by producers.

It is known that during much of the day, weather conditions are adverse for application. It is worth mentioning that, at the time of application, the temperature must be below 30°C, wind speed between 3km/h and 10km/h and relative humidity above 55% (ANDEF, 2004). Therefore, it is necessary to look for moments that present these conditions and that will favor an increase in the life of the droplet on the leaf surface. Thus, application at night appears as a strategy for applications in milder climate conditions. In the vast majority of cases, from the end of the day onwards, climatic extremes attenuate, presenting very interesting conditions for the producer to produce drops and position them on the leaf tissues.

 Regarding nighttime applications, there is a weather phenomenon that needs to be taken into consideration. This phenomenon is thermal inversion that could harm the application. The same occurs especially in the late afternoon and early evening in lower regions and close to forests. (FIGURE 1). Thermal inversion occurs due to the absence of wind and convection of heat that is stored and reflected to the surface, forming a layer of heat that prevents the penetration of drops into the lower canopy of the crop. Therefore, the presence of light wind is essential to remove this layer of heat and prevent the loss of droplets into the environment (FIGURE 2).

When analyzing the plant, it is noted that there are very important particularities for be observedthe quality of night application, especially in soybean cultivation, the nytinastia. This is the name given to the leaf movement that the leaves exert during the night and which can harm the application, if not taken into account. This term derives from the Greek “nyctos” = night and nastos = close, and produces non-directional responses to exogenous stimuli provoked by the presence or absence of light. Leaf movement is the responsibility of the pulvinus, a set of cells located at the base of the petiole of each leaflet responsive to turgidity (FIGURE 3). In this way, they are able to lose water quickly, due to the movement of chlorine and potassium ions through the membranes, causing movement of water into or out of the cell, which provides a change in turgor in the cell (Taiz and Zeiger, 2014). During the day, soybean plants position their leaves open to capture solar radiation and, as the night progresses until the end of the morning, this movement slowly closes and points the leaves towards the ground (FIGURES 4a1 – 16pm, 4a2 – 23pm and 4a3 – 4am; 4b1 – 16pm, 4b2 – 23pm and 4b3 – 4am).

Analyzing this context, it is possible to observe the dichotomy of the two moments, in relation to the day, climatic conditions at certain times are almost always adverse, but the stature of plants with erect leaves and at night, adequate climatic conditions and leaf movement are directed downwards. The great advantage of the latter is that the movement of the pulvinus is slow, and this allows fungicide applications to be carried out until sometime in the morning.

Several research works and field monitoring have directed towards this. In situations where application logistics cannot be achieved under the best conditions of the day, or especially in conditions of water stress due to lack of water, nighttime applications with fungicides are a fundamental strategy for plant protection. However, in nighttime applications, it is necessary to consider and avoid some situations such as late in the morning, rain the next day and surfactant adjuvants (organosilicone). The latter can be avoided under these conditions, as they will break the surface tension of the liquids and, then, there may be some adverse movement, either due to runoff or towards the petiole or stem of the plant.

In all tests conducted, at the end of the morning (between 3am and 6am) the efficiencies of applications for disease control were not satisfactory, remaining below the best times, 9am, 18pm and 23pm (FIGURE 5 and 6). This low performance is not directly linked to dew, as is attributed, but rather to the nyctinasty of the soybean leaves which, being positioned downwards, avoid capturing the drops from the application (FIGURE 7).

Another aspect that demands attention is precipitation after application. For example, nighttime applications that do not suffer the impact of rain the following day, achieve effectiveness very close to those carried out during the day (FIGURE 8). However, if rain occurs after the night application, much of the fungicide will be removed and the effectiveness will be low. This occurs because the penetration of the fungicide into the leaf tissues is slow at night, and requires more time than light conditions.

Therefore, the decision on the appropriate moment of application needs to be technically monitored, to avoid possible loss of effectiveness. Decision-making for nighttime application is extremely positive and of a high technical level, due to the necessary observations, which can optimize operations and in situations of water stress.

Marcelo Gripa Madalosso, Instituto Phytus and URI/Santiago; Marlon Tagliapietra Stefanello, Leandro Nascimento Marques, Pedro Cadore, Ricardo Silveiro Balardin, Federal University of Santa Maria

Article published in issue 214 of Cultivar Grandes Culturas.