**2. Fungicide spray coverage**

118 Soybean Physiology and Biochemistry

when only the cultivation of soybeans used in research and for increasing generations provided by breeding lines is permitted under severe rust control conditions and subject to government control organs (MAPA), the severity of the disease has diminished. Nevertheless, the use of control practices of low efficiency, with inadequate fungicides, the use of reduced doses for lowering costs and inadequate number and duration of applications, unfortunately, contribute to persistence of the disease resulting in significant production losses. Continuous monitoring programs, adequate handling practices and appropriate application technology are necessary in order to guarantee the production of soybean culture. The relationship between lateness in the control of ASR and the severity of the disease is 0.25% for each day in which control is not carried out. The relationship between the return rate of the soybean and the severity of ASR is of -36 kg ha-1 for each

In order to define the strategies to be used for ASR control, regarding application technology, there must be an awareness of the way systemic fungicides move into plants after application and absorption has been carried out. In the present-day market, the majority of fungicides recommended for ASR control move from the base to the top of each leaf, with little chance of moving in the other direction and without the possibility of dislocation from one leaf to another (Antuniassi, 2005). Amongst the fungicides available at present for pathogen control, the triazole fungicides, when used alone, have not presented good performance, as can be seen with ciproconazole, propiconazole and meticonazole (Yorinori et al., 2010). The consistency shown in programmes for chemical control applied in a curative and preventive manner on different soybean varieties and growth stages of the crop has been evaluated by Navarini et al. (2007). The authors established that there was a tendency that higher profit rates were related to preventive applications between the R1 and R3 stages. They also established a low efficiency rate in the control of the pathogen when the fungicide propiconazole was applied in a preventive manner. A deficiency in the control of *P. pachyrhizi* was also observed 30 days after the spraying of the fungicide difeconazole on this crop, in a comparative evaluation of fungicides carried out by Soares et al. (2004). It therefore, becomes evident that triazole fungicides have some limited systemic activity (moving through the plant, especially to newly developed leaves) and are thus somewhat forgiving if the application is less than perfect. When triazole fungicides are mixed with strobilurin fungicides, they show better performance in the control of rust disease. In Brazil, it is believed that the causes of control failures may be related to technical failures in application, predominantly in a population shift which is more tolerant to triazole fungicides in some regions (*Fungicide Resistance Action Committee – FRAC*), instead of developing a tolerance or resistance to the triazole fungicides through genetic mutation of the fungus. As a precautionary measure, class representatives of the producers recommend

the use of triazoles only when mixed with other groups of fungicides.

An estimate of the volume of grain losses and of the economic impact of ASR in the period between 2002 and 2009 reached 34.2 million tons, a value equivalent to more than half a full soybean harvest. On the other hand, the economic impact of ASR, adding up grain losses (US\$ 7.95 billions), control costs (US\$ 5.76 billions) and intake losses (US\$ 1.55 billions) during the same period, totalled US\$ 15.25 billion (Yorinori et al., 2010). If we

**1.4 Economic impact of ASR in Brazil** 

severity percentage point (Calaça, 2007).

**1.3 Control of the pathogenic agent** 

The right moment for application is determined by climate conditions, the presence and severity of the disease, plant growth stage and fungicide efficiency (Yorinori et al., 2004). These factors, together with correct calibration of the application equipment and with correct handling practices aimed at the control of *P. pachyrhizi*, have not been sufficient to impede the advance of the disease in soybean culture. The necessity for more efficient application of phyto-sanitary products has been related to various researchers such as Adam (1977), Matuo (1990) and Van De Zande et al. (1994), amongst others. It can therefore be noted that, in order to obtain the best efficiency, the study and development of new application technologies are indispensable. Phyto-sanitary products must be applied with maximum efficiency and, for this to occur, studies of spray deposition and coverage and spray drift are necessary. This last factor is responsible for losses and is also a cause of environmental contamination (Matthews, 1992).

#### **2.1 Droplet size and spray coverage**

A definition of droplet size and the volume to be applied must be a priority in the planning of an application. Further factors, such as the correct time of application, weather conditions, product recommendations and operational conditions, should be considered as a whole, looking towards maximum performance with the least losses and the least environmental impact (Antuniassi, 2010). Spray volume has the greatest impact on canopy penetration and leaf coverage. Increasing the volume improves penetration and coverage. The recommended spray volume differs for each fungicide. For aerial applications, the minimum recommended volume is 5-7 gallons per acre (47-65 L ha-1). Recent research on soybean canopy coverage for ground applications at different growth stages of soybean (R1, R3 and R5) support recommendations that a spray volume of 15 gpa (140 L ha-1) may provide adequate coverage of the entire canopy early in the growing season (R1 and R3) but 20 gpa (187 L ha-1) is necessary later in the growing season (by R5) when the soybean canopy density and volume have increased (Brown-Rytlewski & Staton, 2010). In Brazil, the spray volume rates for conventional ground spraying of soybean have varied from 100 to 150 L ha-1, but it is possible to have a reduction of 50% in spray volume using the new spray technologies and earlier varieties. In the mid-west region (Cerrado), the use of low application rates with conventional ground sprayers is limited by climatic conditions due to the high temperature (30 to 40°C) and low air humidity (12 to 30%) during the greatest part of the year. Droplet size is the second most important factor affecting canopy penetration and leaf coverage. Research has shown that fine to medium droplets, with median volume diameters (MVD) in the range of 200 to 350 µm, maximise canopy penetration and leaf coverage. Smaller droplets provide better leaf coverage but lack the momentum to penetrate the canopy. Larger droplets have the momentum to penetrate the canopy but do not provide sufficient leaf coverage. Ground speed, nozzle pressure and spray volume should be considered when selecting nozzles for the sprayer. Choose nozzles that will produce 200-350 µm droplets at 15 to 20 gallons per acre (140 to 187 L ha-1) while travelling at the desired speed. In most cases, nozzles for herbicide applications should not be used for fungicide applications as they are designed to generate larger droplets at lower application rates. All nozzle manufacturers use a spray classification system (ASAE standard S-572) of six categories with corresponding colours to classify the droplet size range produced by nozzles under various operating pressures. The colour of the nozzle itself should not be confused with the colours listed in Table 1. The nozzle colour describes the flow rate for the nozzle and the colours on the table describe the nozzle's droplet size range. When using droplet size classification charts, select nozzles that produce droplets near the fine end of the medium (yellow) category.


Michigan State University – Department of Plant Pathology, USA

Table 1. ASAE Standard S-572 Spray Quality Categories

Ground speed affects spray volume and vertical droplet velocity. Taking into consideration that in order to apply fungicides, a fine to medium category of drops are indicated, and that the maximum wind speed during spraying should not surpass 9.6 km h-1 (Andef, 2004), a critical new situation presents itself in the field. Auto-propelled sprayers present innovations that give greater stability to the spray booms and with this, the operational speed increases to values rearing and even above 16 km h-1. The immediate consequence of this operational situation is that the relative wind between the boom in displacement and the air canopy which is present between the spray boom and the intended crop have a braking effect, contrary to the downward speed of the fine droplets generated at the tips of the sprayers. This process help with evaporation and also with the drift of the fine spray droplets and hinders its arrival on the crop canopies to be treated. A second consequence depends on middle-sized droplets that manage to maintain their falling speed in spite of the opposite effect generated by the dislocation speed of the boom. Research carried out recently on winter cereals, by the Institute DLG in Germany, shows that these droplets deposit themselves, on the whole, only on one side of the plants, with the other side ("shady side") consistently lacking in droplets (Boller & Raetano, 2011). The research also revealed that an increase in the displacement speed of the equipment implies in a greater deposit of droplets on the upper third of the plants and fewer droplets deposited on the lower leaves.

The increase in spraying pressure may partially compensate for this effect; however; one cannot emphasise too strongly that excessive working pressure is one of the most important factors that facilitate spray droplet drift. This picture deserves particular attention, due to the fact that the actual and future tendency is the increase in the displacement speed of the spray equipment by land. In the same situation, spray nozzles with flat double spray outlets show a slight increase in the quantity of droplets deposited on the side known as "the shady

considered when selecting nozzles for the sprayer. Choose nozzles that will produce 200-350 µm droplets at 15 to 20 gallons per acre (140 to 187 L ha-1) while travelling at the desired speed. In most cases, nozzles for herbicide applications should not be used for fungicide applications as they are designed to generate larger droplets at lower application rates. All nozzle manufacturers use a spray classification system (ASAE standard S-572) of six categories with corresponding colours to classify the droplet size range produced by nozzles under various operating pressures. The colour of the nozzle itself should not be confused with the colours listed in Table 1. The nozzle colour describes the flow rate for the nozzle and the colours on the table describe the nozzle's droplet size range. When using droplet size classification charts, select nozzles that produce droplets near the fine end of the

**Droplet category Colour Symbol MVD (µm)**  Very fine Red VF <150 Fine Orange F 150-250 Medium Yellow M 250-350 Coarse Blue C 350-450 Very coarse Green VC 450-550 Extremely coarse White XC >550

Ground speed affects spray volume and vertical droplet velocity. Taking into consideration that in order to apply fungicides, a fine to medium category of drops are indicated, and that the maximum wind speed during spraying should not surpass 9.6 km h-1 (Andef, 2004), a critical new situation presents itself in the field. Auto-propelled sprayers present innovations that give greater stability to the spray booms and with this, the operational speed increases to values rearing and even above 16 km h-1. The immediate consequence of this operational situation is that the relative wind between the boom in displacement and the air canopy which is present between the spray boom and the intended crop have a braking effect, contrary to the downward speed of the fine droplets generated at the tips of the sprayers. This process help with evaporation and also with the drift of the fine spray droplets and hinders its arrival on the crop canopies to be treated. A second consequence depends on middle-sized droplets that manage to maintain their falling speed in spite of the opposite effect generated by the dislocation speed of the boom. Research carried out recently on winter cereals, by the Institute DLG in Germany, shows that these droplets deposit themselves, on the whole, only on one side of the plants, with the other side ("shady side") consistently lacking in droplets (Boller & Raetano, 2011). The research also revealed that an increase in the displacement speed of the equipment implies in a greater deposit of droplets on the upper third of the plants and fewer droplets deposited on the lower leaves. The increase in spraying pressure may partially compensate for this effect; however; one cannot emphasise too strongly that excessive working pressure is one of the most important factors that facilitate spray droplet drift. This picture deserves particular attention, due to the fact that the actual and future tendency is the increase in the displacement speed of the spray equipment by land. In the same situation, spray nozzles with flat double spray outlets show a slight increase in the quantity of droplets deposited on the side known as "the shady

Michigan State University – Department of Plant Pathology, USA Table 1. ASAE Standard S-572 Spray Quality Categories

medium (yellow) category.

side". The most balanced situation was obtained when ends with flat double jets, with differentiated angles in relation to the vertical position, were utilised. The results indicate that this type of outlet may be efficient for a more even deposition of the droplets, on both sides of the plants, when the displacement speed of the boom is around 12 km h-1 (Boller & Raetano, 2011). There are basically two ways to increase coverage: 1) reduce droplet size and 2) increase carrier volume (application rate). Large droplets do not provide good coverage and result in chemical wastage. Increasing the application rate may be equally undesirable. It requires frequent refilling of the sprayer tank. This wastes time that may be extremely valuable when there is a short period of opportunity to spray. Ideally, we want to have as many small droplets on the target as possible. However, extremely small droplets have a tendency to drift. Research has shown that there is a rapid decrease in the drift potential of droplets whose diameters are greater than approximately 200 µm. When extremely small droplets are released from the nozzle, they quickly lose the momentum that is needed to push the droplets into the canopy. Also, these extremely small droplets do not last long after they are released from the nozzle. Most of them evaporate within a few seconds (Ozkan, 2010). The single most important factor affecting the control of ASR disease is to get a thorough coverage of soybeans with the fungicide, which is much different and more challenging than spraying for weeds and insects. The most effective coverage on soybean plants can be obtained with both the horizontal as well as vertical distribution of the fungicide on soybean leaves. Asian soybean rust usually shows its symptoms in the lower parts of the plant first and works itself up towards the top of the plant. The most effective spray equipment and methods for applying fungicides on soybean plants to control Asian soybean rust was studied by Ozkan et al. (2006). A second component of the study was to determine the effect of spray quality (fine, medium, coarse) on spray deposition and coverage using three different sizes (8002, 8004 and 8005) of the XR type of a flat fan nozzle operated at different spray pressures. The application rate was kept constant at 145 L ha-1 for all the treatments. The average spray coverage on the middle part of the soybean canopy (0.6 m above the ground) varied from 1.3 to 7.3% among the treatments. The Jacto sprayer provided the highest spray coverage on the middle part of the canopy, followed by Top Air sprayer and the boom sprayer with a TX-18 hollow cone nozzle that produced the lowest spray coverage on the middle part of the canopy, followed by Turbo duo, and then XR 8002 nozzles. The average spray coverage at the bottom part of the soybean canopy (0.3 m above the ground) varied from 0.5 to 3.9% among the treatments. Similarly to the coverage on the middle part of the canopy, the Jacto sprayer provided the highest spray coverage on the bottom part of the canopy, followed by the boom sprayer with the canopy opener and then the Top Air sprayer. The boom sprayer with XR 8002 nozzles produced the lowest spray coverage on the boom part of the canopy, followed by hollow cone TX-18 nozzles. XR 8002 flat fan nozzles and hollow cone nozzles had smaller MVD than other treatments with the boom sprayer. The authors observed that among the three spray qualities (fine, medium and coarse), the medium quality spray provided the highest coverage and the fine quality spray provided the lowest coverage at both middle and bottom parts of the canopy. When compared to the XR 8004 flat fan pattern nozzles with medium spray quality, Twinjet, Turbo dual pattern nozzles and hollow cone nozzles provided very low coverage on the middle and bottom parts of the canopy. Droplets from Twinjet, turbo dual pattern and hollow cone nozzles had poor penetration capabilities because these droplets had horizontal velocities. The horizontal movement of droplets consumed kinetic energy and caused droplets to easily settle on the top leaves. The influence of the size of droplets from different nozzles on soybean spray coverage was studied by Antuniassi et al. (2004). The authors verified that very fine quality spray obtained with hollow cone TX VK6 nozzle and Twinjet flat fan TJ 60 11002 nozzle, and fine quality spray with a flat fan pattern XR 11002 nozzle, provided greater coverage in middle and bottom parts of the soybean plants when compared to the extremely coarse spray quality produced by air induction flat fan nozzles. The effects of spray nozzles (flat fan pattern, pre-orifice flat fan, air induction flat fan and air induction twin flat fan) and volume rates (115 and 160 L ha-1) on chemical control of rust and the deposition of tebuconazole fungicide sprayed on soybeans of the Emgopa 313 variety, were studied by Cunha et al. (2006). The results showed that, despite the fact of the volume rate of 160 L ha-1 and of the use of pattern flat fan nozzles, they provided larger fungicide distribution uniformity in the plant canopy. There was no influence of the nozzle type neither of the application volume in the control of the rust, as well as in the soybean yield. In part, the results described by Raetano & Merlin (2006) ratified those observations that have been made by Cunha et al. (2006). The experiments were conducted in 2004/05 and 2005/06 seasons, using soybean, IAC-19 variety, with the same sprayer equipment and near application volumes (99 and 143 L ha-1; 100 and 150 L ha-1). The values of spray deposition were less influenced by nozzle type (hollow cone, flat fan and twin flat fan), both with fine spray quality. It is recommended for Asian soybean rust control that droplets have a size of 200 to 300 µm (OZKAN, 2005), but droplets smaller than 100 µm can be used with drift control in spraying with air assistance delivery systems near to the sleeve boom.
