Difficult to control, wheat head blight causes great damage to wheat

Fusarium head blight, caused by the fungus Gibberella zeae, It is a disease that can cause quantitative and qualitative losses in wheat, with significant damage to grain yield and hectoliter weight, as well as producing mycotoxins capable of compromising food safety.

In practice, this disease is difficult to control, both through genetic improvement and the use of fungicides. For this reason, efforts are needed to seek gains in the efficiency of FHB control in crops. In In the laboratory, fungicides have already been identified with the potential to control the disease up to 93,2%, however in the field, several factors interfere with the fungitoxic action. It can be deduced from this that control efficiency in crops can be improved by the quality of fungicide deposition at infection sites. This aspect is closely related to the appropriate time for spraying fungicides. The technology considers the beginning and end of the period of wheat predisposition to infection as being the beginning of flowering (anthers loose and attached) to the milky grain stage (anthers attached present), that is, from stage 60 to 75 on the scale by Zadoks et al (1974). This is the period during which the ears of wheat must be protected by fungicides.

The technology for applying phytosanitary products aims to place the active ingredients on the targets where their action is necessary, with maximum economy and minimum waste and contamination of humans and the environment.

Analyzing the low effectiveness of FHB control in crops, it is clear that the difficulties are largely due to the failure to deposit adequate quantities of fungicidal spray drops on the target where they are required (the lateral faces of the ears). ), characterizing a problem that directly involves application technology.

Target of deposition

The plant organs to be protected by the fungicide are the vertically oriented ears that have a cylindrical shape. The infections of Gibberella zeae occur during and after wheat anthesis until the milky grain stage, a period in which the action of fungicides is necessary to control this disease.

The infection sites for FHB are the anthers, especially those that are partially exposed or attached.

In reality, the target of fungicide deposition to control Fusarium head blight appears to have been little considered. It is observed that the spray jets do not reach the lateral sides of the ears, which must be protected, as the infection sites are located there. In part, this occurs because the same spraying equipment (droplets directed vertically) is used to control foliar and ear diseases.

Research has shown that jets facing at an angle of 30^o forward provided greater coverage on the ears than on the flag leaves. The market offers tips that generate double flat jets, one facing 30º forward and the other 30º backwards, in relation to the vertical. Empty conical jet tips are also available, which launch drops in a cone shape, directing them in all directions, including forwards and backwards.

The possibility of using assistance for the spray bar by means of an air curtain should also deserve attention. It is estimated that the study of the dynamics of the drops produced by different spraying equipment and their respective adjustments can be useful tools to help solve the problem.

Experimental works

During the 2011/2012 wheat harvests, trials were carried out in the experimental field at the University of Passo Fundo (UPF). Fungicide applications were compared using a traditional spray boom with nozzles spaced 0,50 m apart with a double boom with and without a wind protection cover (Table 1).

In an attempt to obtain a more uniform distribution of the spray drops around the wheat ears, a double bar capable of directing spray jets both parallel and perpendicular to the line of displacement, towards the lateral faces of the ears, was mounted.

The two bars and their respective nozzles were spaced 0,50 m apart. Considering the direction of travel when spraying, in boom 1 (ahead) the double nozzle bodies were oriented in a north/south direction (parallel to the path) and those in bar 2 (back) were in an east/west direction (perpendicular to the path). ). When empty conical jet tips were used, the bar was covered by a “skirt" composed of plastic film, to reduce wind interference that could cause excessive droplet losses due to drift. The wheat, cultivar Mirante, was trained in plots measuring 20m x 4m. On the eighth day after the beginning of flowering, the treatments were applied, as shown in Table 1. The fungicide pyraclostrobin + metconazole at 0,75L/ha + vegetable oil at 1,0L/ha + organosilicon adjuvant was applied 0,04L/ha.

Table 1 - Compared treatments and application technology parameters

Treatment	Nozzle body	Tips	Volume (L/ha)
1. Traditional bar	simple	Flat jet XR110015	150
2. Double slash	double	Flat jet XR110015	200
3. Double slash + Protective Case	double	Empty conical jet HB1	200*

Treatment

Nozzle body

Tips

Volume (L/ha)

1. Traditional bar

simple

Flat jet XR110015

150

2. Double slash

double

Flat jet XR110015

200

3. Double slash +

Protective Case

double

Empty conical jet HB1

200*

* The wind speed was 5,0km/h in all treatments, however, the presence of the protective cover mitigated the effect of the wind on the drops in treatment 3.

To evaluate droplet deposition, artificial ears (cylinders made from PVC pipes) were used, positioned within the wheat plots at the same height as the real ears. The 1,0cm diameter PVC tubes were cut into 10cm long segments and positioned vertically using steel "pegs" (wires) with a diameter of 1,5mm. The cylinders were wrapped with water-sensitive cardboard. Five artificial ears were distributed randomly within each plot.

After spraying, the images on the hydrosensitive cards were digitized using the CIR 1.5 software. The number of droplet impacts/cm², the volumetric median diameter (DMV - µm) and coverage (%) were determined in each “quadrant” of the card, which was named according to the direction of application in north (N), south (S), east (L) and west (W) The data were subjected to analysis of variance and comparison of means using the Duncan test at 5% probability of error.

Preliminary results

The bars showed differences in coverage uniformity in the N (north), S (south), L (east) and W (west) quadrants of the ears (Table 2). With the traditional bar (vertical jet), there was greater coverage on the N and L sides of the cobs, demonstrating that this equipment does not uniformly cover all sides of the cobs, which can also be explained by the action of the wind causing drift in the N and N directions. L of the ears.

Regardless of the tip model, the double bar provided similar coverage on all faces of the tang. With the empty-cone tip there was a greater number of impacts/cm² and lower DMV around the tang, increasing the chance of reaching the target. The deposition quality was similar with the fan jet tip mounted on the double bar, reinforcing that the direction of the jets must also be perpendicular to the spikes. The conical-filled jet tips (Micron-HB1) showed better performance with the protective cover, indicating that they require this protection, even in conditions of wind speed below 8km/h, due to the greater susceptibility to drift of the fine droplets they generate. , when compared to drops generated by simple flat jet tips (XR11001).

Table 2 - Efficiency of a traditional bar and a double bar equipped with two tip models in the deposition of drops on the sides of artificial wheat ears. FAMV/UPF- Passo Fundo/RS, 2012

Equipment	Tenon face	Impacts/cm2	DVM* (µm)	Covered area** (%)
Traditional bar with series flat jet tips XR 110015	N	256	225,5	18,3 to
	S	22,3	265,9	2,2 b
	L	169,3	222,4	13,8 to
	O	42,3	247,7	3,7 b
	Media	122,4	240,4	9,5
Double bar with series flat jet tips XR 110015	N	233,6	295,2	14,9 to
	S	119,5	312,1	14,0 to
	L	171,5	359,3	16,6 to
	O	256,8	248,1	13,9 to
	Media	195,3	303,7	14,8
Double bar with protection and conical jet tips empty, series HB1	N	260,8	228	15,4 to
	S	196,1	257,3	13,7 to
	L	228,5	222,7	12,7 to
	O	145	241,4	12,0 to
	Media	207,6	237,4	13,4

Equipment

Tenon face

Impacts/cm²

DVM* (µm)

Covered area** (%)

Traditional bar

with series flat jet tips

XR 110015

N

256

225,5

18,3 to

S

22,3

265,9

2,2 b

L

169,3

222,4

13,8 to

O

42,3

247,7

3,7 b

Media

122,4

240,4

9,5

Double bar with series flat jet tips

XR 110015

N

233,6

295,2

14,9 to

S

119,5

312,1

14,0 to

L

171,5

359,3

16,6 to

O

256,8

248,1

13,9 to

Media

195,3

303,7

14,8

Double bar with protection and conical jet tips

empty, series HB1

N

260,8

228

15,4 to

S

196,1

257,3

13,7 to

L

228,5

222,7

12,7 to

O

145

241,4

12,0 to

Media

207,6

237,4

13,4

(*) - Volumetric median diameter of impacts on sensitive card (µm)

(**) – Percentage of the surface of the artificial ears covered by impacts from spray drops, where averages followed by the same letters, within each piece of equipment, do not present significant differences between the faces of the artificial ears - Duncan test at 5% probability error.

With the use of the double bar, the percentage of coverage of the spike surface area was greater (13,4% - conical jets, and 14,8% - flat jets), compared to the traditional bar with flat jet tips ( 9,5%), which corresponds to coverage gains of 41% and 56%, respectively.

These results are preliminary and support the hypothesis that there is a need to direct the spray jets towards the lateral faces of the ears. To prove whether the coverage obtained from 13% to 15% is efficient in controlling Fusarium head blight, field trials are being conducted at the University of Passo Fundo (UPF).

Final considerations

- Anthers attached these are the sites of infection, a critical location, where the potent fungicide must be deposited, guaranteeing protection during the predisposition period;

- The uniform deposition of fungicides on the lateral sides of wheat ears is still the biggest challenge for controlling Fusarium head blight;

- Directing the spray jet in a direction perpendicular to the side of the cob is a strategy that allows it to reach the entire surface of the cobs with greater coverage and homogeneity;

- "Artificial ears" are a useful tool for research in evaluating the quality of coverage on the lateral sides of wheat ears.